Difference between revisions of "Paper - The development of the thymus"

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the histogenesis of the thymus in mammals. Special attention is given to the origin and development of the corpuscles of Hassall, since their mode of formation has never been satisfactorily described in mammals and their significance in all forms is in dispute. An attempt is also made to show in detail the changes that occur during the transformation of the thymus from the epithelial to the lymphoid condition.
  
the histogenesis of the thymus in mammals. Special attention is given
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This work was begun at the suggestion of Dr. D. D. Lewis at the University of Chicago. The greater part of it has been done at the University of Missouri. Special acknowledgments are due Dr. C. M. Jackson for valuable criticism and suggestions. I wish also to thank Mr. Charles H. Miller of the University of Chicago for his kindness in sending me material.
to the origin and development of the corpuscles of Hassall, since their
 
mode of formation has never been satisfactorily described in mammals
 
and their significance in all forms is in dispute. An attempt is also
 
made to show in detail the changes that occur during the transformation
 
of the thymus from the epithelial to the lymphoid condition.
 
 
 
This work was begun at the suggestion of Dr. D. D. Lewis at the
 
University of Chicago. The greater part of it has been done at the
 
University of Missouri. Special acknowledgments are due Dr. C. M.
 
Jackson for valuable criticism and suggestions. I wish also to thank
 
Mr. Charles H. Miller of the University of Chicago for his kindness in
 
sending me material.
 
  
 
MATERIAL AND METHODS.
 
MATERIAL AND METHODS.
  
As material for the greater part of my work, I have used pig embryos
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As material for the greater part of my work, I have used pig embryos from 8 mm. to full term (26 cm. to 30 cm.). These are especially suitable for such work since they may be procured’ in abundance from the large packing houses at almost any stage of development. For special purposes I have studied a few specimens from human foetuses, and from the cat, rat, and guinea pig. The smaller pigs used (8 mm. to 27 mm.‘) belong to the collection in the anatomical laboratory at the University of Missouri. These were stained in bulk with alum-cochineal and mounted in serial sections. In the later stages, which were prepared specially for this work, the ventral half of the cervical and anterior thoracic regions was usually cut out and embedded from pigs from 3 cm. to 8 cm. On specimens from 8 cm. to 30 cm., I dissected out the thymus and used such parts as were desired.
from 8 mm. to full term (26 cm. to 30 cm.). These are especially suitable for such work since they may be procured’ in abundance from the
 
large packing houses at almost any stage of development. For special
 
purposes I have studied a few specimens from human foetuses, and from
 
the cat, rat, and guinea pig. The smaller pigs used (8 mm. to 27 mm.‘)
 
belong to the collection in the anatomical laboratory at the University of
 
Missouri. These were stained in bulk with alum-cochineal and mounted
 
in serial sections. In the later stages, which were prepared specially for
 
this work, the ventral half of the cervical and anterior thoracic regions
 
was usually cut out and embedded from pigs from 3 cm. to 8 cm. On
 
specimens from 8 cm. to 30 cm., I dissected out the thymus and used such
 
parts as were desired.
 
  
 
’ The crown-rump measurement is used in all cases.
 
’ The crown-rump measurement is used in all cases.
  
AMERICAN JOURNAL or A:\'A'roMY.—Vor.. V.
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AMERICAN JOURNAL or A:\'A'roMY.—Vor.. V. 30 The Development of the Thymus
30 The Development of the Thymus
 
  
All pig material was fixed in Zenker’s fluid, embedded in parafiin, and
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All pig material was fixed in Zenker’s fluid, embedded in parafiin, and mounted in serial sections from 3 ,u to 10 ,u. thick. Except those of the young stages (8 mm. to 27 mm.) the sections were stained on the slide. Most of them were stained with haematoxylin or iron-haematoxylin and counterstained with Congo red. For special purposes many other stains were used.
mounted in serial sections from 3 ,u to 10 ,u. thick. Except those of the
 
young stages (8 mm. to 27 mm.) the sections were stained on the slide.
 
Most of them were stained with haematoxylin or iron-haematoxylin and
 
counterstained with Congo red. For special purposes many other stains
 
were used.
 
  
To demonstrate the delicate protoplasmic threads of the syncytium
+
To demonstrate the delicate protoplasmic threads of the syncytium during the later stages of the lymphoid transformation, I stained by the iron—haematoxylin method but omitted the final decolorization in ironalum. Protoplasm is stained deep black; nuclear structure is poorly shown, but the finest cytoplasmic processes may be seen.
during the later stages of the lymphoid transformation, I stained by the
 
iron—haematoxylin method but omitted the final decolorization in ironalum. Protoplasm is stained deep black; nuclear structure is poorly
 
shown, but the finest cytoplasmic processes may be seen.
 
  
For the demonstration of connective tissue fibrillae in the syncytium,
+
For the demonstration of connective tissue fibrillae in the syncytium, I found the method recommended by Jackson (13, S. 39) most satisfactory.
I found the method recommended by Jackson (13, S. 39) most satisfactory.
 
  
 
  
 
TEXT FIG. 1. TEXT FIG. 2.
 
TEXT FIG. 1. TEXT FIG. 2.
  
TEXT FIGURE 1. Cranial view of third gill pouch (thymic anlage); X 33;
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TEXT FIGURE 1. Cranial view of third gill pouch (thymic anlage); X 33; pig embryo, 11 mm.; ec, ectoderm: Z, lumen; n t, nodulus thymicus; ph, connection to pharynx; s p, sinus praacervicalis.
pig embryo, 11 mm.; ec, ectoderm: Z, lumen; n t, nodulus thymicus; ph, connection to pharynx; s p, sinus praacervicalis.
 
  
TEXT FIGURE 2. Ventral view of thymic anlage; X 33; pig embryo, 15 mm.;
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TEXT FIGURE 2. Ventral view of thymic anlage; X 33; pig embryo, 15 mm.; ec, ectoderm; Z, lumen; nt, nodulus thymicus; ph, connection to pharynx; s p, sinus praecervicalis.
ec, ectoderm; Z, lumen; nt, nodulus thymicus; ph, connection to pharynx;
 
s p, sinus praecervicalis.
 
  
To determine the relation of the blood-vessels to the corpuscles of
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To determine the relation of the blood-vessels to the corpuscles of Hassall, I put a young kitten under deep anaesthesia and injected a large quantity of a strong aqueous solution of Prussian blue into the aorta through the common carotid artery. The heart continues to beat even after an amount of fluid twice as great as the total volume of the blood has been injected. An injection made in this way is under a slightly increased blood pressure and easily reaches the finest capillaries. There is therefore a thorough injection with little danger of rupturing delicate blood-vessels.
Hassall, I put a young kitten under deep anaesthesia and injected a large
 
quantity of a strong aqueous solution of Prussian blue into the aorta
 
through the common carotid artery. The heart continues to beat even
 
after an amount of fluid twice as great as the total volume of the blood
 
has been injected. An injection made in this way is under a slightly
 
increased blood pressure and easily reaches the finest capillaries. There
 
is therefore a thorough injection with little danger of rupturing delicate
 
blood-vessels.
 
  
 
ORGANOGENESIS.
 
ORGANOGENESIS.
  
My observations on this phase of development are not sufficiently complete to warrant a full discussion. A brief survey may however prepare
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My observations on this phase of development are not sufficiently complete to warrant a full discussion. A brief survey may however prepare the Way for a better understanding of the histogenesis. The text figures show the shape in outline of the third gill pouch and thymic anlage E. T. Bell 31
the Way for a better understanding of the histogenesis. The text figures
 
show the shape in outline of the third gill pouch and thymic anlage
 
E. T. Bell 31
 
  
from 11 mm. to 27 mm. They are graphic reconstructions. Since this
+
from 11 mm. to 27 mm. They are graphic reconstructions. Since this pouch is nearly all converted into thymus it may be regarded as the thymic anlage from a very early stage.
pouch is nearly all converted into thymus it may be regarded as the
 
thymic anlage from a very early stage.
 
  
At 11 mm. (Text Figure 1), the pouch is a hollow epithelial tube directed from without ventrally and mesially. The lumen (Z) is large
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At 11 mm. (Text Figure 1), the pouch is a hollow epithelial tube directed from without ventrally and mesially. The lumen (Z) is large and communicates freely with the pharynx. On the dorso—lateral aspect of the pouch is a solid epithelial mass (n t) distinctly different in structure from the rest of the pouch. This is the nodulus thymicus (Kastschenko, 14) and will be referred to by that term. This structure has been described by Stieda (26), Prenant” (22), and others as the anlage of the carotid gland.
and communicates freely with the pharynx. On the dorso—lateral aspect
 
of the pouch is a solid epithelial mass (n t) distinctly different in structure from the rest of the pouch. This is the nodulus thymicus
 
(Kastschenko, 14) and will be referred to by that term. This structure
 
has been described by Stieda (26), Prenant” (22), and others as the
 
anlage of the carotid gland.
 
  
It was evidently mistaken by Minot 3 in a 12-mm. pig for the anlage of
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It was evidently mistaken by Minot 3 in a 12-mm. pig for the anlage of the entire thymus. It may be seen as early as the 8 mm. stage budding off from the cranio—lateral aspect of the pouch. Immediately behind the nodulus thymicus, but not connected to it at this stage is the inner blind extremity of the sinus praecervicalis (s p). These become fused at 12 mm. or 13 mm.
the entire thymus. It may be seen as early as the 8 mm. stage budding
 
off from the cranio—lateral aspect of the pouch. Immediately behind the
 
nodulus thymicus, but not connected to it at this stage is the inner blind
 
extremity of the sinus praecervicalis (s p). These become fused at
 
12 mm. or 13 mm.
 
  
At 15 mm. (Text Figure 2) the thymic anlage is more elongated. It
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At 15 mm. (Text Figure 2) the thymic anlage is more elongated. It now projects ventrally and medianwards, its free end lying immediately caudad to the median thyroid anlage and just craniad to the pericardium. Its lumen (Z) is still in communication with the pharynx. The ‘sinus praecervicalis (s p) is drawn out, its lumen being smaller and longer. It is now fused to the outer two-thirds of the posterior aspect of the nodulus thymicus (n t).
now projects ventrally and medianwards, its free end lying immediately
 
caudad to the median thyroid anlage and just craniad to the pericardium.
 
Its lumen (Z) is still in communication with the pharynx. The ‘sinus
 
praecervicalis (s p) is drawn out, its lumen being smaller and longer.
 
It is now fused to the outer two-thirds of the posterior aspect of the
 
nodulus thymicus (n t).
 
  
At 18 mm. (Text Figure 3) the anlage is growing rapidly in a caudal
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At 18 mm. (Text Figure 3) the anlage is growing rapidly in a caudal direction and just entering the thoracic cavity. It is connected to the pharynx by a delicate epithelial cord. There is still a lumen in its caudal part. The sinus praacervicalis has lost its connection to the nodulus thymicus.‘ The outer part of its lumen has disappeared and it seems about to lose its connection with the ectoderm.
direction and just entering the thoracic cavity. It is connected to the
 
pharynx by a delicate epithelial cord. There is still a lumen in its caudal
 
part. The sinus praacervicalis has lost its connection to the nodulus
 
thymicus.‘ The outer part of its lumen has disappeared and it seems
 
about to lose its connection with the ectoderm.
 
  
At 20 mm. (Text Figure 4) the thymus extends well into the thoracic
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At 20 mm. (Text Figure 4) the thymus extends well into the thoracic cavity. Its thoracic segment (25 h) has increased considerably in size and is united to the gland of the opposite side.
cavity. Its thoracic segment (25 h) has increased considerably in size and
 
is united to the gland of the opposite side.
 
  
The nodulus thymicus still forms the greater part of the head. The
+
The nodulus thymicus still forms the greater part of the head. The anlage has no connection with the pharynx or the epidermis. There is
anlage has no connection with the pharynx or the epidermis. There is
 
  
’Prenant is said to have since abandoned this idea and accepted Kestschenko’s view. (v. Ebner in K'ci11iker’s Gewebelehre des Menchen. Aufl.
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’Prenant is said to have since abandoned this idea and accepted Kestschenko’s view. (v. Ebner in K'ci11iker’s Gewebelehre des Menchen. Aufl. 6, Bd. 3, 1, S. 340.)
6, Bd. 3, 1, S. 340.)
 
  
 
’Laboratory Text of Embryology, p. 191; also p. 209 and Fig. 124.
 
’Laboratory Text of Embryology, p. 191; also p. 209 and Fig. 124.
  
‘ On the opposite side in this specimen, these structures were fused over a.
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‘ On the opposite side in this specimen, these structures were fused over a. very small area. 32 The Development of the Thymus
very small area.
 
32 The Development of the Thymus
 
  
now an elongated mass of thymic tissue extending upwards behind the
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now an elongated mass of thymic tissue extending upwards behind the nodulus thymicus, and fused with it. Its upper pointed extremity curves outwards around the hypoglossal nerve. This is the “thymus superficialis ” (t s) of Kastschenko and is regarded by him as being formed from the sinus praecervicalis. Kast-schenko describes this elongated portion as being always separate from the rest of the head, being connected only by connective tissue. In my preparations it is clearly continuous with the rest of the anlage, and seems to have formed by growing out from it. The presence of a lumen in its lower end favors Kast—
nodulus thymicus, and fused with it. Its upper pointed extremity curves
 
outwards around the hypoglossal nerve. This is the “thymus superficialis ” (t s) of Kastschenko and is regarded by him as being formed
 
from the sinus praecervicalis. Kast-schenko describes this elongated
 
portion as being always separate from the rest of the head, being connected only by connective tissue. In my preparations it is clearly continuous with the rest of the anlage, and seems to have formed by growing
 
out from it. The presence of a lumen in its lower end favors Kast—
 
  
 
/is
 
/is
  
/E ’_}”
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/E ’_}” \ EC 310 I
\ EC 310 I
 
  
 
]
 
]
Line 173: Line 91:
 
TEXT Frc. 3. TEXT FIG. 4.
 
TEXT Frc. 3. TEXT FIG. 4.
  
Tnxr FIGURE 3. Ventral view of thymic anlage; X 33; pig embryo, 18 mm.;
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Tnxr FIGURE 3. Ventral view of thymic anlage; X 33; pig embryo, 18 mm.; 60, ectoderm; Z, lumen; nt, nodulus thymicus; ph, connection to pharynx; s 12, sinus przecervicalis.
60, ectoderm; Z, lumen; nt, nodulus thymicus; ph, connection to pharynx;
 
s 12, sinus przecervicalis.
 
  
Tnxr FIGURE 4. Ventral view of thymic anlagef X 33; pig embryo, 20 mm.;
+
Tnxr FIGURE 4. Ventral view of thymic anlagef X 33; pig embryo, 20 mm.; 7‘, area fused with gland of opposite side; I, lumen; nt, nodulus thymicus; th, thoracic segment; ts, thymus superflcialis.
7‘, area fused with gland of opposite side; I, lumen; nt, nodulus thymicus;
 
th, thoracic segment; ts, thymus superflcialis.
 
  
schenko’s view, for there is no lumen in the head at 18 mm. in my preparations. On the other hand the separation from the head at the 18 mm.
+
schenko’s view, for there is no lumen in the head at 18 mm. in my preparations. On the other hand the separation from the head at the 18 mm. stage favors the idea that the sinus praecervicalis degenerates. I have not studied a sufficient number of specimens at this transition stage to enable me to decide this point, though I believe the ectoderm takes no part in the formation of the thymic anlage. Kastschenko’s results are opposed by nearly all other students of this problem, but his work should not be discarded before the development of the thymus superficialis in the pig has been accurately determined.
stage favors the idea that the sinus praecervicalis degenerates. I have not
 
studied a sufficient number of specimens at this transition stage to
 
enable me to decide this point, though I believe the ectoderm takes no
 
part in the formation of the thymic anlage. Kastschenko’s results are
 
opposed by nearly all other students of this problem, but his work should
 
not be discarded before the development of the thymus superficialis in
 
the pig has been accurately determined.
 
  
At 27 mm. (Text Figure 5) the thymus is much longer and extends
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At 27 mm. (Text Figure 5) the thymus is much longer and extends E. T. Bell 33
E. T. Bell 33
 
  
well down into the thoracic cavity in relation to the base of the heart
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well down into the thoracic cavity in relation to the base of the heart where it has fused with the gland of the opposite side. Buds are now beginning to form through the greater part of its extent. Traces of the original lumen (Z) may still be seen in several places. The “thymus superficialis ” is bent around the twelfth nerve. A delicate cord of thymic
where it has fused with the gland of the opposite side. Buds are now
 
beginning to form through the greater part of its extent. Traces of the
 
original lumen (Z) may still be seen in several places. The “thymus
 
superficialis ” is bent around the twelfth nerve. A delicate cord of thymic
 
  
 
TEXT FIG. 5.
 
TEXT FIG. 5.
  
TEXT FIGURE 5. Ventral View of thymic anlage; X 33; pig embryo, 27 min;
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TEXT FIGURE 5. Ventral View of thymic anlage; X 33; pig embryo, 27 min; f, area fused with gland of opposite side; I, lumen; nt, nodulus thymicus; th, thoracic segment; ts, thymus superflclalis.
f, area fused with gland of opposite side; I, lumen; nt, nodulus thymicus;
 
th, thoracic segment; ts, thymus superflclalis.
 
  
 
tissue fused with the back of the nodulus thymicus connects the thymus
 
tissue fused with the back of the nodulus thymicus connects the thymus
  
superficialis to the rest of the head.
+
superficialis to the rest of the head. From about 3 cm. until toward the end of foetal life the thymus shows the two constrictions, described by Prenant (22) (for the sheep) as the
From about 3 cm. until toward the end of foetal life the thymus shows
 
the two constrictions, described by Prenant (22) (for the sheep) as the
 
  
intermediary and the cervico-thoracic cords. These cords connect three
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intermediary and the cervico-thoracic cords. These cords connect three 3 34 The Development of the Thymus
3
 
34 The Development of the Thymus
 
  
enlargements which we may call the head, the mid-cervical segment, and
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enlargements which we may call the head, the mid-cervical segment, and the thoracic segment. I will consider each part separately. The thoracic segment develops rapidly, spreading out above and in front of the heart. The glands of the two sides fuse completely in this region. The lymphoid transformation is noticeable at 3.5 cm. and well advanced at 4.5 cm. The medulla begins to form at 8 cm. The cervico—thoracic cord is at first very narrow but soon thickens and joins the c_ord of the opposite side. At full term they form a sharp constriction, 3 mm. to 4 mm. wide and 5 mm. to 6 mm. long, situated at the superior aperture of the thorax, and connecting the mid-cervica.l and thoracic segments. The histological changes take place here later than in the enlargements.
the thoracic segment. I will consider each part separately. The thoracic
 
segment develops rapidly, spreading out above and in front of the heart.
 
The glands of the two sides fuse completely in this region. The lymphoid
 
transformation is noticeable at 3.5 cm. and well advanced at 4.5 cm. The
 
medulla begins to form at 8 cm. The cervico—thoracic cord is at first
 
very narrow but soon thickens and joins the c_ord of the opposite side.
 
At full term they form a sharp constriction, 3 mm. to 4 mm. wide and
 
5 mm. to 6 mm. long, situated at the superior aperture of the thorax,
 
and connecting the mid-cervica.l and thoracic segments. The histological
 
changes take place here later than in the enlargements.
 
  
The mid-cervical segment develops like the thoracic segment but somewhat more slowly. Budding, lymphoid transformation, and formation
+
The mid-cervical segment develops like the thoracic segment but somewhat more slowly. Budding, lymphoid transformation, and formation of the medulla all begin here a little later than in the head and thoracic segment. Its caudal end is slightly in advance of its cranial end. The intermediary cord is well marked at 4 cm. It -soon becomes very attenuated, having at 6 cm. in many places a diameter of only 15 p.. Prenant suggests that this drawing out of the gland is caused by the rapid growth of the neck. Later it increases in size and at full term is noticeable only as a very slight constriction between the head and the mid-cervical segment. The histological changes are much later here than in other parts of the gland.
of the medulla all begin here a little later than in the head and thoracic
 
segment. Its caudal end is slightly in advance of its cranial end. The
 
intermediary cord is well marked at 4 cm. It -soon becomes very attenuated, having at 6 cm. in many places a diameter of only 15 p.. Prenant
 
suggests that this drawing out of the gland is caused by the rapid growth
 
of the neck. Later it increases in size and at full term is noticeable only
 
as a very slight constriction between the head and the mid-cervical
 
segment. The histological changes are much later here than in other
 
parts of the gland.
 
  
The greater part of the head in the early stages is formed by the
+
The greater part of the head in the early stages is formed by the nodulus thymicus. This body grows slowly, attaining a diameter in crosssection of .5 mm. at 8 cm. Its cross-sectional area at 8 cm. is about onethird that of the rest of the head of the thymus. At this stage the nodulus thymicus is a rounded body lying on the inner aspect of the head in relation to the carotid artery. A small area of its outer surface is fused with the lymphoid tissue of the thymus. Its histological structure has been fully described by Prenant (22). From the earliest stages, it consists of cords of epithelial cells separated by blood capillaries. At 8 cm. the thymus superficialis (Kastschenko) is a large body lying craniad and dorsal to the rest of the head but connected to it at its caudal extremity.
nodulus thymicus. This body grows slowly, attaining a diameter in crosssection of .5 mm. at 8 cm. Its cross-sectional area at 8 cm. is about onethird that of the rest of the head of the thymus. At this stage the nodulus
 
thymicus is a rounded body lying on the inner aspect of the head in relation to the carotid artery. A small area of its outer surface is fused
 
with the lymphoid tissue of the thymus. Its histological structure has
 
been fully described by Prenant (22). From the earliest stages, it
 
consists of cords of epithelial cells separated by blood capillaries. At
 
8 cm. the thymus superficialis (Kastschenko) is a large body lying craniad
 
and dorsal to the rest of the head but connected to it at its caudal
 
extremity.
 
  
I have no observations on the head of the thymus between 8 cm. and
+
I have no observations on the head of the thymus between 8 cm. and full term, having overlooked it in collecting my material. In a full term pig (30 cm.), the cervical part of the thymus is in two distinct parts. The postero-ventral part, representing part of the head, the intermediary cord, and the mid-cervical segment, is about 3 cm. long and 1 cm. wide. It extends from the upper border of the thyroid cartilage to the thorax, and with the corresponding part of the opposite gland encloses the E. '1‘. Bell 35
full term, having overlooked it in collecting my material. In a full term
 
pig (30 cm.), the cervical part of the thymus is in two distinct parts.
 
The postero-ventral part, representing part of the head, the intermediary
 
cord, and the mid-cervical segment, is about 3 cm. long and 1 cm. wide.
 
It extends from the upper border of the thyroid cartilage to the thorax,
 
and with the corresponding part of the opposite gland encloses the
 
E. '1‘. Bell 35
 
  
trachea, thyroid, and lower part of the larynx. A slight narrowing at
+
trachea, thyroid, and lower part of the larynx. A slight narrowing at the junction of the upper and middle thirds indicates the position of the intermediary cord. The antero-dorsal part, the thymus superficialis, is rounded in cross-section, tapering to a point behind. Its anterior end is about '7 mm. in diameter and’ loops around the twelfth nerve as in the earliest stages. A lobule hangs over ventral to the nerve, a thin cord being dorsal to it. The posterior end of the thymus superficialis extends to the cricoid cartilage dorsal to the postero—ventral part of the gland. It is united to this part of the gland by a very delicate band of thymic tissue. I did not find the nodulus thymicus at full term. It has either moved away from the head or degenerated.
the junction of the upper and middle thirds indicates the position of the
 
intermediary cord. The antero-dorsal part, the thymus superficialis, is
 
rounded in cross-section, tapering to a point behind. Its anterior end is
 
about '7 mm. in diameter and’ loops around the twelfth nerve as in the
 
earliest stages. A lobule hangs over ventral to the nerve, a thin cord
 
being dorsal to it. The posterior end of the thymus superficialis extends
 
to the cricoid cartilage dorsal to the postero—ventral part of the gland.
 
It is united to this part of the gland by a very delicate band of thymic
 
tissue. I did not find the nodulus thymicus at full term. It has either
 
moved away from the head or degenerated.
 
  
The duct of the thymus is the lumen of the third gill pouch. A
+
The duct of the thymus is the lumen of the third gill pouch. A glance at the Text Figures will show its development. It is broken up into segments and finally obliterated. I could find no traces of it at 3.7 cm. or later. On the theory of the exclusively endodermal origin of the thymus, I cannot explain the absence of a lumen in the head at 18 mm. and its presence at 20 mm. and 27 mm. unless it be due to individual variation in different specimens.
glance at the Text Figures will show its development. It is broken up
 
into segments and finally obliterated. I could find no traces of it at
 
3.7 cm. or later. On the theory of the exclusively endodermal origin of
 
the thymus, I cannot explain the absence of a lumen in the head at
 
18 mm. and its presence at 20 mm. and 27 mm. unless it be due to
 
individual variation in different specimens.
 
  
It appears from the foregoing that in the pig the exclusively endodermal origin of the thymus from the third gill pouch is probable, but
+
It appears from the foregoing that in the pig the exclusively endodermal origin of the thymus from the third gill pouch is probable, but a slight participation by the ectoderm has not been satisfactorily excluded. Kastschenko’s conclusions, however, as to the ectodermal origin of the thymus are unwarranted by his recorded observations. Prenant, after his careful work (on the sheep), was not sure that a small mass of ectoderm did not enter into the formation of the head. Practically all other investigators of this problem maintain that the ectoderm takes no part in the formation of the thymus. The epithelial body (nodulus thymieus) developing in connection with the head of the thymus from the third gill pouch does not form the carotid gland. Kastschenko’s description of the origin of the carotid gland in mammals from the adventitia of the internal carotid is now accepted by the majority of anatomists, and it therefore has nothing to do with the thymus in origin.
a slight participation by the ectoderm has not been satisfactorily excluded.
 
Kastschenko’s conclusions, however, as to the ectodermal origin of the
 
thymus are unwarranted by his recorded observations. Prenant, after
 
his careful work (on the sheep), was not sure that a small mass of ectoderm did not enter into the formation of the head. Practically all other
 
investigators of this problem maintain that the ectoderm takes no part
 
in the formation of the thymus. The epithelial body (nodulus thymieus)
 
developing in connection with the head of the thymus from the third
 
gill pouch does not form the carotid gland. Kastschenko’s description of
 
the origin of the carotid gland in mammals from the adventitia of the
 
internal carotid is now accepted by the majority of anatomists, and it
 
therefore has nothing to do with the thymus in origin.
 
  
 
THE HISTOLOGY or THE FULLY-FORMED THYMUS.
 
THE HISTOLOGY or THE FULLY-FORMED THYMUS.
  
Before taking up the histogenesis, I shall briefly consider the histology
+
Before taking up the histogenesis, I shall briefly consider the histology of the gland as shown in a 24-em. embryo. At this stage the gland may be regarded as fully formed. As is well known, the thymic lobule consists of a cortical and a medullary portion,—the medulla of all the lobules being united by the medullary cord. The cortex. consists of a delicate reticulum with its spaces well filled by cells, usually lymphocytes. The reticulum may be regarded as composed of small branched anastom36 The Development of the Thymus
of the gland as shown in a 24-em. embryo. At this stage the gland may
 
be regarded as fully formed. As is well known, the thymic lobule consists of a cortical and a medullary portion,—the medulla of all the
 
lobules being united by the medullary cord. The cortex. consists of a
 
delicate reticulum with its spaces well filled by cells, usually lymphocytes.
 
The reticulum may be regarded as composed of small branched anastom36 The Development of the Thymus
 
  
osing cells, though of course no cell boundaries are distinguishable. The
+
osing cells, though of course no cell boundaries are distinguishable. The nuclei are poor in chromatin, rounded, and usually 4.5;; to 5.5,; in diameter. The amount of cytoplasm around the nuclei and connecting them is usually very small. At some nodes there is a greater amount of cytoplasm giving the appearance of a large reticulum cell. In connection with the blood-vessels, which are numerous in the cortex, are often found branched cells with pale nuclei and cytoplasm that stains more intensely than that of the rest of the reticulum. By a modification of Mallory’s method used by Jackson (13, S. 39), I have been able to demonstrate numerous fibrillae in the reticulum. In some cortical areas at this stage there are a great many erythroblasts. Masses of free erythrocytes are often found, usually near a comparatively large vessel, but such cells occur singly anywhere in the cortex.
nuclei are poor in chromatin, rounded, and usually 4.5;; to 5.5,; in
 
diameter. The amount of cytoplasm around the nuclei and connecting
 
them is usually very small. At some nodes there is a greater amount of
 
cytoplasm giving the appearance of a large reticulum cell. In connection
 
with the blood-vessels, which are numerous in the cortex, are often found
 
branched cells with pale nuclei and cytoplasm that stains more intensely
 
than that of the rest of the reticulum. By a modification of Mallory’s
 
method used by Jackson (13, S. 39), I have been able to demonstrate
 
numerous fibrillae in the reticulum. In some cortical areas at this
 
stage there are a great many erythroblasts. Masses of free erythrocytes
 
are often found, usually near a comparatively large vessel, but such cells
 
occur singly anywhere in the cortex.
 
  
In the medulla, the syncytial character of the stroma is much more
+
In the medulla, the syncytial character of the stroma is much more pronounced. The cytoplasm is much more abundant than in the cortex, and the spaces are smaller and not so numerous. There is not so much room for lymphocytes as in the cortex, hence the lighter color of the medulla in stained preparations. The nuclei of the syncytium are either pale or dark, both types showing wide variations in size. By J ackson’s method (13, S. 39), fibrillaa may be readily demonstrated in the syncytium. In Plate I, Fig. 6, is shown the arrangement of these fibrillae (s f) in the medulla at 24 cm. They often may be traced into the areas which are forming the concentric corpuscles. In some parts of the medulla the fibrillae are very numerous; in a few places, entirely absent. In both cortex and medulla eosinophile cells are often found. These occur in groups in the interlobular tissue around the blood-vessels, around some of the corpuscles of Hassall, and singly in the reticulum. These have been described by Schaffer (24). Free erythrocytes are rarely found in the medulla. In the medulla are also found the corpuscles of Hassall. Since the structure of these bodies depends largely upon their age, it may be better understood from the consideration of their development.
pronounced. The cytoplasm is much more abundant than in the cortex,
 
and the spaces are smaller and not so numerous. There is not so much
 
room for lymphocytes as in the cortex, hence the lighter color of the
 
medulla in stained preparations. The nuclei of the syncytium are either
 
pale or dark, both types showing wide variations in size. By J ackson’s
 
method (13, S. 39), fibrillaa may be readily demonstrated in the syncytium. In Plate I, Fig. 6, is shown the arrangement of these fibrillae
 
(s f) in the medulla at 24 cm. They often may be traced into the
 
areas which are forming the concentric corpuscles. In some parts of the
 
medulla the fibrillae are very numerous; in a few places, entirely absent.
 
In both cortex and medulla eosinophile cells are often found. These
 
occur in groups in the interlobular tissue around the blood-vessels, around
 
some of the corpuscles of Hassall, and singly in the reticulum. These
 
have been described by Schaffer (24). Free erythrocytes are rarely found
 
in the medulla. In the medulla are also found the corpuscles of Hassall.
 
Since the structure of these bodies depends largely upon their age, it may
 
be better understood from the consideration of their development.
 
  
 
THE LYMPHOID TRANSFORMATION.‘
 
THE LYMPHOID TRANSFORMATION.‘
  
Kolliker, in 79, first advanced the idea that the leucocytes are formed
+
Kolliker, in 79, first advanced the idea that the leucocytes are formed directly from the epithelial cells of the thymic anlage. According to Minot (in human embryology, p. 878), “ he records for the rabbit that between the twentieth and twenty-third days the cells of the thymus become smaller and their outlines disappear, so that the organ appears to
directly from the epithelial cells of the thymic anlage. According to
 
Minot (in human embryology, p. 878), “ he records for the rabbit that
 
between the twentieth and twenty-third days the cells of the thymus become smaller and their outlines disappear, so that the organ appears to
 
  
‘This term will be used to include those changes that occur in the thymus
+
‘This term will be used to include those changes that occur in the thymus during its passage from an epithelial to a characteristic lymphoid structure. E. T. Bell 37
during its passage from an epithelial to a characteristic lymphoid structure.
 
E. T. Bell 37
 
  
be an accumulation of small round nuclei. At about the same period
+
be an accumulation of small round nuclei. At about the same period blood-vessels and connective tissue grow into the epithelial anlage.”
blood-vessels and connective tissue grow into the epithelial anlage.”
 
  
His (12 b), 80, and Stieda (26), 81, claimed that the corpuscles of
+
His (12 b), 80, and Stieda (26), 81, claimed that the corpuscles of Hassall are the only remnants of the epithelial anlage, that the lymphocytes, reticulum, etc., are of mesenchymal origin.
Hassall are the only remnants of the epithelial anlage, that the lymphocytes, reticulum, etc., are of mesenchymal origin.
 
  
Maurer (17 a), 86, described the leucocytes as arising directly from the
+
Maurer (17 a), 86, described the leucocytes as arising directly from the cells of the epithelial anlagc in the thymus of teleosts. In the amphibian thymus (17 b), 88, he thinks that the leucocytes are probably of 1nesenchymal origin. He was unwilling to conclude that they arose from the epithelium because he could not find transition forms. In lizards (17 c), 99, he records that even before the separation of the epithelial anlage of the thymus from the pharynx, changes begin. The peripheral cells are closely crowded together and show many mitoses. There arises between the central cells, or is formed in vacuoles in their protoplasm, a fluid which accumulates until the nuclei surrounded by a thin zone of protoplasm are connected only by protoplasmic threads. A loose medulla is thus form.ed which looks like a cellular reticulum. The cortex is still solid. The lymphocytes are formed from the epithelial cells; none come from without. Later, blood-vessels and connective tissue grow in. He believes that the reticulum is of mesenchymal origin in all forms. Maurer (17 d), 02, still holds that in amphibians the lymphocytes are probably of mesenchymal origin.
cells of the epithelial anlagc in the thymus of teleosts. In the amphibian
 
thymus (17 b), 88, he thinks that the leucocytes are probably of 1nesenchymal origin. He was unwilling to conclude that they arose from the
 
epithelium because he could not find transition forms. In lizards (17 c),
 
99, he records that even before the separation of the epithelial anlage of
 
the thymus from the pharynx, changes begin. The peripheral cells are
 
closely crowded together and show many mitoses. There arises between
 
the central cells, or is formed in vacuoles in their protoplasm, a fluid which
 
accumulates until the nuclei surrounded by a thin zone of protoplasm are
 
connected only by protoplasmic threads. A loose medulla is thus form.ed
 
which looks like a cellular reticulum. The cortex is still solid. The
 
lymphocytes are formed from the epithelial cells; none come from without. Later, blood-vessels and connective tissue grow in. He believes
 
that the reticulum is of mesenchymal origin in all forms. Maurer
 
(17 d), 02, still holds that in amphibians the lymphocytes are probably
 
of mesenchymal origin.
 
  
Hermann et Tourneux (11), 87, find that in man and other mammals
+
Hermann et Tourneux (11), 87, find that in man and other mammals the epithelial anlage of the thymus is gradually converted into leucocytes and reticulum cells. Vacuoles appear during the transformation which seem to be formed by the absorption of large cells. In a sheep embryo of 130 mm., the clear epithelial cells have all disappeared, giving rise to small round cells and reticulum cells. Prolongations of connective tissue, each containing a blood-vessel, grow into the anlage during the transformation. They are not sure that all the thymic elements are epithelial in origin, being especially in doubt about the origin of some of the reticulum cells.
the epithelial anlage of the thymus is gradually converted into leucocytes
 
and reticulum cells. Vacuoles appear during the transformation which
 
seem to be formed by the absorption of large cells. In a sheep embryo
 
of 130 mm., the clear epithelial cells have all disappeared, giving rise
 
to small round cells and reticulum cells. Prolongations of connective
 
tissue, each containing a blood-vessel, grow into the anlage during the
 
transformation. They are not sure that all the thymic elements are
 
epithelial in origin, being especially in doubt about the origin of some of
 
the reticulum cells.
 
  
Gulland (8), 91, describes the development of the tonsil in the rabbit.
+
Gulland (8), 91, describes the development of the tonsil in the rabbit. Leucocytes first appear in the connective tissue around the thymus. Later they appear in the connective tissue around the tonsil. They infiltrate the tonsillar epithelium. N o leucocytes are of epithelial origin. After studying the tonsil he examined the thymus in a few specimens and concluded that the same process of leucocyte infiltration occurred there. He does not give the details of their infiltration, and did not see any of the transition forms of nuclei in the thymus at that period.
Leucocytes first appear in the connective tissue around the thymus. Later
 
they appear in the connective tissue around the tonsil. They infiltrate
 
the tonsillar epithelium. N o leucocytes are of epithelial origin. After
 
studying the tonsil he examined the thymus in a few specimens and
 
concluded that the same process of leucocyte infiltration occurred there.
 
He does not give the details of their infiltration, and did not see any of
 
the transition forms of nuclei in the thymus at that period.
 
  
Prenant (22), 94, made a careful study of the development of the
+
Prenant (22), 94, made a careful study of the development of the 38 The Development of the Thymus
38 The Development of the Thymus
 
  
thymus in sheep embryos. His results are as follows. At 25 mm. the
+
thymus in sheep embryos. His results are as follows. At 25 mm. the gland is composed of distinct polyhedral cells with nuclei of only one kind. A few mitoses and an occasional direct division are to be seen. At 26 mm. mitoses are numerous (one nucleus in fifty). Nuclei are regularly rounded or elliptical and some small nuclei occur juxtaposed to large nuclei. At 28 mm. many mitoses are present and irregular spaces have appeared. These spaces are not blood-vessels nor parts of the thymic duct but vacuoles. Some nuclei, noticeably small and darkly colored, lie close to the large, clear nuclei and seem to be budded off from them. Some nuclei (rare) are broken into three or four chromatic bodies. Embryos of 40 mm. have undergone in great part the lymphoid transformation. All transitions are found between the large, pale elliptical nuclei of clear reticular structure and the small, deeply colored rounded nuclei whose sap is strongly stained. These last are certainly lymphocytes and constitute an immense majority of the cellular elements. Large, clear nuclei are found joined to small dark ones——nuclear couples.
gland is composed of distinct polyhedral cells with nuclei of only one
 
kind. A few mitoses and an occasional direct division are to be seen. At
 
26 mm. mitoses are numerous (one nucleus in fifty). Nuclei are regularly rounded or elliptical and some small nuclei occur juxtaposed to
 
large nuclei. At 28 mm. many mitoses are present and irregular spaces
 
have appeared. These spaces are not blood-vessels nor parts of the
 
thymic duct but vacuoles. Some nuclei, noticeably small and darkly
 
colored, lie close to the large, clear nuclei and seem to be budded off from
 
them. Some nuclei (rare) are broken into three or four chromatic
 
bodies. Embryos of 40 mm. have undergone in great part the lymphoid
 
transformation. All transitions are found between the large, pale elliptical nuclei of clear reticular structure and the small, deeply colored
 
rounded nuclei whose sap is strongly stained. These last are certainly
 
lymphocytes and constitute an immense majority of the cellular elements.
 
Large, clear nuclei are found joined to small dark ones——nuclear couples.
 
  
At 85 mm. the medulla appears; the cortex corresponds to the entire
+
At 85 mm. the medulla appears; the cortex corresponds to the entire thymic mass of preceding stages. The cortex contains a great many lymphocytes separated by islands and rows of pale nuclei- There are about thirty lymphocytes to one pale nucleus. Mitoses are numerous in the cortex. In the medulla at this stage, the large clear, and small dark, nuclei are about equal in number, and mitoses are rarer than in the cortex. In later embryonic stages avclear peripheral zone is present where cell proliferation takes place. Mitoses are now more numerous in the medulla than in the cortex. It is probable that a certain number of the epithelial cells persist as reticulum cells in the fully-formed organ.
thymic mass of preceding stages. The cortex contains a great many
 
lymphocytes separated by islands and rows of pale nuclei- There are
 
about thirty lymphocytes to one pale nucleus. Mitoses are numerous in
 
the cortex. In the medulla at this stage, the large clear, and small dark,
 
nuclei are about equal in number, and mitoses are rarer than in the
 
cortex. In later embryonic stages avclear peripheral zone is present
 
where cell proliferation takes place. Mitoses are now more numerous in
 
the medulla than in the cortex. It is probable that a certain number of
 
the epithelial cells persist as reticulum cells in the fully-formed organ.
 
  
J. Beard (3 a), 94, (3 b), 99, thinks that the function of the thymus
+
J. Beard (3 a), 94, (3 b), 99, thinks that the function of the thymus is to form the first leucocytes. He finds that in 'the skate the epithelial cells are converted early into lymphocytes which emigrate into the blood. There are many breaks in the gland where the lymphocytes escape in masses. The thymus is the only source of leucocytes until the other lymphoid organs are formed.
is to form the first leucocytes. He finds that in 'the skate the epithelial
 
cells are converted early into lymphocytes which emigrate into the blood.
 
There are many breaks in the gland where the lymphocytes escape in
 
masses. The thymus is the only source of leucocytes until the other
 
lymphoid organs are formed.
 
  
Vcr Eecke (28), 99, finds that in the frog the epithelial thymus is
+
Vcr Eecke (28), 99, finds that in the frog the epithelial thymus is invaded by lymphocytes and connective tissue. The epithelial cells are not destroyed but merely dispersed by the mcsenchymal elements. He calls the resulting tissue lympho-epithelial. This idea of the commingling of the two tissues had already been advanced by Retterer.
invaded by lymphocytes and connective tissue. The epithelial cells are
 
not destroyed but merely dispersed by the mcsenchymal elements. He
 
calls the resulting tissue lympho-epithelial. This idea of the commingling of the two tissues had already been advanced by Retterer.
 
  
Nusbaum and Prymak (20), OI, on teleosts, agree with Maurer that
+
Nusbaum and Prymak (20), OI, on teleosts, agree with Maurer that the lymphocytes are of epithelial origin but disagree on the details of their formation. They find that the epithelial anlage is at first composed of cells with distinct boundaries. It is not difierent from the epithelium E. T. Bell 39
the lymphocytes are of epithelial origin but disagree on the details of their
 
formation. They find that the epithelial anlage is at first composed of
 
cells with distinct boundaries. It is not difierent from the epithelium
 
E. T. Bell 39
 
  
of the pharynx. Before any blood-vessels or connective tissue have invaded the organ, changes begin in the central part. These changes consist in the breaking up of the cytoplasm so that the cells become
+
of the pharynx. Before any blood-vessels or connective tissue have invaded the organ, changes begin in the central part. These changes consist in the breaking up of the cytoplasm so that the cells become branched and connected by delicate processes. These processes finally break apart leaving a nucleus surrounded by a thin layer of protoplasm— a lymphocyte. The peripheral epithelial layer multiplies rapidly, forming nuclei somewhat smaller and darker than their own. These nuclei become gradually changed into the nuclei of lymphocytes and break away from the other cells. All transitions are present between the large, clear epithelial nuclei and the lymphocytes. Blood-vessels and connective tissue grow in from the outside.
branched and connected by delicate processes. These processes finally
 
break apart leaving a nucleus surrounded by a thin layer of protoplasm—
 
a lymphocyte. The peripheral epithelial layer multiplies rapidly, forming
 
nuclei somewhat smaller and darker than their own. These nuclei become gradually changed into the nuclei of lymphocytes and break away
 
from the other cells. All transitions are present between the large, clear
 
epithelial nuclei and the lymphocytes. Blood-vessels and connective
 
tissue grow in from the outside.
 
  
It appears from a survey of the literature that, of those who have
+
It appears from a survey of the literature that, of those who have studied the origin of lymphocytes in the mamalian thymus, Hi-s, Stieda, and Gulland have advocated the idea that they invade the gland from without, and that the original epithelial anlage persists only as remnants, the corpuscles of Hassall. They also consider the stroma of mesenchymal origin. On the other hand, Ktilliker, Herm-ann and Toumeux, and Prenant, have described the lymphocytes as derived directly from the epithelial cells of the anlage. Hermann and Tourneux and Prenant ascribed a similar origin to part of the reticulum.
studied the origin of lymphocytes in the mamalian thymus, Hi-s, Stieda,
 
and Gulland have advocated the idea that they invade the gland from
 
without, and that the original epithelial anlage persists only as remnants,
 
the corpuscles of Hassall. They also consider the stroma of mesenchymal
 
origin. On the other hand, Ktilliker, Herm-ann and Toumeux, and Prenant, have described the lymphocytes as derived directly from the epithelial
 
cells of the anlage. Hermann and Tourneux and Prenant ascribed a
 
similar origin to part of the reticulum.
 
  
Neither His, Stieda, nor Gulland made a detailed histological study
+
Neither His, Stieda, nor Gulland made a detailed histological study of the changes that take place in the thymus during the transformation. They did not see the vacuolization of the cytoplasm, the changes in the epithelial nuclei, etc.—processes which undoubtedly occur. Gulland made nearly all his observations on the tonsil and then from a superficial examination of the thymus concluded that the process is the same there. The conclusions of these men are therefore not to be compared on this point with those obtained by the thorough and careful work of Prenant.
of the changes that take place in the thymus during the transformation.
 
They did not see the vacuolization of the cytoplasm, the changes in the
 
epithelial nuclei, etc.—processes which undoubtedly occur. Gulland
 
made nearly all his observations on the tonsil and then from a superficial
 
examination of the thymus concluded that the process is the same there.
 
The conclusions of these men are therefore not to be compared on this
 
point with those obtained by the thorough and careful work of Prenant.
 
  
On amphibians, Maurer hesitatingly agrees with His, and Ver Eecke
+
On amphibians, Maurer hesitatingly agrees with His, and Ver Eecke accepts the mesenchymal origin of the leucoeytes ; while on fishes Maurer, Beard, and Nusbaum and Prymak believe in the epithelial origin of lymphocytes. Maurer’s work on reptiles is in agreement with his work on teleosts.
accepts the mesenchymal origin of the leucoeytes ; while on fishes Maurer,
 
Beard, and Nusbaum and Prymak believe in the epithelial origin of
 
lymphocytes. Maurer’s work on reptiles is in agreement with his work
 
on teleosts.
 
  
As to the origin of leucocytes in the lymphoid organs of the alimentary
+
As to the origin of leucocytes in the lymphoid organs of the alimentary canal, opinion is divided. Retterer, v. Davidoff, Rudinger, Klaatsch, and others have described the leucoeytes as arising from epithelium and being invaded by mesenchymal elements forming adenoid tissue. Stiihr, Gulland, Tomarkin, and others describe them as penetrating the epithelium from without.
canal, opinion is divided. Retterer, v. Davidoff, Rudinger, Klaatsch,
 
and others have described the leucoeytes as arising from epithelium and
 
being invaded by mesenchymal elements forming adenoid tissue. Stiihr,
 
Gulland, Tomarkin, and others describe them as penetrating the epithelium from without.
 
  
I hall now discuss my own observations on the lymphoid transformation in the thymus of the pig. From a very early stage (11 mm.), the
+
I hall now discuss my own observations on the lymphoid transformation in the thymus of the pig. From a very early stage (11 mm.), the 40 The Development of the Thymus
40 The Development of the Thymus
 
  
epithelium of the third gill pouch is a syncytium. No cell boundaries
+
epithelium of the third gill pouch is a syncytium. No cell boundaries exist. The nuclei, large and irregular in shape, are embedded in a common mass of cytoplasm. In the thymus at 20 mm. I find a syncytium of dense cytoplasm embedded in which are large nuclei of irregular shape and size. No distinct types of nuclei are present yet; all stain with medium intensity. A few mitoses are to be seen.
exist. The nuclei, large and irregular in shape, are embedded in a common mass of cytoplasm. In the thymus at 20 mm. I find a syncytium
 
of dense cytoplasm embedded in which are large nuclei of irregular shape
 
and size. No distinct types of nuclei are present yet; all stain with
 
medium intensity. A few mitoses are to be seen.
 
  
In a section of the mid—cervical segment at 3.7 cm. (Plate I, Fig. 1),
+
In a section of the mid—cervical segment at 3.7 cm. (Plate I, Fig. 1), I find evidence that the lymphoid transformation has begun. ‘The syncytium is composed of coarsely reticulated cytoplasm much looser in texture than that of the preceding stage. It contains a few irregular spaces (3 s) which are evidently of the nature of vacuoles. These may be formed, as Maurer suggests, by liquefaction of the cytoplasm. There is no reason to suppose that cells degenerate and form them as Hermann and Tourneux believed. Three types of nuclei may be distinguished; large pale nuclei (Z pn) large dark nuclei (Z (In), and small dark nuclei (lymphoblasts) (lb). Transition’ forms occur between these types. The l-arge dark nuclei are intermediate forms between the pale nuclei and the lymphoblasts. A few mitoses (m) occur. No blood-vessels are present inside the anlage but they may be seen between the buds just outside. At this stage, the head and the thoracic segment have areas that are somewhat farther advanced than this. The intermediary and cervico-thoracic cords show no changes.
I find evidence that the lymphoid transformation has begun. ‘The syncytium is composed of coarsely reticulated cytoplasm much looser in
 
texture than that of the preceding stage. It contains a few irregular
 
spaces (3 s) which are evidently of the nature of vacuoles. These may
 
be formed, as Maurer suggests, by liquefaction of the cytoplasm. There
 
is no reason to suppose that cells degenerate and form them as Hermann
 
and Tourneux believed. Three types of nuclei may be distinguished;
 
large pale nuclei (Z pn) large dark nuclei (Z (In), and small dark nuclei
 
(lymphoblasts) (lb). Transition’ forms occur between these types.
 
The l-arge dark nuclei are intermediate forms between the pale nuclei
 
and the lymphoblasts. A few mitoses (m) occur. No blood-vessels are
 
present inside the anlage but they may be seen between the buds just
 
outside. At this stage, the head and the thoracic segment have areas
 
that are somewhat farther advanced than this. The intermediary and
 
cervico-thoracic cords show no changes.
 
  
At a later stage than the above (Plate I, Fig. 2), in the thoracic
+
At a later stage than the above (Plate I, Fig. 2), in the thoracic segment of a 4.5-cm. pig, the spaces of the syncytium (s s) have increased greatly in number and size. The anlage is now a cellular reticulum. The large pale nuclei are somewhat less numerous than the dark nuclei and many have become angular, adapting themselves to the nodes of the syncytium. They contain less chromatin than in the preceding stage. Large dark nuclei and lymphoblasts are present; the lymphoblasts are much more numerous than in the preceding stage. A very few small dark nuclei are completely separated from the syncytium. These are lymphocytes. There are no lymphocytes in the connective tissue around the thymus or in the blood at this stage. I did not examine the tonsil or spleen at any stage. A few small blood-vessels are to be seen; their walls consist of endothelium only. There are more mitoses than at the preceding stage, but none happened to be present in the area shown in the figure. During mitosis, at all stages of development, except the early epithelial condition, the chromosomes are so closely packed that it is very difficult to distinguish them individually.
segment of a 4.5-cm. pig, the spaces of the syncytium (s s) have increased greatly in number and size. The anlage is now a cellular reticulum. The large pale nuclei are somewhat less numerous than the
 
dark nuclei and many have become angular, adapting themselves to the
 
nodes of the syncytium. They contain less chromatin than in the
 
preceding stage. Large dark nuclei and lymphoblasts are present; the
 
lymphoblasts are much more numerous than in the preceding stage. A
 
very few small dark nuclei are completely separated from the syncytium.
 
These are lymphocytes. There are no lymphocytes in the connective
 
tissue around the thymus or in the blood at this stage. I did not
 
examine the tonsil or spleen at any stage. A few small blood-vessels are
 
to be seen; their walls consist of endothelium only. There are more
 
mitoses than at the preceding stage, but none happened to be present in
 
the area shown in the figure. During mitosis, at all stages of development, except the early epithelial condition, the chromosomes are so
 
closely packed that it is very difficult to distinguish them individually.
 
  
In a section through the mid—cervical segment of a 7-cm. pig
+
In a section through the mid—cervical segment of a 7-cm. pig (Plate I, Fig. 5), we see a stage somewhat later than the one shown in E. T. Bell 41
(Plate I, Fig. 5), we see a stage somewhat later than the one shown in
 
E. T. Bell 41
 
  
Fig. 2. In various parts of the section lymphocytes (Z) are completely
+
Fig. 2. In various parts of the section lymphocytes (Z) are completely formed. The great majority of the small round nuclei are inithe lymphoblast (Z b) condition, i. e.,they are not yet completely separated from the syncytium. There are a few lymphocytes outside the thymus in the interlobular tissue in this region; around the head and the thoracic segment at 7 cm. they are numerous, these parts being in a later stage of transformation. I have never seen lymphocytes outside the thymus, where there were none inside it ; but they appear outside shortly after they are formed here. Those formed next the interlobular septa seem to pass out very early. Of course the lymphoblasts, which are distinguishable from the lymphocytes only by being imbedded in the syncytium, are to be seen in the thymus long before any appear outside.
formed. The great majority of the small round nuclei are inithe lymphoblast (Z b) condition, i. e.,they are not yet completely separated from the
 
syncytium. There are a few lymphocytes outside the thymus in the
 
interlobular tissue in this region; around the head and the thoracic
 
segment at 7 cm. they are numerous, these parts being in a later stage
 
of transformation. I have never seen lymphocytes outside the thymus,
 
where there were none inside it ; but they appear outside shortly after they
 
are formed here. Those formed next the interlobular septa seem to pass
 
out very early. Of course the lymphoblasts, which are distinguishable
 
from the lymphocytes only by being imbedded in the syncytium, are to
 
be seen in the thymus long before any appear outside.
 
  
At the stage shown in Fig. 5, a great many nuclei are in mitosis. I
+
At the stage shown in Fig. 5, a great many nuclei are in mitosis. I have not seen at any stage, the amitoses and nuclear couples described by Prenant for the sheep. In some parts of the section comparatively large solid epithelial areas occur. These are found as often in the central as in the peripheral part. Many of the pale nuclei are smaller than those shown in Fig. 2. The blood-vessels are somewhat larger and more numerous than those at 4.5 cm.
have not seen at any stage, the amitoses and nuclear couples described
 
by Prenant for the sheep. In some parts of the section comparatively
 
large solid epithelial areas occur. These are found as often in the
 
central as in the peripheral part. Many of the pale nuclei are smaller
 
than those shown in Fig. 2. The blood-vessels are somewhat larger and
 
more numerous than those at 4.5 cm.
 
  
It is to be noted that the epithelial anlage does not at any stage become
+
It is to be noted that the epithelial anlage does not at any stage become converted entirely into small round cells as many observers have stated. Distinctly pale angular reticular nuclei can always be seen.
converted entirely into small round cells as many observers have stated.
 
Distinctly pale angular reticular nuclei can always be seen.
 
  
In the mid-cervical segment at 8.5 cm. (Plate I, Fig. 4), a great many
+
In the mid-cervical segment at 8.5 cm. (Plate I, Fig. 4), a great many lymphocytes (Z) are formed. These lie between the persisting epithelial cells which are now arranged in irregular cords and islands. In these epithelial masses, lymphoblasts may still be seen indicating -that the formation of lymphocytes is still in progress. Many of the pale nuclei are now small. The heavy haematoxylin stain in this case makes the nuclei darker than they would appear with an ordinary‘ stain. A few nuclei are in mitosis.
lymphocytes (Z) are formed. These lie between the persisting epithelial
 
cells which are now arranged in irregular cords and islands. In these
 
epithelial masses, lymphoblasts may still be seen indicating -that the
 
formation of lymphocytes is still in progress. Many of the pale nuclei
 
are now small. The heavy haematoxylin stain in this case makes the
 
nuclei darker than they would appear with an ordinary‘ stain. A few
 
nuclei are in mitosis.
 
  
This figure shows also the first appearance of the medulla (md). The
+
This figure shows also the first appearance of the medulla (md). The medulla is formed directly, as shown in the figure, from persisting parts of the epithelial syncytium. Certain centrally situated masses of this syncytium undergo changes of such a nature that they stain readily with cytoplasmic stains such as Congo red. In sections stained with haematoxylin and Congo red, the medulla is first recognized as a brightly colored area situated usually about the center of the lobule. These epithelial masses that give rise to the medulla seem to increase in size about the time of the change in staining capacity. The first differentiation of the medulla is chemical rather than morphological, for there'are other persisting epithelial mvasses even larger than it in the same section 42 The Development of the Thymus
medulla is formed directly, as shown in the figure, from persisting parts
 
of the epithelial syncytium. Certain centrally situated masses of this
 
syncytium undergo changes of such a nature that they stain readily with
 
cytoplasmic stains such as Congo red. In sections stained with haematoxylin and Congo red, the medulla is first recognized as a brightly
 
colored area situated usually about the center of the lobule. These
 
epithelial masses that give rise to the medulla seem to increase in size
 
about the time of the change in staining capacity. The first differentiation of the medulla is chemical rather than morphological, for there'are
 
other persisting epithelial mvasses even larger than it in the same section
 
42 The Development of the Thymus
 
  
that do not react in the same way with the cytoplasmic stains. The
+
that do not react in the same way with the cytoplasmic stains. The medulla appears in the head and the thoracic segment at 7.5 cm. to 8 cm. All the gland except this small central area forms the cortex. Bloodvessels now reach all parts of the gland, but are still few in number. I cannot distinguish any wall except the endothelium on those actually inside the gland.
medulla appears in the head and the thoracic segment at 7.5 cm. to 8 cm.
 
All the gland except this small central area forms the cortex. Bloodvessels now reach all parts of the gland, but are still few in number. I
 
cannot distinguish any wall except the endothelium on those actually
 
inside the gland.
 
  
In a 9.5-cm. pig, the medulla is larger. It contains pale nuclei of
+
In a 9.5-cm. pig, the medulla is larger. It contains pale nuclei of various sizes, large dark nuclei, and lymphoblasts. Its spaces are smaller than those of the cortex. The early stages in the formation of the corpuscles of Hassall appear as soon as the medulla begins to form. The epithelial cords in the cortex have become less conspicuous, but are still forming lymphocytes. A few nuclei are in mitosis. Many blood cap illaries may now be seen penetrating the gland from the periphery:
various sizes, large dark nuclei, and lymphoblasts. Its spaces are smaller
 
than those of the cortex. The early stages in the formation of the corpuscles of Hassall appear as soon as the medulla begins to form. The
 
epithelial cords in the cortex have become less conspicuous, but are still
 
forming lymphocytes. A few nuclei are in mitosis. Many blood cap
 
illaries may now be seen penetrating the gland from the periphery:
 
  
These vessels run in the epithelial masses and have a wall of large
+
These vessels run in the epithelial masses and have a wall of large endothelial cells which gives them the appearance of radiating cords. When these vessels first appear, as at this stage, they have only an endothelial wall. The blood-vessels grow in as small c-ap-illaries which, after their entrance, increase in size and branch; they do not break in as large Vessels surrounded by mesenchymal tissue. I am fairly sure that aside from the endothelial cells few or no mesenchymal cells come into the thymus. Around the greater part of the periphery of the gland is a solid zone of syncytium two or three nuclei deep which is in transformation like the epithelial cords inside. This zone, described by Prenant as a zone of proliferation, grows rapidly, as the frequent mitoses indicate. Its inner boundary is forming lymphocytes and reticulum cells.
endothelial cells which gives them the appearance of radiating cords.
 
When these vessels first appear, as at this stage, they have only an
 
endothelial wall. The blood-vessels grow in as small c-ap-illaries which,
 
after their entrance, increase in size and branch; they do not break in as
 
large Vessels surrounded by mesenchymal tissue. I am fairly sure that
 
aside from the endothelial cells few or no mesenchymal cells come into
 
the thymus. Around the greater part of the periphery of the gland is
 
a solid zone of syncytium two or three nuclei deep which is in transformation like the epithelial cords inside. This zone, described by Prenant as a zone of proliferation, grows rapidly, as the frequent mitoses
 
indicate. Its inner boundary is forming lymphocytes and reticulum cells.
 
  
In a 14-cm. pig, the lymphoid transformation is practically at an end
+
In a 14-cm. pig, the lymphoid transformation is practically at an end except in the medulla. The peripheral zone of proliferation has disappeared. The cortex has about the same structure as at 24 cm., as previously described. In the medulla, lymphoblasts, large pale nuclei, and the large dark intermediate types are still present. There are a few mitoses here. It is very probable therefore that the formation of lymphocytes is still in progress in the medulla. The medulla at 24 cm. shows a similar structure except that there are fewer lymphoblasts. These facts persuade me to regard the medulla as a center for lymphocyte formation at least as late as birth. Connective tissue fibrillae begin to appear in the gland along the large blood-vessels and the interlobular septa as early as 10.5 cm. They are only a little farther in at 16 cm. ; but near full term they are present in nearly all parts of the stroma. (See Plate I, Fig. 6.)
except in the medulla. The peripheral zone of proliferation has disappeared. The cortex has about the same structure as at 24 cm., as
 
previously described. In the medulla, lymphoblasts, large pale nuclei,
 
and the large dark intermediate types are still present. There are a few
 
mitoses here. It is very probable therefore that the formation of lymphocytes is still in progress in the medulla. The medulla at 24 cm.
 
shows a similar structure except that there are fewer lymphoblasts. These
 
facts persuade me to regard the medulla as a center for lymphocyte
 
formation at least as late as birth. Connective tissue fibrillae begin to
 
appear in the gland along the large blood-vessels and the interlobular
 
septa as early as 10.5 cm. They are only a little farther in at 16 cm. ;
 
but near full term they are present in nearly all parts of the stroma.
 
(See Plate I, Fig. 6.)
 
  
The above account may be summarized as follows: In the pig the
+
The above account may be summarized as follows: In the pig the epithelial syncytium of the thymic anlage becomes loosened up by the E. T. Bell 43
epithelial syncytium of the thymic anlage becomes loosened up by the
 
E. T. Bell 43
 
  
formation of vacuoles in it. These vacuoles increase in number and
+
formation of vacuoles in it. These vacuoles increase in number and size, converting the anlage into a cellular reticulum. While this vacuolization is in progress, the nuclei, which at first are of one kind with a medium amount of chrom-atin, differentiate into large clear, large dark, and small dark (lymphoblast) forms. The large dark nuclei probably divide by mitosis and form the lymphoblasts. The lymphoblasts gradually break loose from the syncytium, passing into its spaces and becoming lymphocytes. Shortly after lymphocytes begin to be formed, some of them pass out of the gland into the surrounding connective tissue. The lymphoid transformation begins in embryos of 2.5 cm. to 3 cm. and continues in the cortex until 12 cm. or 13 cm. In the medulla it is not complete at birth. Since the thymus increases greatly in size during this period the epithelial syncytium must grow rapidly. Lymphocytes are constantly being formed at the expense of the growing syncytium. A peripheral zone of proliferation is present from about 8 cm. to 12 em. The medulla is formed as a chemical differentiation of certain centrally situated areas of the epithelial syncytium. The histological changes occur earlier in the head and thoracic segment than in the mid-cervical segment and very much earlier than in the cords. The reticulum of both cortex and medulla is practically all of epithelial origin. Some branched cells around the blood-vessels in the cortex may be of mesenchymal origin. ‘ T
size, converting the anlage into a cellular reticulum. While this vacuolization is in progress, the nuclei, which at first are of one kind with a
 
medium amount of chrom-atin, differentiate into large clear, large dark,
 
and small dark (lymphoblast) forms. The large dark nuclei probably
 
divide by mitosis and form the lymphoblasts. The lymphoblasts gradually break loose from the syncytium, passing into its spaces and becoming
 
lymphocytes. Shortly after lymphocytes begin to be formed, some of
 
them pass out of the gland into the surrounding connective tissue. The
 
lymphoid transformation begins in embryos of 2.5 cm. to 3 cm. and continues in the cortex until 12 cm. or 13 cm. In the medulla it is not complete at birth. Since the thymus increases greatly in size during this
 
period the epithelial syncytium must grow rapidly. Lymphocytes are
 
constantly being formed at the expense of the growing syncytium. A
 
peripheral zone of proliferation is present from about 8 cm. to 12 em.
 
The medulla is formed as a chemical differentiation of certain centrally
 
situated areas of the epithelial syncytium. The histological changes
 
occur earlier in the head and thoracic segment than in the mid-cervical
 
segment and very much earlier than in the cords. The reticulum of both
 
cortex and medulla is practically all of epithelial origin. Some branched
 
cells around the blood-vessels in the cortex may be of mesenchymal
 
origin. ‘ T
 
  
My reasons for regarding the lymphocytes as of epithelial origin are
+
My reasons for regarding the lymphocytes as of epithelial origin are as follows:
as follows:
 
  
A. The lymphoblasts are true epithelial nuclei, because ( 1) there are
+
A. The lymphoblasts are true epithelial nuclei, because ( 1) there are numerous transition forms between them and the large dark nuclei which later cannot be regarded as invading lymphocytes; (2) they are closely embedded in the syncytium and show no evidence of having eaten their way through the protoplasm; (3) they are present from a very early stage and increase in number as development proceeds; (4) they are present before blood-vessels invade the gland and have no constant relation to blood-vessels or to the surface of the gland that indicates an invasion from either of these directions; ( 5) they are present before lymphocytes appear in the connective-tissue around the thymus.
numerous transition forms between them and the large dark nuclei which
 
later cannot be regarded as invading lymphocytes; (2) they are closely
 
embedded in the syncytium and show no evidence of having eaten their
 
way through the protoplasm; (3) they are present from a very early
 
stage and increase in number as development proceeds; (4) they are
 
present before blood-vessels invade the gland and have no constant relation to blood-vessels or to the surface of the gland that indicates an invasion from either of these directions; ( 5) they are present before lymphocytes appear in the connective-tissue around the thymus.
 
  
B. Some observers admit that the small dark nuclei (lymphoblasts)
+
B. Some observers admit that the small dark nuclei (lymphoblasts) are of epithelial origin but do not admit that they form lymphocytes. The considerations that lead me to believe that the lymphoblasts do form the lymphocytes are: (1) the small dark nuclei (l.ymphoblasts) show every possible relation to the syncytium from being completely embedded in it to lying free in the syncytial spaces. A comparison with later stages shows that this appearance is not due to poor fixation or to the 44 The Development of the Thymus
are of epithelial origin but do not admit that they form lymphocytes.
 
The considerations that lead me to believe that the lymphoblasts do form
 
the lymphocytes are: (1) the small dark nuclei (l.ymphoblasts) show
 
every possible relation to the syncytium from being completely embedded
 
in it to lying free in the syncytial spaces. A comparison with later
 
stages shows that this appearance is not due to poor fixation or to the
 
44 The Development of the Thymus
 
  
adherence of the nuclei to the reticulum; (2) the first free nuclei often
+
adherence of the nuclei to the reticulum; (2) the first free nuclei often appear in the center of the gland when there are no other free nuclei in the periphery at that level; (3) there is good evidence that lymphocytes emigrate from the thymus in large numbers. If we examine the thymus of a '7-cm. pig in serial sections we find that the lymphoid transformation is less advanced in the mid—cervical segment than in the head. In the mid-cervical segment there are a few lymphocytes in the interlobular tissue. In the lower end of the head where there are more lymphocytes inside the gland, lymphocytes pack the interlobular tissue and form a thin zone around the periphery of the gland. In the middle of the head where the transformation is far advanced, lymphocytes pack the interlobular tissue and form a thick zone around the entire gland. Indeed, in some sections, there are more lymphocytes in the zone outside than are present inside the gland. If this zone of lymphocytes be passing into the gland, it is not easy to understand why it is formed from within outwards, and why it is thickest where the greatest number of lymphocytes are already present inside. No satisfactory suggestion has yet been made as to why lymphocytes should thus suddenly pour into the thymus at a time when if present at all elsewhere they are rare. They do not come to break up the thymic epithelium, for that is already a reticulum before free cells are present (Fig. 2, Plate I). Where lymphocytes invade intestinal epithelium as in the tonsil they eat paths through it leaving spaces. The epithelial reticulum of the thymus is not formed in that way. On the other hand it is not diificult to believe that this zone of lymphocytes is formed by cells passing out the periphery of the thymus and that the gland thus contributes a great number of lymphocytes to the organism; (4) I have not been able to find lymphocytes in the connectivetissue around the thymus before they are present inside. An invasion by way of the blood-vessels may be excluded, since the thick zone of lymphocytes formed around the gland shows that these cells either enter or leave it through the preiphery.
appear in the center of the gland when there are no other free nuclei in
 
the periphery at that level; (3) there is good evidence that lymphocytes
 
emigrate from the thymus in large numbers. If we examine the thymus
 
of a '7-cm. pig in serial sections we find that the lymphoid transformation
 
is less advanced in the mid—cervical segment than in the head. In the
 
mid-cervical segment there are a few lymphocytes in the interlobular
 
tissue. In the lower end of the head where there are more lymphocytes
 
inside the gland, lymphocytes pack the interlobular tissue and form a
 
thin zone around the periphery of the gland. In the middle of the head
 
where the transformation is far advanced, lymphocytes pack the interlobular tissue and form a thick zone around the entire gland. Indeed,
 
in some sections, there are more lymphocytes in the zone outside than
 
are present inside the gland. If this zone of lymphocytes be passing into
 
the gland, it is not easy to understand why it is formed from within outwards, and why it is thickest where the greatest number of lymphocytes
 
are already present inside. No satisfactory suggestion has yet been made
 
as to why lymphocytes should thus suddenly pour into the thymus at a
 
time when if present at all elsewhere they are rare. They do not come
 
to break up the thymic epithelium, for that is already a reticulum before
 
free cells are present (Fig. 2, Plate I). Where lymphocytes invade
 
intestinal epithelium as in the tonsil they eat paths through it leaving
 
spaces. The epithelial reticulum of the thymus is not formed in that
 
way. On the other hand it is not diificult to believe that this zone of
 
lymphocytes is formed by cells passing out the periphery of the thymus
 
and that the gland thus contributes a great number of lymphocytes to the
 
organism; (4) I have not been able to find lymphocytes in the connectivetissue around the thymus before they are present inside. An invasion by
 
way of the blood-vessels may be excluded, since the thick zone of lymphocytes formed around the gland shows that these cells either enter or leave
 
it through the preiphery.
 
  
 
THE CoRPUsoLEs or HASSALL.
 
THE CoRPUsoLEs or HASSALL.
  
These bodies were first mentioned by Hassall (10) in 46. He speaks
+
These bodies were first mentioned by Hassall (10) in 46. He speaks of them as being composed of mother cells which enclose the newlyformed daughter cells and nuclei. He thought the central mass was formed by the outer enclosing layers. He found bodies which he regarded of the" same nature in fibrous coagulations in the heart.
of them as being composed of mother cells which enclose the newlyformed daughter cells and nuclei. He thought the central mass was
 
formed by the outer enclosing layers. He found bodies which he regarded
 
of the" same nature in fibrous coagulations in the heart.
 
  
Virchow (29), 51. in a discussion of endogenous cell formation, compares Hassall’s corpuscles to carcinoma pearls. He had about the same
+
Virchow (29), 51. in a discussion of endogenous cell formation, compares Hassall’s corpuscles to carcinoma pearls. He had about the same E. T. Bell 45
E. T. Bell 45
 
  
conception of the nature of the corpuscles as Hassall. This oft—quoted
+
conception of the nature of the corpuscles as Hassall. This oft—quoted comparison was therefore not based upon a deep insight into their nature.
comparison was therefore not based upon a deep insight into their nature.
 
  
Giinzburg (9), 57, did not advance beyond Hassall’s conception that
+
Giinzburg (9), 57, did not advance beyond Hassall’s conception that the central mass is formed by the peripheral layers.
the central mass is formed by the peripheral layers.
 
  
Paulitzky (21), 63, described the center of the corpuscles as homogeneous or granular. They sometimes contain an elliptical nucleus, sometimes fat droplets. The larger ones have in the center several nuclei or
+
Paulitzky (21), 63, described the center of the corpuscles as homogeneous or granular. They sometimes contain an elliptical nucleus, sometimes fat droplets. The larger ones have in the center several nuclei or cell-like forms. The central part is formed from masses of epithelial cells. Connective tissue cells grow around them and are transformed into epithelial cells forming the peripheral part of the corpuscle.
cell-like forms. The central part is formed from masses of epithelial
 
cells. Connective tissue cells grow around them and are transformed into
 
epithelial cells forming the peripheral part of the corpuscle.
 
  
The term “ concentric corpuscles ” was introduced by Ecker (6), who
+
The term “ concentric corpuscles ” was introduced by Ecker (6), who described them as arising directly from gland cells by fatty metamorphosis. He distinguished (1) simple corpuscles, round vesicles with thick concentric hulls, containing inside a fatty opalescent mass, and (2) compound corpuscles, which consist of several vesicles with a common hull. The peripheral layers of a corpuscle consist of flattened cells.
described them as arising directly from gland cells by fatty metamorphosis. He distinguished (1) simple corpuscles, round vesicles with
 
thick concentric hulls, containing inside a fatty opalescent mass, and (2)
 
compound corpuscles, which consist of several vesicles with a common
 
hull. The peripheral layers of a corpuscle consist of flattened cells.
 
  
His (12 a, 12 b), 60, 80, described the corpuscles as consisting of an
+
His (12 a, 12 b), 60, 80, described the corpuscles as consisting of an outer striated shell, probably composed of nucleated cells, and containing lymphocyte-like cells inside. He supposed them to be the original cells of the epithelial anlage which become entangled in the reticulum in some way. Their rapid growth in their narrow confines causes the concentric form.
outer striated shell, probably composed of nucleated cells, and containing lymphocyte-like cells inside. He supposed them to be the original
 
cells of the epithelial anlage which become entangled in the reticulum in
 
some way. Their rapid growth in their narrow confines causes the concentric form.
 
  
Cornil et Ranvier (5), 69, considered the corpuscles as arising from
+
Cornil et Ranvier (5), 69, considered the corpuscles as arising from the endothelium of blood-vessels and compared them to the spheres of their “ Sarcome angiolithique.”
the endothelium of blood-vessels and compared them to the spheres of their
 
“ Sarcome angiolithique.”
 
  
This suggestion of a vascular origin, made by Cornil et Ranvier, was
+
This suggestion of a vascular origin, made by Cornil et Ranvier, was elaborated by Afanassiew (la), 77.
elaborated by Afanassiew (la), 77.
 
  
Afanassiew held that the corpuscles of Hassall arise from the endothelium of the smaller veins and capillaries. The endothelial cells increase in size, become cubical, and later fill the lumen of the vessel.
+
Afanassiew held that the corpuscles of Hassall arise from the endothelium of the smaller veins and capillaries. The endothelial cells increase in size, become cubical, and later fill the lumen of the vessel. During the proliferation of the endothelium, the blood-vessels break up into segments which are now nearly solid cords.) These cords are at first connected to each other and to blood-vessels, but they soon break apart. The surest proof that the corpuscles are of vascular origin is that erythrocytes may be found -inside them. Vascular injections, however, do not go into a corpuscle except in a very early stage, since the lumen is soon obliterated by the endothelial plugs. The corpuscles are formed
During the proliferation of the endothelium, the blood-vessels break up
 
into segments which are now nearly solid cords.) These cords are at first
 
connected to each other and to blood-vessels, but they soon break apart.
 
The surest proof that the corpuscles are of vascular origin is that erythrocytes may be found -inside them. Vascular injections, however, do
 
not go into a corpuscle except in a very early stage, since the lumen is
 
soon obliterated by the endothelial plugs. The corpuscles are formed
 
  
entirely by the endothelial cells. Afanassiew worked on embryos of man,
+
entirely by the endothelial cells. Afanassiew worked on embryos of man, the rabbit, and the calf.
the rabbit, and the calf.
 
  
Stieda (26), 81, in sheep embryos, describes the epithelial mass of the
+
Stieda (26), 81, in sheep embryos, describes the epithelial mass of the 46 The Development of the Thymus
46 The Development of the Thymus
 
  
thymic anlage as being broken up by ingrowing adenoid tissue. From
+
thymic anlage as being broken up by ingrowing adenoid tissue. From 50 mm. to 60 mm., there are no large epithelial cells; but later at 100 mm. he finds in the adenoid tissue large cells 9 ,u. to 15 a in diameter, isolated or united in groups, whose protoplasm colors light-red with carmine. These large cells have a concentric structure. Some of them are enclosed by large cells whose cytoplasm does not color with carmine, giving rise to a yellowish mass of irregular form and stratified appearance. In older embryos (250 mm.), the cellular masses are numerous but the large colored cells are rare. The yellowish masses are groups of the large cells which have undergone a transformation like that of the stratum corneum of the epidermis. Stieda considers the large colored cells which form the corpuscles as remnants of the epithelial anlage, although he admits that for a long period during development he found no trace of them. He explains the formation of the corpuscles in accordance with (Johnheim’s hypothesis that most tumors‘ arise from unused tissue remnants.
50 mm. to 60 mm., there are no large epithelial cells; but later at 100 mm.
 
he finds in the adenoid tissue large cells 9 ,u. to 15 a in diameter, isolated
 
or united in groups, whose protoplasm colors light-red with carmine.
 
These large cells have a concentric structure. Some of them are enclosed
 
by large cells whose cytoplasm does not color with carmine, giving rise to
 
a yellowish mass of irregular form and stratified appearance. In older
 
embryos (250 mm.), the cellular masses are numerous but the large
 
colored cells are rare. The yellowish masses are groups of the large cells
 
which have undergone a transformation like that of the stratum corneum
 
of the epidermis. Stieda considers the large colored cells which form
 
the corpuscles as remnants of the epithelial anlage, although he admits
 
that for a long period during development he found no trace of them.
 
He explains the formation of the corpuscles in accordance with (Johnheim’s hypothesis that most tumors‘ arise from unused tissue remnants.
 
  
Ammann (2) 82, made most of his observations on human foetuses.
+
Ammann (2) 82, made most of his observations on human foetuses. He describes the corpuscles as arising from connective tissue. The corpuscles are cellular in structure and are formed of one, two, or three central cells around which -a Variable number of cells, increasing with age, are arranged like the coats of an onion. The corpuscles are formed from reticulum cells and leucocytes. Growth consists in the apposition of cells from without. The life of a corpuscle consists usually of four stages: ( 1) Stadium der Transparenz; (2) Stadium der colloiden Entartung; (3) Stadium der Verkalkung; (4) Stadium des Zerfalls. The nucleus of a reticulum cell or leucocyte increases in size at the expense of the cell body. Its increase in- size establishes the concentric form. The corpuscle undergoes colloid and usually calcareous degeneration. Fat droplets, cholesterin crystals, and colloid granules are found together in the degenerating corpuscles. Breaking up in this way makes absorption possible. No epithelial remnants are to be observed. No erythrocytes are found in the corpuscles.
He describes the corpuscles as arising from connective tissue. The corpuscles are cellular in structure and are formed of one, two, or three central cells around which -a Variable number of cells, increasing with age,
 
are arranged like the coats of an onion. The corpuscles are formed from
 
reticulum cells and leucocytes. Growth consists in the apposition of cells
 
from without. The life of a corpuscle consists usually of four stages:
 
( 1) Stadium der Transparenz; (2) Stadium der colloiden Entartung;
 
(3) Stadium der Verkalkung; (4) Stadium des Zerfalls. The nucleus
 
of a reticulum cell or leucocyte increases in size at the expense of the
 
cell body. Its increase in- size establishes the concentric form. The
 
corpuscle undergoes colloid and usually calcareous degeneration. Fat
 
droplets, cholesterin crystals, and colloid granules are found together in
 
the degenerating corpuscles. Breaking up in this way makes absorption
 
possible. No epithelial remnants are to be observed. No erythrocytes
 
are found in the corpuscles.
 
  
In four cases of atrophic thymus gland which yet contained lymphoid
+
In four cases of atrophic thymus gland which yet contained lymphoid tissue Ammann found corpuscles in all stages of development. He also found that the corpuscles are formed most rapidly when the thymus is at the height of its development. From these facts he concluded that they are not connected with the involution of _the thymus as Afanassiew thought. He thought that their formation is due to a physiological decrease in the intensity of growth of the medulla, due to the rapid growth of the cortex.
tissue Ammann found corpuscles in all stages of development. He also
 
found that the corpuscles are formed most rapidly when the thymus is
 
at the height of its development. From these facts he concluded that
 
they are not connected with the involution of _the thymus as Afanassiew
 
thought. He thought that their formation is due to a physiological decrease in the intensity of growth of the medulla, due to the rapid growth
 
of the cortex.
 
  
Watney (31), 83, agreed with Ammann that the corpuscles arise from
+
Watney (31), 83, agreed with Ammann that the corpuscles arise from connective tissue cells. E. T. Bell 47
connective tissue cells.
 
E. T. Bell 47
 
  
 
Monguidi (18), 85, distinguished true and false concentric corpuscles—the latter being only sections of blood-véssels.
 
Monguidi (18), 85, distinguished true and false concentric corpuscles—the latter being only sections of blood-véssels.
  
Hermann et Tourneux (11), 87, gave a description of the structure
+
Hermann et Tourneux (11), 87, gave a description of the structure and formation of the concentric corpuscles about like that given by Ammann except that they regard the reticulum cells from which the corpuscles develop as of epithelial origin.
and formation of the concentric corpuscles about like that given by
 
Ammann except that they regard the reticulum cells from which the
 
corpuscles develop as of epithelial origin.
 
  
Gulland (8), 91, regarded the corpuscles as epithelial remnants and
+
Gulland (8), 91, regarded the corpuscles as epithelial remnants and compared them to the epithelial pearls of the tonsil.
compared them to the epithelial pearls of the tonsil.
 
  
Maurer (17 c), 99, described the corpuscles as epithelial in origin.
+
Maurer (17 c), 99, described the corpuscles as epithelial in origin. His description of their formation is however different from that of His. All the cells of the epithelial anlage at first assume a lymphoid character. Later, some of these cells reassume their epithelial nature and then form the corpuscles. His conclusions for teleosts and amphibians are similar to the above results which he obtained from the lizard.
His description of their formation is however different from that of His.
 
All the cells of the epithelial anlage at first assume a lymphoid character.
 
Later, some of these cells reassume their epithelial nature and then form
 
the corpuscles. His conclusions for teleosts and amphibians are similar
 
to the above results which he obtained from the lizard.
 
  
Ver Eecke (28), 99, for the frog, describes the leucocytes and connective tissue cells as invading the thymic anlage and separating the epithelial cells. The epithelial cells, separated by the mesenchymal elements,
+
Ver Eecke (28), 99, for the frog, describes the leucocytes and connective tissue cells as invading the thymic anlage and separating the epithelial cells. The epithelial cells, separated by the mesenchymal elements, lie at first in groups or singly. They go through a cycle of two phases, a stage of growth, and a stage of involution. In the former stage, they increase to three or four'times their original size and their cytoplasm differentiates into circular layers like the coats of an onion. The majority are monocellular. Some cells grow together making a more complex multicellular type. There are some intermediate forms, cells with a dense dark protoplasmic body, indistinct striations, and a nucleus partly or completely hidden in a precocious degeneration. In the stage of involution, which sets in early, th'e.cytoplasm degenerates by the formation of vacuoles containing a hyaline liquid. The liquefaction may be in the form of a diffuse vacuolization, a large central vacuole, or a peripheral vacuole circular in section. The nucleus loses its affinity for stains, becomes deformed, breaks up, and finally disappears. The corpuscles are finally absorbed. They never contain erythrocytes. The cells do not degenerate to form a corpuscle. The liquefaction forms an internal secretion which is forced out by the muscle tissue in the reticulum.
lie at first in groups or singly. They go through a cycle of two phases,
 
a stage of growth, and a stage of involution. In the former stage, they
 
increase to three or four'times their original size and their cytoplasm
 
differentiates into circular layers like the coats of an onion. The majority
 
are monocellular. Some cells grow together making a more complex
 
multicellular type. There are some intermediate forms, cells with a dense
 
dark protoplasmic body, indistinct striations, and a nucleus partly or
 
completely hidden in a precocious degeneration. In the stage of involution, which sets in early, th'e.cytoplasm degenerates by the formation
 
of vacuoles containing a hyaline liquid. The liquefaction may be in the
 
form of a diffuse vacuolization, a large central vacuole, or a peripheral
 
vacuole circular in section. The nucleus loses its affinity for stains, becomes deformed, breaks up, and finally disappears. The corpuscles are
 
finally absorbed. They never contain erythrocytes. The cells do not degenerate to form a corpuscle. The liquefaction forms an internal secretion which is forced out by the muscle tissue in the reticulum.
 
  
Entirely different results on amphibians are reported by Nusbaum and
+
Entirely different results on amphibians are reported by Nusbaum and Machowski (19), 02. These investigators revive the old idea of Afanassiew, accepting his results except that they think the adventitia as well as the endothelium of the blood-vessels takes part in the formation of the corpuscles. They find erythrocytes in the corpuscles. These erythrocytes either gradually shrivel and disappear, or they are absorbed by leucocytes or endothelial cells. The leucocytes after digesting the haemoglobin of the erythrocytes become eosinophile cells which are numerous in the thymus. 48 The Development of the Thymus
Machowski (19), 02. These investigators revive the old idea of Afanassiew, accepting his results except that they think the adventitia as well
 
as the endothelium of the blood-vessels takes part in the formation of the
 
corpuscles. They find erythrocytes in the corpuscles. These erythrocytes
 
either gradually shrivel and disappear, or they are absorbed by leucocytes
 
or endothelial cells. The leucocytes after digesting the haemoglobin of the
 
erythrocytes become eosinophile cells which are numerous in the thymus.
 
48 The Development of the Thymus
 
  
Wallisch (30), 03, measured the volume of the human thymus and of
+
Wallisch (30), 03, measured the volume of the human thymus and of the corpuscles of Hassall at various stages. He finds that the total volume of the corpuscles of a 7-mo. embryo is 4.6 mm.,’‘ and of those of a 6-mo. child, 174.6 mm? The total volume of the thymus of a 78-mm. embryo, when it has already been partly transformed into adenoid tissue is only 6.8 mm? Since there is no evidence that the cells of the corpuscles multiply, he concludes that they cannot be regarded merely as remnants of the original epithelial anlage.
the corpuscles of Hassall at various stages. He finds that the total volume
 
of the corpuscles of a 7-mo. embryo is 4.6 mm.,’‘ and of those of a 6-mo.
 
child, 174.6 mm? The total volume of the thymus of a 78-mm. embryo,
 
when it has already been partly transformed into adenoid tissue is only
 
6.8 mm? Since there is no evidence that the cells of the corpuscles
 
multiply, he concludes that they cannot be regarded merely as remnants
 
of the original epithelial anlage.
 
  
Disregarding the crude observations of the earliest investigators, there
+
Disregarding the crude observations of the earliest investigators, there remain three distinct theories of the formation of the corpuscles of Hassall.
remain three distinct theories of the formation of the corpuscles of
 
Hassall.
 
  
1. The epithelial anlage of the thymus is broken up by the invading
+
1. The epithelial anlage of the thymus is broken up by the invading mesenchymal elements. The separated masses of epithelial cells undergo further changes mainly of a degenerative nature to form the corpuscles. This was the belief of His and Kolliker. According to this interpretation, the corpuscles are to be regarded as remnants that have nothing further to do with the gland. Stieda, Maurer, and Ver Eecke held this View in a modified form. Stieda regarded the cells forming the corpuscles as epithelial remnants but admitted that they go through a stage in Which, for a time, they lose their epithelial character. This is -substantially the same as Maurer’s view. He thinks that the cells of the epithelial anlage all become lymphoid, and that some of them afterwards reassume their epithelial nature and form the corpuscles. Ver Eecke regards the corpuseles as epithelial remnants but thinks that they are glandular in nature, not mere useless remains.
mesenchymal elements. The separated masses of epithelial cells undergo
 
further changes mainly of a degenerative nature to form the corpuscles.
 
This was the belief of His and Kolliker. According to this interpretation,
 
the corpuscles are to be regarded as remnants that have nothing further to
 
do with the gland. Stieda, Maurer, and Ver Eecke held this View in a
 
modified form. Stieda regarded the cells forming the corpuscles as
 
epithelial remnants but admitted that they go through a stage in Which,
 
for a time, they lose their epithelial character. This is -substantially the
 
same as Maurer’s view. He thinks that the cells of the epithelial anlage
 
all become lymphoid, and that some of them afterwards reassume their
 
epithelial nature and form the corpuscles. Ver Eecke regards the corpuseles as epithelial remnants but thinks that they are glandular in nature,
 
not mere useless remains.
 
  
2. The corpuscles are formed from the proliferating ‘Walls of bloodvessels. This idea was suggested by Cornil and Ranvier and elaborated by
+
2. The corpuscles are formed from the proliferating ‘Walls of bloodvessels. This idea was suggested by Cornil and Ranvier and elaborated by Afanassievv. Nusbaum and Maehowski accept Afanassiew’s view except that they believe the adventitia of the blood-vessels as well as their endothelium takes part in the formation of a corpuscle. These investigators thought that the formation of the corpuscles is connected with the involution of the thymus.
Afanassievv. Nusbaum and Maehowski accept Afanassiew’s view except
 
that they believe the adventitia of the blood-vessels as well as their endothelium takes part in the formation of a corpuscle. These investigators
 
thought that the formation of the corpuscles is connected with the involution of the thymus.
 
  
3. The corpuscles are formed from reticulum cells of the medulla
+
3. The corpuscles are formed from reticulum cells of the medulla and grow by apposition of the surrounding cells. This View was advanced by Ammann. Ammann thought that the reticulum is of connective tissue origin. He also believed that leucocytes formed the central part at least of some corpuscles. Hermann -and Tourneux accepted Amma.nn’s results, except that they ascribed an epithelial origin to the reticulum. (I do not know Whether they accepted the origin from leucocytes described by Ammann.) Ammann thought that the corpuscles formed because of a physiological decrease in the rate of growth in the medulla. E. T. Bell 49
and grow by apposition of the surrounding cells. This View was advanced
 
by Ammann. Ammann thought that the reticulum is of connective tissue
 
origin. He also believed that leucocytes formed the central part at least
 
of some corpuscles. Hermann -and Tourneux accepted Amma.nn’s results,
 
except that they ascribed an epithelial origin to the reticulum. (I do not
 
know Whether they accepted the origin from leucocytes described by
 
Ammann.) Ammann thought that the corpuscles formed because of a
 
physiological decrease in the rate of growth in the medulla.
 
E. T. Bell 49
 
  
My own observations on the development of the corpuscles of Hassall
+
My own observations on the development of the corpuscles of Hassall in pig embryos, will now be considered. The medulla, as previously described, begins to form from the epithelial syncytium usually near the center of the lobule. It is first recognized by its more marked reaction with cytoplasmic stains such as Congo red. Shortly after the medulla begins to form, the earliest stages of the corpuscles may be observed. A few corpuscles have appeared at 9.5 cm. I did not find them earlier. They are all formed from the epithelial syncytium of the medulla.
in pig embryos, will now be considered. The medulla, as previously described, begins to form from the epithelial syncytium usually near the
 
center of the lobule. It is first recognized by its more marked reaction
 
with cytoplasmic stains such as Congo red. Shortly after the medulla
 
begins to form, the earliest stages of the corpuscles may be observed. A
 
few corpuscles have appeared at 9.5 cm. I did not find them earlier.
 
They are all formed from the epithelial syncytium of the medulla.
 
  
Before beginning this discussion I will explain the use of my terms,
+
Before beginning this discussion I will explain the use of my terms, By a corpuscle of Hassall, I mean a modified area of the epithelial syncytium of the medulla, containing at some period of its development, one or more nuclei, and whose cytoplasm has been in part or entirely transformed into colloid. The term colloid is applied to various substances probably of widely different chemical nature, but is fairly adapted to our imperfect knowledge. I shall use the term here in the restricted sense «employed by Ziegler,‘ i. e., hyaline substances of epithelial origin, that do not give the reactions of mucin.
By a corpuscle of Hassall, I mean a modified area of the epithelial syncytium of the medulla, containing at some period of its development, one
 
or more nuclei, and whose cytoplasm has been in part or entirely transformed into colloid. The term colloid is applied to various substances
 
probably of widely different chemical nature, but is fairly adapted to our
 
imperfect knowledge. I shall use the term here in the restricted sense
 
«employed by Ziegler,‘ i. e., hyaline substances of epithelial origin, that do
 
not give the reactions of mucin.
 
  
Colloid in the corpuscles of Hassall does not usually appear as solid
+
Colloid in the corpuscles of Hassall does not usually appear as solid "masses in its early formation, but as fibers, granules, or sheets which are separated by more or less cytoplasm that is not yet changed. This stage I have called, “colloid in formation” (c f). It later assumes a more solid homogeneous appearance which I call solid colloid (c s). Often the solid colloid stains intensely with cytoplasmic stains. I call this kind solid deeply-staining colloid (c s d). In later tages, the colloid often loses its affinity for cytoplasmic stains, staining a very pale color or not staining at all. I call this variety old colloid (0 0).
"masses in its early formation, but as fibers, granules, or sheets which are
 
separated by more or less cytoplasm that is not yet changed. This stage
 
I have called, “colloid in formation” (c f). It later assumes a more
 
solid homogeneous appearance which I call solid colloid (c s). Often
 
the solid colloid stains intensely with cytoplasmic stains. I call this
 
kind solid deeply-staining colloid (c s d). In later tages, the colloid
 
often loses its affinity for cytoplasmic stains, staining a very pale color or
 
not staining at all. I call this variety old colloid (0 0).
 
  
According to their mode of development, the corpuscles of Hasall may
+
According to their mode of development, the corpuscles of Hasall may be classified as follows:
be classified as follows:
 
  
A. Concentric Corpuscles.
+
A. Concentric Corpuscles. a. Simple. 1. Ordinary type. 2. Epithelioid type. 3. Cystic type. b. Compound. 13. Irregular Corpuscles. a. Compact type. b. Reticular type.
a. Simple.
 
1. Ordinary type.
 
2. Epithelioid type.
 
3. Cystic type.
 
b. Compound.
 
13. Irregular Corpuscles.
 
a. Compact type.
 
b. Reticular type.
 
  
 
‘Gen. Pathology, 10th ed., Warth1n’s translation, p. 205.
 
‘Gen. Pathology, 10th ed., Warth1n’s translation, p. 205.
  
4
+
4 50 The Development of the Thymus
50 The Development of the Thymus
 
  
A. The concentric corpuscles include those that from their earliest
+
A. The concentric corpuscles include those that from their earliest appearance are concentric in structure. Adopting Ecker’s classification., I distinguish simple concentric corpuscles and compound concentric corpuscles.
appearance are concentric in structure. Adopting Ecker’s classification.,
 
I distinguish simple concentric corpuscles and compound concentric
 
corpuscles.
 
  
(a) Three types of simple concentric corpuscles are to be considered.
+
(a) Three types of simple concentric corpuscles are to be considered. (1) The ordinary type is far more numerous than any other. The earliest recognizable stage is shown in Plate II, Fig. 11. A nucleus (n) of the syncytium of the medulla has enlarged to perhaps twice its ordinary volume and has lost the ability to stain in the characteristic way with haematoxylin. Its sap is clear and a few reddish stained granules represent its chromatin. Around it in the cytoplasm is an indistinct uneven layer of colloid (c f). The colloid is not yet solid and is being formed in concentric fibers or sheets. A slightly later stage is shown in Plate II, Fig. 14 and Fig. 15 (left side of figure). Some of the colloid (c s) next to the nucleus is now solid. The next stage is shown in Plate II, Fig. 15 (right side of figure). These corpuscles show a thick layer of colloid (c s d) that stains intensely with Congo red. Just outside the deeply staining colloid, colloid in formation may be seen. The nuclei are clear, and have become smaller and irregular in outline. The colloid seems to be pressing upon them and obliterating them. The colloid transformation gradually involves the adjacent cytoplasm of the syncytium until other nuclei are involved. The corpuscle has now reached the condition shown in Plate II, Fig. 12. The central area (0 c) is solid, the nucleus having disappeared entirely. Another (72!) is nearly obliterated by the colloid. Part of the central area (0 c) no longer stains intensely, and it is breaking loose by the formation of a concentric space. Several nuclei are surrounded by colloid in formation. Their long axes are nearly in a tangential direction. These nuclei are clear but only
(1) The ordinary type is far more numerous than any other. The earliest recognizable stage is shown in Plate II, Fig. 11. A nucleus (n)
 
of the syncytium of the medulla has enlarged to perhaps twice its ordinary
 
volume and has lost the ability to stain in the characteristic way with
 
haematoxylin. Its sap is clear and a few reddish stained granules represent its chromatin. Around it in the cytoplasm is an indistinct uneven
 
layer of colloid (c f). The colloid is not yet solid and is being formed
 
in concentric fibers or sheets. A slightly later stage is shown in Plate II,
 
Fig. 14 and Fig. 15 (left side of figure). Some of the colloid (c s)
 
next to the nucleus is now solid. The next stage is shown in Plate II,
 
Fig. 15 (right side of figure). These corpuscles show a thick layer of
 
colloid (c s d) that stains intensely with Congo red. Just outside the
 
deeply staining colloid, colloid in formation may be seen. The nuclei
 
are clear, and have become smaller and irregular in outline. The colloid
 
seems to be pressing upon them and obliterating them. The colloid
 
transformation gradually involves the adjacent cytoplasm of the syncytium until other nuclei are involved. The corpuscle has now reached
 
the condition shown in Plate II, Fig. 12. The central area (0 c) is
 
solid, the nucleus having disappeared entirely. Another (72!) is nearly obliterated by the colloid. Part of the central area (0 c) no longer stains
 
intensely, and it is breaking loose by the formation of a concentric space.
 
Several nuclei are surrounded by colloid in formation. Their long axes
 
are nearly in a tangential direction. These nuclei are clear but only
 
  
 
moderately swollen.
 
moderately swollen.
  
In the further development of the corpuscle (Plate III, Fig. 17 and
+
In the further development of the corpuscle (Plate III, Fig. 17 and Plate II, Fig. 7), the central area (c s (1) increases in size. The nuclei involved in this area become obliterated probably by the pressure of the colloid and are no longer distinguishable. This central area usually splits off and may break up into many smaller masses. The peripheral part of the corpuscle increases by extension of the colloid formation into the adjacent part of the syncytium. This extension takes place in- the early stages by direct progressive involvement of the immediately adjacent cytoplasm; in later stages (Fig. 7), by the formation of concentric lamellae which cut ofi’ unchanged areas of cytoplasm. The lamellm increase in size and number, the cytoplasm _included between them is changed into colloid. They finally become closely packed, giving the characteristic and THE DEVELOPMENT OF THE THYMUS. PLATE I.
Plate II, Fig. 7), the central area (c s (1) increases in size. The nuclei
 
involved in this area become obliterated probably by the pressure of the
 
colloid and are no longer distinguishable. This central area usually splits
 
off and may break up into many smaller masses. The peripheral part of
 
the corpuscle increases by extension of the colloid formation into the
 
adjacent part of the syncytium. This extension takes place in- the early
 
stages by direct progressive involvement of the immediately adjacent cytoplasm; in later stages (Fig. 7), by the formation of concentric lamellae
 
which cut ofi’ unchanged areas of cytoplasm. The lamellm increase in
 
size and number, the cytoplasm _included between them is changed into
 
colloid. They finally become closely packed, giving the characteristic and
 
THE DEVELOPMENT OF THE THYMUS. PLATE I.
 
  
 
E. 1'. BELL.
 
E. 1'. BELL.
  
AMIRIOAN JOURNAL OF ANATOMY--VOL V. 5- 7- BEL‘-y DEL
+
AMIRIOAN JOURNAL OF ANATOMY--VOL V. 5- 7- BEL‘-y DEL NELIOTVPE 00., IOITON. THE DEVELOPMENT OF THE THYMUS. PLATEII.
NELIOTVPE 00., IOITON.
 
THE DEVELOPMENT OF THE THYMUS. PLATEII.
 
  
 
E. T. BELL.
 
E. T. BELL.
  
AMERICAN aounnu. or ANATOMY--VOL v. 5- 7- “Lt: “Et
+
AMERICAN aounnu. or ANATOMY--VOL v. 5- 7- “Lt: “Et HELIOTVPE 00., BOSTON‘ THE DEVELOPMENT OF THE THYMUS. PLATEIII.
HELIOTVPE 00., BOSTON‘
 
THE DEVELOPMENT OF THE THYMUS. PLATEIII.
 
  
 
E. T. BELL.
 
E. T. BELL.
Line 944: Line 301:
 
AMERICAN JOURNAL OF ANATOMY--VOL V.
 
AMERICAN JOURNAL OF ANATOMY--VOL V.
  
E. T. BELL, DEL.
+
E. T. BELL, DEL. E. T. Bell 51
E. T. Bell 51
 
  
Well-known onion-like structure found in the fully-formed corpuscle.
+
Well-known onion-like structure found in the fully-formed corpuscle. The nuclei that are enclosed between the lamellae gradually lose their chromatin and become flattened out. They do not swell and are not obliterated. It seems that swelling occurs only in nuclei that are surrounded by deeply staining colloid, and that this change is preparatory to their obliteration by or transformation into colloid. The amount of the corpuscle that breaks up to form the softer center i-s very variable. The size of the center usually seems to increase with the age of the corpuscle.
The nuclei that are enclosed between the lamellae gradually lose their
 
chromatin and become flattened out. They do not swell and are not
 
obliterated. It seems that swelling occurs only in nuclei that are surrounded by deeply staining colloid, and that this change is preparatory to
 
their obliteration by or transformation into colloid. The amount of the
 
corpuscle that breaks up to form the softer center i-s very variable. The
 
size of the center usually seems to increase with the age of the corpuscle.
 
  
Plate III, Fig. 21, shows a variation from the ordinary concentric
+
Plate III, Fig. 21, shows a variation from the ordinary concentric type. The central nucleus (n) stains reddish but is not enlarged. Most of the other nuclei are unchanged. All the colloid (c f) is in the early fibrous and granular stage.
type. The central nucleus (n) stains reddish but is not enlarged. Most
 
of the other nuclei are unchanged. All the colloid (c f) is in the early
 
fibrous and granular stage.
 
  
From 20 cm. to full term many corpuscles show masses of calcareous
+
From 20 cm. to full term many corpuscles show masses of calcareous material in or near the center. This material rarely appears in younger corpuscles (Plate III, Fig. 1?’, cl). It stains a violet blue with Delafield’s haematoxylin.
material in or near the center. This material rarely appears in younger
 
corpuscles (Plate III, Fig. 1?’, cl). It stains a violet blue with Delafield’s haematoxylin.
 
  
The majority of the corpuscles of Hassall belong to the ordinary type
+
The majority of the corpuscles of Hassall belong to the ordinary type of simple concentric corpuscles described above. It is very clear that they have nothing to do with blood—vessels. ‘They never contain erythrocytes nor anything resembling them. Rarely a lymphoblast or leuco— cyte is found inside the corpuscle. These seem to be usually involved in the corpuscle like ordinary stroma nuclei during the formation of the lamellae. (Their occurrence in other types will be discussed later.) It is also clear that these corpuscles arise from the syncytium of the medulla. They are epithelial in origin, since the entire stroma of the gland is derived from epithelium, but they are certainly not remnants of the original epithelial anlage. Neither are they formed from lymphoidlike elements that reassume their epithelial nature as Maurer described for the lizard.
of simple concentric corpuscles described above. It is very clear that
 
they have nothing to do with blood—vessels. ‘They never contain erythrocytes nor anything resembling them. Rarely a lymphoblast or leuco—
 
cyte is found inside the corpuscle. These seem to be usually involved in
 
the corpuscle like ordinary stroma nuclei during the formation of the
 
lamellae. (Their occurrence in other types will be discussed later.) It
 
is also clear that these corpuscles arise from the syncytium of the
 
medulla. They are epithelial in origin, since the entire stroma of
 
the gland is derived from epithelium, but they are certainly not remnants
 
of the original epithelial anlage. Neither are they formed from lymphoidlike elements that reassume their epithelial nature as Maurer described
 
for the lizard.
 
  
Some of Ammann’s observations are in accord with my results. The
+
Some of Ammann’s observations are in accord with my results. The swelling of the nucleus was noted by Ammann as the first step toward the formation of the corpuscle. It should be noted, however, that rarely a corpuscle begins to form as a mass of colloid out in the cytoplasm and involves nuclei secondarily. I cannot distinguish his “Stadium der Transparenz ” for I cannot be sure that a corpuscle is beginning to form until some colloid is present. The formation of the colloid is associated with the swelling of the nucleus. His other three stages, “ Stadium der colloiden Entartung,” “ Stadium der Verkalkung,” and “Stadium des Zerfalls ” are easily seen. I have never seen corpuscles begin in leucocytes as Ammann described. His statement that the corpuscle grows by apposition of reticulum cells is true in a modified sense. He thought that 52 The Development of the Thymus
swelling of the nucleus was noted by Ammann as the first step toward the
 
formation of the corpuscle. It should be noted, however, that rarely a
 
corpuscle begins to form as a mass of colloid out in the cytoplasm and
 
involves nuclei secondarily. I cannot distinguish his “Stadium der
 
Transparenz ” for I cannot be sure that a corpuscle is beginning to form
 
until some colloid is present. The formation of the colloid is associated
 
with the swelling of the nucleus. His other three stages, “ Stadium der
 
colloiden Entartung,” “ Stadium der Verkalkung,” and “Stadium des
 
Zerfalls ” are easily seen. I have never seen corpuscles begin in leucocytes
 
as Ammann described. His statement that the corpuscle grows by apposition of reticulum cells is true in a modified sense. He thought that
 
52 The Development of the Thymus
 
  
the outer part of a corpuscle is formed of reticulum cells that have moved
+
the outer part of a corpuscle is formed of reticulum cells that have moved up and flattened themselves out around it. The description just given shows that the corpuseles are never composed of distinct cells, and that the increase in size is due to an extension outward of the colloid formation and not to a moving in of the adjacent tissue.
up and flattened themselves out around it. The description just given
 
shows that the corpuseles are never composed of distinct cells, and that
 
the increase in size is due to an extension outward of the colloid formation
 
and not to a moving in of the adjacent tissue.
 
  
The concentric form of this type of corpuscle is due at first to its being
+
The concentric form of this type of corpuscle is due at first to its being formed around a spherical or ellipsoidal nucleus. The swelling of this nucleus creates a centrifugal pressure in the adjacent cytoplasm. Before or during its transformation into colloid, the cytoplasm also increases in quantity. That the cytoplasm increases in quantity is shown by the fact that the nuclei are fewer in the corpuscle than in any adjacent area of the syncytium of equal size. This centrifugal pressure presses the newly formed colloid into concentric lamellae. It at first /turns the long axes of the nuclei tangentially, and later flattens them and makes them concave toward the center.
formed around a spherical or ellipsoidal nucleus. The swelling of this
 
nucleus creates a centrifugal pressure in the adjacent cytoplasm. Before
 
or during its transformation into colloid, the cytoplasm also increases in
 
quantity. That the cytoplasm increases in quantity is shown by the fact
 
that the nuclei are fewer in the corpuscle than in any adjacent area of
 
the syncytium of equal size. This centrifugal pressure presses the newly
 
formed colloid into concentric lamellae. It at first /turns the long axes of
 
the nuclei tangentially, and later flattens them and makes them concave
 
toward the center.
 
  
2. The epithetliovld type of corpuscle is characterized by large areas of
+
2. The epithetliovld type of corpuscle is characterized by large areas of cytoplasm so marked off by colloid lamellae as to give the appearance of a mass of large epithelial cells. They may contain only one nucleus embedded in a well-defined area of cytoplasm (Plate III, Figs. 18 and 20). These correspond to the monocellular corpuseles that have been described for lizards and amphibians. They are rare in the pig. I have not been able to trace these very far, as they soon become indistinguishable from other forms. The only difference I have noted is that the outer colloid lamellae begin to form early, causing the peculiar appearance of a large epithelial cell. Again the epithelioid type may present an appearance such as shown in Plate I, Fig. 3. These do not seem to be formed around any special nucleus. The outer colloid lamellaa form before any center has been established, marking off large cytoplasmic areas that may look like large cells. The centrifugal pressure of expansion caused by the great increase of cytoplasm in this area determines the concentric form in these corpuseles. Pure epithelioid corpuseles are very rare, but epithelioid areas in other corpuseles are not uncommon. The occurrence of epithelioid areas in corpuseles of the ordinary type shows that it is due to variations in a fundamentally similar process.
cytoplasm so marked off by colloid lamellae as to give the appearance of a
 
mass of large epithelial cells. They may contain only one nucleus embedded in a well-defined area of cytoplasm (Plate III, Figs. 18 and 20).
 
These correspond to the monocellular corpuseles that have been described
 
for lizards and amphibians. They are rare in the pig. I have not been
 
able to trace these very far, as they soon become indistinguishable from
 
other forms. The only difference I have noted is that the outer colloid
 
lamellae begin to form early, causing the peculiar appearance of a large
 
epithelial cell. Again the epithelioid type may present an appearance
 
such as shown in Plate I, Fig. 3. These do not seem to be formed around
 
any special nucleus. The outer colloid lamellaa form before any center
 
has been established, marking off large cytoplasmic areas that may look
 
like large cells. The centrifugal pressure of expansion caused by the
 
great increase of cytoplasm in this area determines the concentric form
 
in these corpuseles. Pure epithelioid corpuseles are very rare, but epithelioid areas in other corpuseles are not uncommon. The occurrence of
 
epithelioid areas in corpuseles of the ordinary type shows that it is due
 
to variations in a fundamentally similar process.
 
  
3. In the cystic type of corpuscle, the central part, instead of becoming
+
3. In the cystic type of corpuscle, the central part, instead of becoming transformed into colloid, undergoes early liquefaction, forming vacuoles. The central nucleus does not increase in size as in the ordinary type, but shrivels up and disappears. The corpuscle begins by the formation of outer colloid lamellae—the central mass is not changed into colloid. In Plate II, Fig. 10, the central area (p m) is undergoing a diffuse liquefaction. The nucleus (n) is colorless and shrunken. In Plate II. E. T. Bell 53
transformed into colloid, undergoes early liquefaction, forming vacuoles.
 
The central nucleus does not increase in size as in the ordinary type, but
 
shrivels up and disappears. The corpuscle begins by the formation of
 
outer colloid lamellae—the central mass is not changed into colloid.
 
In Plate II, Fig. 10, the central area (p m) is undergoing a diffuse
 
liquefaction. The nucleus (n) is colorless and shrunken. In Plate II.
 
E. T. Bell 53
 
  
Fig. 8 (right side of figure), the central area has formed two large vacuoles (zz). On the left side of the same figure, a concentric vacuole (12)
+
Fig. 8 (right side of figure), the central area has formed two large vacuoles (zz). On the left side of the same figure, a concentric vacuole (12) has formed, separating off a central spherical nucleated mass of protoplasm. The nucleus of this mass of protoplasm is shrunken and the cytoplasm shows many small vacuoles. The corpuscle shown in Plate II, Fig. 9, is probably a later stage of the form just described. The central protoplasmic mass has become converted into an ellipsoidal pale body (p m). The small circular body in this shriveled mass is probably the nucleus. Some corpuscles like the one shown in Fig. 9 are found in which the central mass has entirely disappeared. The further growth of corpuscles of this type seems to be by formation of colloid lamellae as in the ordinary type. They soon become indistinguishable from other forms.
has formed, separating off a central spherical nucleated mass of protoplasm. The nucleus of this mass of protoplasm is shrunken and the
 
cytoplasm shows many small vacuoles. The corpuscle shown in Plate II,
 
Fig. 9, is probably a later stage of the form just described. The central
 
protoplasmic mass has become converted into an ellipsoidal pale body
 
(p m). The small circular body in this shriveled mass is probably the
 
nucleus. Some corpuscles like the one shown in Fig. 9 are found in
 
which the central mass has entirely disappeared. The further growth
 
of corpuscles of this type seems to be by formation of colloid lamellae as
 
in the ordinary type. They soon become indistinguishable from other
 
forms.
 
  
The cystic type of corpuscle is rare in the pig. This evidently corresponds to the form in amphibia that misled Nusbaum and Machowski
+
The cystic type of corpuscle is rare in the pig. This evidently corresponds to the form in amphibia that misled Nusbaum and Machowski into reviving Afanassiew’s theory. The central masses, in Figs. 8 and 9, might readily be mistaken for red corpuscles in animals in which these cells are nucleated. But the red cells of the blood of the pig are not nucleated at this stage. I have traced a number of these corpuscles (as well as those of other types) in serial sections and have never seen any indications of a connection to blood-vessels. Nusbaum and Machowski (19), (Fig. 1, c, S. 116) show a corpuscle which is similar to my Fig. 9. It will be noted that the central space in neither of these figures is lined by endothelium. The early form of corpuscle shown by Nusbaum and Machowski (Fig. 1, d, S. 116) is very probably a normal bloodvessel with cubical endothelium. I have often found such vessels with cubical endothelium in the interlobular tissue of the pig’s thymus at 10 cm. to 12 cm. They probably may be found at other stages also. In the thymus of a kitten, injected by the inrtraavitam Prussian blue method previously described, the majority of the corpuscles were found to be in early stages. The injection did not penetrate any corpuscle. I had a somewhat better opportunity to study the relations of the corpuscles to the blood-vessels in a 14 cm. human embryo. Here the vessels of the thymus were all very much distended with blood and the corpuscles were in early stages. No blood cells were found in the corpuscles.
into reviving Afanassiew’s theory. The central masses, in Figs. 8 and 9,
 
might readily be mistaken for red corpuscles in animals in which these
 
cells are nucleated. But the red cells of the blood of the pig are not
 
nucleated at this stage. I have traced a number of these corpuscles (as
 
well as those of other types) in serial sections and have never seen any
 
indications of a connection to blood-vessels. Nusbaum and Machowski (19), (Fig. 1, c, S. 116) show a corpuscle which is similar to my
 
Fig. 9. It will be noted that the central space in neither of these figures
 
is lined by endothelium. The early form of corpuscle shown by Nusbaum and Machowski (Fig. 1, d, S. 116) is very probably a normal bloodvessel with cubical endothelium. I have often found such vessels with
 
cubical endothelium in the interlobular tissue of the pig’s thymus at 10
 
cm. to 12 cm. They probably may be found at other stages also. In the
 
thymus of a kitten, injected by the inrtraavitam Prussian blue method previously described, the majority of the corpuscles were found to be in
 
early stages. The injection did not penetrate any corpuscle. I had a
 
somewhat better opportunity to study the relations of the corpuscles to
 
the blood-vessels in a 14 cm. human embryo. Here the vessels of the
 
thymus were all very much distended with blood and the corpuscles were
 
in early stages. No blood cells were found in the corpuscles.
 
  
(b) C'o~mpo/und concentric corpuscles are formed whenever two or more
+
(b) C'o~mpo/und concentric corpuscles are formed whenever two or more simple concentric corpuscles begin to form so close together that they come in contact during their later growth. An early stage of such a corpuscle is shown in Plate II, Fig. 15. The colloid lamellae are formed around each center until they come in contact; they are then formed around both centers. In Plate III, Fig. 22, a compound concentric cor54 The Development of the Thymus
simple concentric corpuscles begin to form so close together that they
 
come in contact during their later growth. An early stage of such a
 
corpuscle is shown in Plate II, Fig. 15. The colloid lamellae are formed
 
around each center until they come in contact; they are then formed
 
around both centers. In Plate III, Fig. 22, a compound concentric cor54 The Development of the Thymus
 
  
puscle is shown. There are three simple concentric corpuscles in it—
+
puscle is shown. There are three simple concentric corpuscles in it— one of them (the lowest in the figure.) in a very early stage. Several l-amellae are common to the older corpuscles, and one is common to all three. This arrangement of the lamellae is a mechanical effect of the tension in the cytoplasm, due to the centrifugal pressure from the two centers. The size of the separate centers in a compound corpuscle depends upon the stage they have reached when they come in contact. If a compound corpuscle be formed by the union of two simple corpuscles in an early stage, as in Plate III, Fig. 19, all indications of its coinpound nature are soon lost. A corpuscle originally compound may, then, in later stages, become indistinguishable from simple corpuscles. The simple corpuscles uniting to form a compound concentric corpuscle may be of any of the types previously described.
one of them (the lowest in the figure.) in a very early stage. Several
 
l-amellae are common to the older corpuscles, and one is common to all
 
three. This arrangement of the lamellae is a mechanical effect of the
 
tension in the cytoplasm, due to the centrifugal pressure from the two
 
centers. The size of the separate centers in a compound corpuscle depends upon the stage they have reached when they come in contact. If
 
a compound corpuscle be formed by the union of two simple corpuscles
 
in an early stage, as in Plate III, Fig. 19, all indications of its coinpound nature are soon lost. A corpuscle originally compound may, then,
 
in later stages, become indistinguishable from simple corpuscles. The
 
simple corpuscles uniting to form a compound concentric corpuscle may
 
be of any of the types previously described.
 
  
 
B. IRREGULAR CORPUSCLES.
 
B. IRREGULAR CORPUSCLES.
  
This group includes those corpuscles which are not at first concentric.
+
This group includes those corpuscles which are not at first concentric. Concentric areas may appear later. According to the classification previously given, I distinguish a compact type and la reticular type.
Concentric areas may appear later. According to the classification previously given, I distinguish a compact type and la reticular type.
 
  
(a) The compact type (Plate III, Fig. 16) first appears as a compact
+
(a) The compact type (Plate III, Fig. 16) first appears as a compact area of syncytium of irregular shape. It is recognizable by the colloid it contains. The nuclei are not noticeably increased in size and have no regular arrangement. Their chromatin still stains dark with nuclear stains. The colloid (c f) is not yet solid. The corpuscle has no distinct center. These corpuscles grow by direct colloid transformation of the adjacent syncytium. No distinct lamellae are formed. The colloid may remain in the fibrous condition shown in the figure (cf) or it may become solid, but it never reaches the deeply staining condition unless a concentric area be established.
area of syncytium of irregular shape. It is recognizable by the colloid
 
it contains. The nuclei are not noticeably increased in size and have no
 
regular arrangement. Their chromatin still stains dark with nuclear
 
stains. The colloid (c f) is not yet solid. The corpuscle has no distinct
 
center. These corpuscles grow by direct colloid transformation of the
 
adjacent syncytium. No distinct lamellae are formed. The colloid may
 
remain in the fibrous condition shown in the figure (cf) or it may
 
become solid, but it never reaches the deeply staining condition unless a
 
concentric area be established.
 
  
 
A later stage of this type is -shown in Plate III, Fig. 23. The cor-v
 
A later stage of this type is -shown in Plate III, Fig. 23. The cor-v
  
puscle is sharply marked off from the syncytium. Some of its colloid is
+
puscle is sharply marked off from the syncytium. Some of its colloid is solid. A concentric area (as) is beginning to form. The nuclei are not markedly different from those of the adjacent syncytium. These corpuscles may become large and branched. Often one or more concentric areas are developed after the corpuscle has attained considerable size. By the growth of these concentric areas, irregular corpuscles may become converted into concentric corpuscles.
solid. A concentric area (as) is beginning to form. The nuclei are
 
not markedly different from those of the adjacent syncytium. These
 
corpuscles may become large and branched. Often one or more concentric areas are developed after the corpuscle has attained considerable
 
size. By the growth of these concentric areas, irregular corpuscles may
 
become converted into concentric corpuscles.
 
  
(b) The reticular type is produced by colloid formation in the ordinary reticulum of the medulla. In the types previously described, the
+
(b) The reticular type is produced by colloid formation in the ordinary reticulum of the medulla. In the types previously described, the spaces of the reticulum are usually obliterated as the colloid form-ation advances; but in this form the spaces persist as a part of the corpuscle. Pure reticular corpuscles vary greatly in size, sometimes involving only E. T. Bell 55
spaces of the reticulum are usually obliterated as the colloid form-ation
 
advances; but in this form the spaces persist as a part of the corpuscle.
 
Pure reticular corpuscles vary greatly in size, sometimes involving only
 
E. T. Bell 55
 
  
one node of the syncytium. They are never concentric, and never form
+
one node of the syncytium. They are never concentric, and never form lamellae. Reticular areas often occur in other forms of corpuscles. In this way leucocytes are often involved in the eorpusele, since they lie in the spaces of the reticulum. Lymphocytes often get into a eorpusele in the lymphoblast condition, being cut off by the formation of lamellze outside them (Plate III, Fig. 22). The leucocytes shut in the corpuscle in this way during development may not degenerate. '1‘hey probably persist and help to remove the eorpusele in its final stages of degeneration.
lamellae. Reticular areas often occur in other forms of corpuscles. In
 
this way leucocytes are often involved in the eorpusele, since they lie in
 
the spaces of the reticulum. Lymphocytes often get into a eorpusele in
 
the lymphoblast condition, being cut off by the formation of lamellze
 
outside them (Plate III, Fig. 22). The leucocytes shut in the corpuscle in this way during development may not degenerate. '1‘hey probably persist and help to remove the eorpusele in its final stages of degeneration.
 
  
The amount of expansion of the eytopla-sm before or during the colloid
+
The amount of expansion of the eytopla-sm before or during the colloid transformation is probably small in the irregular reticular corpuscles, since it does not obliterate the spaces of the syncytium. In the compact type, the spaces of the syncytium are obliterated and there is evidence of some expansive force (note the arrangement of the nuclei in the upper part of Fig. 2-3, Plate III). In the figure referred to, the number of nuclei in any part of the eorpusele is less than in an equal area of the adjacent reticulum. These facts indicate that there is an expansion of the cytoplasm. That this expansive force does not produce a concentric form is due primarily to the fact that there is no expansion of a nucleus and distinct center of formation as is present in concentric corpuscles of the ordinary type. The absence of the onion-like structure in irregular corpuscles is due to the fact that the colloid is not laid ‘down in lamellze.
transformation is probably small in the irregular reticular corpuscles,
 
since it does not obliterate the spaces of the syncytium. In the compact
 
type, the spaces of the syncytium are obliterated and there is evidence
 
of some expansive force (note the arrangement of the nuclei in the
 
upper part of Fig. 2-3, Plate III). In the figure referred to, the number
 
of nuclei in any part of the eorpusele is less than in an equal area of the
 
adjacent reticulum. These facts indicate that there is an expansion of
 
the cytoplasm. That this expansive force does not produce a concentric
 
form is due primarily to the fact that there is no expansion of a nucleus
 
and distinct center of formation as is present in concentric corpuscles of
 
the ordinary type. The absence of the onion-like structure in irregular
 
corpuscles is due to the fact that the colloid is not laid ‘down in lamellze.
 
  
Significance of the corpuscles of I-Iassall. It has been shown in the
+
Significance of the corpuscles of I-Iassall. It has been shown in the preceding pages that the corpuscles of Hassall in the pig are not epithelial remnants, and also that they are not_ formed from blood-vessels. There is no evidence connecting their development with the involution of the thymus, for they begin to form before the lymphoid transformation is complete a.nd are most numerous when the thymus is at the height of its development. I have not been able to see the decrease in the rate of growth of the medulla described by Ammann, and even if such dirl occur it is difilcult to understand how it could cause the formation of 9. eorpusele.
preceding pages that the corpuscles of Hassall in the pig are not epithelial remnants, and also that they are not_ formed from blood-vessels.
 
There is no evidence connecting their development with the involution
 
of the thymus, for they begin to form before the lymphoid transformation is complete a.nd are most numerous when the thymus is at the height
 
of its development. I have not been able to see the decrease in the rate
 
of growth of the medulla described by Ammann, and even if such dirl
 
occur it is difilcult to understand how it could cause the formation of 9.
 
eorpusele.
 
  
The above theories are, therefore, inconsistent with the facts of development in the pig. It seems to me that the formation of a eorpusele is
+
The above theories are, therefore, inconsistent with the facts of development in the pig. It seems to me that the formation of a eorpusele is not to be regarded as a process of degeneration. The fact that the formation of colloid is an essential feature in the development of every eorpusele is a strong argument that it is a form of secretion such as occurs in its neighboring branchial derivative, the thyroid. The "fact that the corpuscles differentiate in an apparently uniform syiicytium is further evidence against a theory of degeneration. Since the lymphocyte-forming function of the thymus is probably secondary, it is not 56 The Development of the Thymus
not to be regarded as a process of degeneration. The fact that the formation of colloid is an essential feature in the development of every
 
eorpusele is a strong argument that it is a form of secretion such as
 
occurs in its neighboring branchial derivative, the thyroid. The "fact
 
that the corpuscles differentiate in an apparently uniform syiicytium is
 
further evidence against a theory of degeneration. Since the lymphocyte-forming function of the thymus is probably secondary, it is not
 
56 The Development of the Thymus
 
  
unreasonable to suppose that its primitive function was the formation
+
unreasonable to suppose that its primitive function was the formation of a colloid secretion such as occurs in the thyroid, and that ‘the corpuscles are abortive expressions of this primitive function.’
of a colloid secretion such as occurs in the thyroid, and that ‘the corpuscles
 
are abortive expressions of this primitive function.’
 
  
 
GIANT CELLS.
 
GIANT CELLS.
  
Polykaryocytes may often be seen in the medulla. These bodies develop from the syncytium of the medulla. They are first noticeable as
+
Polykaryocytes may often be seen in the medulla. These bodies develop from the syncytium of the medulla. They are first noticeable as groups of small spherical nuclei in a solid area of the syncytium. These nuclei stain with medium intensity and are all very similar in size.and color. The area containing this group of nuclei becomes a well-defined node of the reticulum and persists as such. 'A polykaryocyte is, therefore, a large node of the reticulum containing a number of small nuclei very similar in appearance. These cells often occur in groups. They are entirely distinct from the corpuscles. They are evidently similar to the polykaryocytes found in bone marrow and other lymphoid tissues.
groups of small spherical nuclei in a solid area of the syncytium. These
 
nuclei stain with medium intensity and are all very similar in size.and
 
color. The area containing this group of nuclei becomes a well-defined
 
node of the reticulum and persists as such. 'A polykaryocyte is, therefore, a large node of the reticulum containing a number of small nuclei
 
very similar in appearance. These cells often occur in groups. They
 
are entirely distinct from the corpuscles. They are evidently similar
 
to the polykaryocytes found in bone marrow and other lymphoid tissues.
 
  
 
SUMMARY.
 
SUMMARY.
  
The following is a resume of the development of the thymus in the
+
The following is a resume of the development of the thymus in the P18 3
P18 3
 
  
 
The thymus of the pig is probably developed entirely from the endoderm of the third gill pouch.
 
The thymus of the pig is probably developed entirely from the endoderm of the third gill pouch.
  
By a gradual process of vacuolization and liquefaction of the cytoplasm, the epithelial syncytium of the thymic anlage is converted into a
+
By a gradual process of vacuolization and liquefaction of the cytoplasm, the epithelial syncytium of the thymic anlage is converted into a cellular reticulum.
cellular reticulum.
 
  
From the first appyarance of vacuolization, three types of nuclei are
+
From the first appyarance of vacuolization, three types of nuclei are present: large pale nuclei; small dark nuclei (lymphoblasts), and large dark intermediate forms.
present: large pale nuclei; small dark nuclei (lymphoblasts), and large
 
dark intermediate forms.
 
  
The lymphobla.sts gradually break loose from the cellular reticulum,
+
The lymphobla.sts gradually break loose from the cellular reticulum, moving into its spaces and forming lymphocytes. Mitoses are most numerous at the period of the most rapid formation of lymphocytes. The medulla continues to form lymphocytes at least as late as birth.
moving into its spaces and forming lymphocytes. Mitoses are most numerous at the period of the most rapid formation of lymphocytes. The
 
medulla continues to form lymphocytes at least as late as birth.
 
  
Lymphocytes appear in the connective tissue around the thymus
+
Lymphocytes appear in the connective tissue around the thymus shortly after they are formed; and lymphoblasts, which are distinguishable from lymphocytes only by being embedded in the syncytium, are present in the thymus a long period before lymphocytes are found anywhere in the neighborhood of the thymus.
shortly after they are formed; and lymphoblasts, which are distinguishable from lymphocytes only by being embedded in the syncytium, are
 
present in the thymus a long period before lymphocytes are found anywhere in the neighborhood of the thymus.
 
  
The cellular reticulum of the earlier stages persists in a modified form
+
The cellular reticulum of the earlier stages persists in a modified form as the reticulum of both cortex and medulla. It retains more cytoplasm
as the reticulum of both cortex and medulla. It retains more cytoplasm
 
  
‘Ver Eecke (28) believes that the corpuscles in amphibians are of a.
+
‘Ver Eecke (28) believes that the corpuscles in amphibians are of a. glandular nature. E. T. Bell 5'?’
glandular nature.
 
E. T. Bell 5'?’
 
  
 
in the medulla. Practically all the reticulum of both cortex and medulla., as well as the lymphocytes, are, therefore, of epithelial origin.
 
in the medulla. Practically all the reticulum of both cortex and medulla., as well as the lymphocytes, are, therefore, of epithelial origin.
  
The corpuscles of Hassall develop from the syncytium and are, there-.
+
The corpuscles of Hassall develop from the syncytium and are, there-. fore, epithelial in origin. They are, however, not to be considered as remnants of the original epithelial anlage.’
fore, epithelial in origin. They are, however, not to be considered as
 
remnants of the original epithelial anlage.’
 
  
In development various types of corpuscles are distinguished. The
+
In development various types of corpuscles are distinguished. The ordinary type of concentric corpuscles first appears as an enlarged clear nucleus around which colloid is being formed. Before or during the formation of colloid, the cytoplasm increases in quantity, filling the spaces of the reticulum and producing a centrifugal pressure which shapes the newly—formed colloid into concentric lamellae and flattens the neighboring nuclei, making them concave toward the center. The central nuclei usually become obliterated.
ordinary type of concentric corpuscles first appears as an enlarged clear
 
nucleus around which colloid is being formed. Before or during the
 
formation of colloid, the cytoplasm increases in quantity, filling the
 
spaces of the reticulum and producing a centrifugal pressure which
 
shapes the newly—formed colloid into concentric lamellae and flattens the
 
neighboring nuclei, making them concave toward the center. The central nuclei usually become obliterated.
 
  
The epithelioid type is distinguished by its resemblance to large epithelial cells, this appearance being due to the formation of colloid
+
The epithelioid type is distinguished by its resemblance to large epithelial cells, this appearance being due to the formation of colloid lamellae around large areas of clear cytoplasm. The central part of the corpuscle usually remains unchanged until after some of the colloid lamellae are formed.
lamellae around large areas of clear cytoplasm. The central part of the
 
corpuscle usually remains unchanged until after some of the colloid
 
lamellae are formed.
 
  
The cystic type differs from the ordinary type only in that the central
+
The cystic type differs from the ordinary type only in that the central part undergoes vacuolization instead of colloid transformation. Those with concentric vacuoles may simulate blood-vessels containing nucleated red cells. Corpuscles never contain erythrocytes; neither can they be injected at any stage of development. Serial sections also show that there is no connection to blood-vessels at any stage.
part undergoes vacuolization instead of colloid transformation. Those
 
with concentric vacuoles may simulate blood-vessels containing nucleated
 
red cells. Corpuscles never contain erythrocytes; neither can they be
 
injected at any stage of development. Serial sections also show that
 
there is no connection to blood-vessels at any stage.
 
  
Compound concentric corpuscles are formed by the union of two or
+
Compound concentric corpuscles are formed by the union of two or more simple concentric corpuscles during development.
more simple concentric corpuscles during development.
 
  
Irregular corpuscles are not concentric in arrangement, and are
+
Irregular corpuscles are not concentric in arrangement, and are formed in the syncytium in an irregular manner. In the compact type of irregular corpuscles, concentric areas may form.
formed in the syncytium in an irregular manner. In the compact type
 
of irregular corpuscles, concentric areas may form.
 
  
The formation of colloid is an essential feature in the development of
+
The formation of colloid is an essential feature in the development of every corpuscle, and is not to be considered as a process of degeneration.
every corpuscle, and is not to be considered as a process of degeneration.
 
  
Since the conclusion of my work and after my manuscript was given to the
+
Since the conclusion of my work and after my manuscript was given to the publishers, two articles dealing with the thymus have appeared.
publishers, two articles dealing with the thymus have appeared.
 
  
Ph. Stohr (Ueber die Thymus, Sitzungsberichte der phys.-med. Gesellschaft
+
Ph. Stohr (Ueber die Thymus, Sitzungsberichte der phys.-med. Gesellschaft zu Wiirzburg, June 8, 1905) believes that the thymus first epithelial in nature becomes converted entirely into small cells of lymphoid appearance. Later the large reticulum cells are formed from these by enlargement. The corpuscles of I-Iassall.are formed by the massing together and enlargement of these lymphoid—like cells. The small round cells of the gland are epithelial in origin but are to be regarded not as lymphocytes but as epithelial cells. The thymus is not a. source of lymphocytes. 58 The Development of the Thymus
zu Wiirzburg, June 8, 1905) believes that the thymus first epithelial in nature
 
becomes converted entirely into small cells of lymphoid appearance. Later
 
the large reticulum cells are formed from these by enlargement. The corpuscles of I-Iassall.are formed by the massing together and enlargement of
 
these lymphoid—like cells. The small round cells of the gland are epithelial
 
in origin but are to be regarded not as lymphocytes but as epithelial cells.
 
The thymus is not a. source of lymphocytes.
 
58 The Development of the Thymus
 
  
The author apparently believes that none of the small round cells leave
+
The author apparently believes that none of the small round cells leave the gland though he admits that lymphocytes enter. But as mentioned above the zone of connective tissue immediately around the head at 7 cm. may contain even more lymphocytes than are present inside the gland at that time If these are all entering the gland then it is probable that most of the small round cells are really lymphocytes. This conception then does not simplify the problem but is only a theoretical compromise between the two views as to the origin of the lymphocytes.
the gland though he admits that lymphocytes enter. But as mentioned above
 
the zone of connective tissue immediately around the head at 7 cm. may
 
contain even more lymphocytes than are present inside the gland at that time
 
If these are all entering the gland then it is probable that most of the small
 
round cells are really lymphocytes. This conception then does not simplify
 
the problem but is only a theoretical compromise between the two views as
 
to the origin of the lymphocytes.
 
  
J. Aug. Hammar (Zur Histogenese und Involution der Thymusdriise, Anat.
+
J. Aug. Hammar (Zur Histogenese und Involution der Thymusdriise, Anat. Anz. Bd. XXVII, June 17, 1905) regards the reticulum as formed from the epithelial anlage but thinks the evidence at hand insufiicient to decide the question as to the origin of the lymphocytes. He finds lymphocytes outside the thymus in many animals (man, cat, chick, frog) before any are present inside the gland. The corpuscles of Hassall develop from the epithelial reticulum and undergo hyaline (colloid?) degeneration.
Anz. Bd. XXVII, June 17, 1905) regards the reticulum as formed from the
 
epithelial anlage but thinks the evidence at hand insufiicient to decide the
 
question as to the origin of the lymphocytes. He finds lymphocytes outside
 
the thymus in many animals (man, cat, chick, frog) before any are present
 
inside the gland. The corpuscles of Hassall develop from the epithelial
 
reticulum and undergo hyaline (colloid?) degeneration.
 
  
My description of the formation of the corpuscles of Hassall differs essentially from Hammar’s, in that I believe the formation of the corpuscle consists
+
My description of the formation of the corpuscles of Hassall differs essentially from Hammar’s, in that I believe the formation of the corpuscle consists in the expansion of the cytoplasm of the syncytium and its conversion into colloid. Hammar did not recognize “colloid in formation,” though he speaks of the coarse fibrillar structure of the protoplasm. He did not describe such corpuscles as are shown in Fig. 7, Plate II.
in the expansion of the cytoplasm of the syncytium and its conversion into
 
colloid. Hammar did not recognize “colloid in formation,” though he speaks
 
of the coarse fibrillar structure of the protoplasm. He did not describe such
 
corpuscles as are shown in Fig. 7, Plate II.
 
  
The considerations presented above in favor of the epithelial origin of the
+
The considerations presented above in favor of the epithelial origin of the lymphocytes seem to me much stronger than those given by Hammar. His statements as to the presence of lymphocytes around the thymus before they are present inside are to be taken with some reservation inasmuch as he mentions small round cells separate from the syncytium earlier, but regards them as degenerating epithelial cells (S. 65). His figure from the human foetus (Fig. 18, S. 66) does not seem to be strong support for his statement. Certainly many lymphocytes are present in the pig thymus when the reticulum is broken up as much as shown in the figure referred to. It is also to be borne in mind that the different parts of the thymus undergo the lymphoid transformation at different times and that a single section may therefore be
lymphocytes seem to me much stronger than those given by Hammar. His
 
statements as to the presence of lymphocytes around the thymus before they
 
are present inside are to be taken with some reservation inasmuch as he
 
mentions small round cells separate from the syncytium earlier, but regards
 
them as degenerating epithelial cells (S. 65). His figure from the human
 
foetus (Fig. 18, S. 66) does not seem to be strong support for his statement.
 
Certainly many lymphocytes are present in the pig thymus when the reticulum is broken up as much as shown in the figure referred to. It is also to
 
be borne in mind that the different parts of the thymus undergo the lymphoid
 
transformation at different times and that a single section may therefore be
 
  
misleading.
+
misleading. LITERATURE.
LITERATURE.
 
  
la. AFANASSIEW, B.—Ueber die concentrischen Korper der Thymus. Archiv
+
la. AFANASSIEW, B.—Ueber die concentrischen Korper der Thymus. Archiv f. mlkr. Anat., Bd. XIV, 1877.
f. mlkr. Anat., Bd. XIV, 1877.
 
  
Weitere Untersuchungen iiber den Bau und die Entwickelung
+
Weitere Untersuchungen iiber den Bau und die Entwickelung der Thymus und der Wintcrschlafdriise der Siiugethiere. Archiv f. mikr. Anat., Bd. XIV, 1877.
der Thymus und der Wintcrschlafdriise der Siiugethiere. Archiv f.
 
mikr. Anat., Bd. XIV, 1877.
 
  
 
2. AMMANN, A.—Beitrage zur Anatomie der Thymusdriise. Basel, 1882.
 
2. AMMANN, A.—Beitrage zur Anatomie der Thymusdriise. Basel, 1882.
  
3a. BEARD.—The development and probable function of the thymus. Anat.
+
3a. BEARD.—The development and probable function of the thymus. Anat. Anz., Bd. IX, 1894.
Anz., Bd. IX, 1894.
 
  
 
The true function of the thymus. Lancet, 1899.
 
The true function of the thymus. Lancet, 1899.
  
4. BORN, G.—Ueber die Derivate der embryonalen Schlundbogen und
+
4. BORN, G.—Ueber die Derivate der embryonalen Schlundbogen und Schlundspalten bel Saugethleren. Archiv f. mikr. Anat., Bd. XXII, 1883.
Schlundspalten bel Saugethleren. Archiv f. mikr. Anat., Bd. XXII,
 
1883.
 
  
5. CORNIL et RANVIER.—MaI1l181 d’histologie pathologique. Paris, 1869, p.
+
5. CORNIL et RANVIER.—MaI1l181 d’histologie pathologique. Paris, 1869, p. 135 (cited from Ammann).
135 (cited from Ammann).
 
  
lb.
+
lb. 10.
10.
 
  
 
11.
 
11.
  
129..
+
129.. 1.2b. 13.
1.2b.
 
13.
 
  
 
14.
 
14.
Line 1,324: Line 437:
 
17d.
 
17d.
  
18.
+
18. 19.
19.
 
  
 
20.
 
20.
Line 1,337: Line 449:
 
E. T. Bell 59
 
E. T. Bell 59
  
EcKER.—Art. “ Blutgeffissdriisen,” Wa.gner’s Handw. der Phys., III (cited
+
EcKER.—Art. “ Blutgeffissdriisen,” Wa.gner’s Handw. der Phys., III (cited from Ammann). FRIEDLEBEN, A.—Die Physiol. der Thymusdriise. Frankfurt, 1858.
from Ammann).
 
FRIEDLEBEN, A.—Die Physiol. der Thymusdriise. Frankfurt, 1858.
 
  
G-ULLANl).—The Development of adenoid tissue with special reference
+
G-ULLANl).—The Development of adenoid tissue with special reference to the tonsil and thymus. Laboratory Reports issued by the Royal College Phys., Edinburgh, Vol. III, 1891.
to the tonsil and thymus. Laboratory Reports issued by the Royal
 
College Phys., Edinburgh, Vol. III, 1891.
 
  
GtiNzsURe.—Ueber die geschichteten Kiirper der Thymus. Zeitschr. f.
+
GtiNzsURe.—Ueber die geschichteten Kiirper der Thymus. Zeitschr. f. klin. Med., Bd. VI, 1857, S. 456 (cited from Henle und Meissner. Berlcht iiber die Fortschritte der Anat. u. Physiol.).
klin. Med., Bd. VI, 1857, S. 456 (cited from Henle und Meissner.
 
Berlcht iiber die Fortschritte der Anat. u. Physiol.).
 
  
HASSALL.—-The microscropical anatomy of the human body in health and
+
HASSALL.—-The microscropical anatomy of the human body in health and disease. London, 1846 (cited from Ammann).
disease. London, 1846 (cited from Ammann).
 
  
 
HERMANN et ToUnNEUx.—-Article thymus, Dict. encycl. des Sciences Médicales. Troisieme Série, 17, 1887.
 
HERMANN et ToUnNEUx.—-Article thymus, Dict. encycl. des Sciences Médicales. Troisieme Série, 17, 1887.
Line 1,358: Line 463:
 
Anatomie menschlicher Embryonen. Leipzig, 1880, S. 56.
 
Anatomie menschlicher Embryonen. Leipzig, 1880, S. 56.
  
JACKSON, 0. M.—-Zur I-Iistologie und Histogenese des Knochenmarkes.
+
JACKSON, 0. M.—-Zur I-Iistologie und Histogenese des Knochenmarkes. Archiv f. Anat. und Physio1., Anat. Abth., 1904.
Archiv f. Anat. und Physio1., Anat. Abth., 1904.
 
  
KASTSCI-IENKO.—-Das Schicksal der embryonalen Schlundspalten bei
+
KASTSCI-IENKO.—-Das Schicksal der embryonalen Schlundspalten bei Siugethieren. Archiv f. mikr. Anat., Bd. XXX, 1887.
Siugethieren. Archiv f. mikr. Anat., Bd. XXX, 1887.
 
  
KL:«:1N.—-—Neuere Arbeiten iiber die Glandula Thymus. Centralbl. f. allg.
+
KL:«:1N.—-—Neuere Arbeiten iiber die Glandula Thymus. Centralbl. f. allg. Pathol. u. pathol. Anat., 1898.
Pathol. u. pathol. Anat., 1898.
 
  
LANGERKANS und SAvEL1r.w.—Beitr§.ge zur Physiologie der Brustdriise.
+
LANGERKANS und SAvEL1r.w.—Beitr§.ge zur Physiologie der Brustdriise. Virchow’s Archiv,»Bd. 134, 1893.
Virchow’s Archiv,»Bd. 134, 1893.
 
  
MAUREn.—Schi1ddriise und Thymus der Teleostier. Morph. Jahrb., Bd.
+
MAUREn.—Schi1ddriise und Thymus der Teleostier. Morph. Jahrb., Bd. XI, 1886.
XI, 1886.
 
  
 
Schilddriise, Thymus, und Kiemenreste bei Amphibien. Morph.
 
Schilddriise, Thymus, und Kiemenreste bei Amphibien. Morph.
Line 1,377: Line 477:
 
Jahrb., Bd. XIII, 1888. 4
 
Jahrb., Bd. XIII, 1888. 4
  
Schilddriise, Thymus, und andere Schlundspaltenderlvate bei der
+
Schilddriise, Thymus, und andere Schlundspaltenderlvate bei der Eidechse. Morph. Jahrb., Bd. XXVII, 1899.
Eidechse. Morph. Jahrb., Bd. XXVII, 1899.
 
  
——— In I-Iertwig’s Handbuch der Entwickelungslehre der Wirbelthiere,
+
——— In I-Iertwig’s Handbuch der Entwickelungslehre der Wirbelthiere, Lief. 6-8, S. 131 111., 1902.
Lief. 6-8, S. 131 111., 1902.
 
  
 
MoNoUmI.——-Sulla glandula timo. P,arma, 1885 (cited from Prenant-).
 
MoNoUmI.——-Sulla glandula timo. P,arma, 1885 (cited from Prenant-).
  
NUSBAUM, J., und MACHOWSKI.-—Die Bildung der concentrischen K6rperchen und die phagocytotischen Vorgiinge bei der Involution der
+
NUSBAUM, J., und MACHOWSKI.-—Die Bildung der concentrischen K6rperchen und die phagocytotischen Vorgiinge bei der Involution der Amphlbienthymus, etc. Anat. Anz., Bd. XXI, 1902.
Amphlbienthymus, etc. Anat. Anz., Bd. XXI, 1902.
 
  
NUSBAUM, J., und PRYMAK, T.-— Zur Entwickelungsgeschichte der lymphoiden Elemente der Thymu bei den Knochenflschen. Anat. Anz.,
+
NUSBAUM, J., und PRYMAK, T.-— Zur Entwickelungsgeschichte der lymphoiden Elemente der Thymu bei den Knochenflschen. Anat. Anz., Bd. XIX, 1901.
Bd. XIX, 1901.
 
  
PAULITzKY.—DiSqulS. de stratis glandulae thymi corpusculi. Habi1itationsschr., Halis, 1863 (cited from Henle und 'Meissner’s Bericht fiber
+
PAULITzKY.—DiSqulS. de stratis glandulae thymi corpusculi. Habi1itationsschr., Halis, 1863 (cited from Henle und 'Meissner’s Bericht fiber die Fottschritte der Anat. und Physiol.).
die Fottschritte der Anat. und Physiol.).
 
  
P3ENAN:r.—Développement organique et histologique du thymus, de la
+
P3ENAN:r.—Développement organique et histologique du thymus, de la glande thyroide, et de la glande carotidienne. La Cellule, Tome X, 1894.
glande thyroide, et de la glande carotidienne. La Cellule, Tome X,
 
1894.
 
  
PRYMAK, T.—Beitria'.ge zur Kenntnis des feineren Baues und der Involution der Thymusdriise bei den Teleostieren. Anat. Anz., Bd. XXI,
+
PRYMAK, T.—Beitria'.ge zur Kenntnis des feineren Baues und der Involution der Thymusdriise bei den Teleostieren. Anat. Anz., Bd. XXI, 1902. 60 The Development of the Thymus
1902.
 
60 The Development of the Thymus
 
  
24. SCHAFFER, J.—Ueber den feineren Bau der Thymus und deren Beziehung
+
24. SCHAFFER, J.—Ueber den feineren Bau der Thymus und deren Beziehung zur Blutbildung. Sitzungsber. d. K. Acad. d. Wissensch. Math.naturw. Kl. Wien., Bd. CII, Abt. III, 1893.
zur Blutbildung. Sitzungsber. d. K. Acad. d. Wissensch. Math.naturw. Kl. Wien., Bd. CII, Abt. III, 1893.
 
  
25. SCHEDEL, J.——Zellvermehrung in der Thymusdriise. Archiv f. mikr.
+
25. SCHEDEL, J.——Zellvermehrung in der Thymusdriise. Archiv f. mikr. Anat., Bd. XXIV.
Anat., Bd. XXIV.
 
  
26. STIEDA, L.—Untersuchungen iiber die Entwickelung der Glandula_Thymus, Glandula. Thyroidea, und Glandula Carotica. Leipzig, 1881
+
26. STIEDA, L.—Untersuchungen iiber die Entwickelung der Glandula_Thymus, Glandula. Thyroidea, und Glandula Carotica. Leipzig, 1881 (cited from Hermann et Tourneux).
(cited from Hermann et Tourneux).
 
  
27. SUL'rAN.—Beitrag zur Involution der Thymusdriise. Virchow’s Archiv,
+
27. SUL'rAN.—Beitrag zur Involution der Thymusdriise. Virchow’s Archiv, Bd. 144, 1896.
Bd. 144, 1896.
 
  
28. VER EEcK1~:.—Structure et modifications fonctionelles du thymus de la.
+
28. VER EEcK1~:.—Structure et modifications fonctionelles du thymus de la. grenouille. Bulletin de l’Académie royale de Medicine de Belgique,
grenouille. Bulletin de l’Académie royale de Medicine de Belgique,
 
  
1899.
+
1899. 29. Vmcnow, R.—Kritisches fiber den oberschlesischen Typhus. Archiv,Bd. 3, 1851, S. 222.
29. Vmcnow, R.—Kritisches fiber den oberschlesischen Typhus. Archiv,Bd.
 
3, 1851, S. 222.
 
  
30. VVALLISCI-I.—-Z111‘ Bedeutung der Ha.ssail’schen Kdrperchen. Archiv f.
+
30. VVALLISCI-I.—-Z111‘ Bedeutung der Ha.ssail’schen Kdrperchen. Archiv f. mikr. Anat., 1903.
mikr. Anat., 1903.
 
  
31. WATNEY.—The minute anatomy of the thymus. Philos. Transact. of the
+
31. WATNEY.—The minute anatomy of the thymus. Philos. Transact. of the Royal Society of London, Vol. 173, Part III, 1883 (cited from Pre nant).
Royal Society of London, Vol. 173, Part III, 1883 (cited from Pre
 
nant).
 
  
 
EXPLANATION OF PLATES.
 
EXPLANATION OF PLATES.
  
All the figures were drawn with Leitz obj. 1/12, 0c. 4, and camera lucida.
+
All the figures were drawn with Leitz obj. 1/12, 0c. 4, and camera lucida. The magnification after the reduction of the plates is about 1060 diameters. All drawings were made from transverse sections of the mid-cervical segment of ‘the thymus unless they are otherwise indicated.
The magnification after the reduction of the plates is about 1060 diameters.
 
All drawings were made from transverse sections of the mid-cervical segment of ‘the thymus unless they are otherwise indicated.
 
  
The following abbreviations designate the structures indicated in all the
+
The following abbreviations designate the structures indicated in all the figures:
figures:
 
  
c f—colloid in formation. 11; n——large pale nucleus.
+
c f—colloid in formation. 11; n——large pale nucleus. cl-calcareous deposit. m—nucleus in mitosis. c s——solid colloid. md—beginning of medulla. csd—solid colloid that stains n—nucleus. deeply. 0 c—old colloid. e—erythrocyte. pm—protoplasmic mass. end—endothelial nucleus. sf—-fibril in syncytium. I-—-lymphocyte. " ss~—-space in syncytium. lb—iymphoblast. 12-—va.cuole.
cl-calcareous deposit. m—nucleus in mitosis.
 
c s——solid colloid. md—beginning of medulla.
 
csd—solid colloid that stains n—nucleus.
 
deeply. 0 c—old colloid.
 
e—erythrocyte. pm—protoplasmic mass.
 
end—endothelial nucleus. sf—-fibril in syncytium.
 
I-—-lymphocyte. " ss~—-space in syncytium.
 
lb—iymphoblast. 12-—va.cuole.
 
  
 
ldn——large dark nucleus.
 
ldn——large dark nucleus.
Line 1,451: Line 521:
 
PLATE I.
 
PLATE I.
  
FIG. 1. From a 3.7~cm. pig embryo. Stained with iron-haematoxylin and
+
FIG. 1. From a 3.7~cm. pig embryo. Stained with iron-haematoxylin and Congo red. Vacuolization of the cytoplasm and differentiation of the nuclei have begun.
Congo red. Vacuolization of the cytoplasm and differentiation of the nuclei
 
have begun.
 
  
FIG. 2. From the thoracic segment of a 4.5-cm. pig embryo. Stained with
+
FIG. 2. From the thoracic segment of a 4.5-cm. pig embryo. Stained with iron-haematoxylin and Congo red. A cellular reticulum is now formed. Large pale nuclei, lymphoblats, and the large dark intermediate forms are present.
iron-haematoxylin and Congo red. A cellular reticulum is now formed. Large
 
pale nuclei, lymphoblats, and the large dark intermediate forms are present.
 
  
 
FIG. 3. Epithelioid type of concentric corpuscle. From a 16-cm. pig embryo. Stained with haematoxylin and Congo red. Colloid lamellae (c s d)
 
FIG. 3. Epithelioid type of concentric corpuscle. From a 16-cm. pig embryo. Stained with haematoxylin and Congo red. Colloid lamellae (c s d)
  
  
separate large areas of clear cytoplasm, causing the appearance of large
+
separate large areas of clear cytoplasm, causing the appearance of large epithelial cells. Colloid is being formed between the lamellee and around several nuclei.
epithelial cells. Colloid is being formed between the lamellee and around
 
several nuclei.
 
  
FIG. 4. From a 8.5-cm. pig embryo. Stained with iron-haamatoxylin (not
+
FIG. 4. From a 8.5-cm. pig embryo. Stained with iron-haamatoxylin (not decolorized). The medulla has appeared. Lymphocytes are present between the epithelial cords.
decolorized). The medulla has appeared. Lymphocytes are present between
 
the epithelial cords.
 
  
FIG. 5. From a 7-cm. pig embryo. Stained with iron-haematoxylin and
+
FIG. 5. From a 7-cm. pig embryo. Stained with iron-haematoxylin and Congo red. A few lymphocytes have been formed. In the cellular reticulum are large pale nuclei, lymphoblasts, and large dark intermediate nuclei. The nuclei in mitosis are very compact.
Congo red. A few lymphocytes have been formed. In the cellular reticulum
 
are large pale nuclei, lymphoblasts, and large dark intermediate nuclei. The
 
nuclei in mitosis are very compact.
 
  
FIG. 6. From the medulla of a 24-cm. pig embryo. Stained with Jackson’s
+
FIG. 6. From the medulla of a 24-cm. pig embryo. Stained with Jackson’s modification of Mallory’s method (ref. in text). Many fibrillae are seen in the syncytium.
modification of Mallory’s method (ref. in text). Many fibrillae are seen in
 
the syncytium.
 
  
 
PLATE II.
 
PLATE II.
  
FIG. 7. Ordinary type of simple concentric corpuscle. From a 16.5-cm.
+
FIG. 7. Ordinary type of simple concentric corpuscle. From a 16.5-cm. pig embryo. Stained with haematoxylin and Congo red. The corpuscle is well advanced in development. Concentric lamellze of colloid have been formed. The cytoplasm between the lamellae is in an early stage of colloid transformation. Colloid fibers cut transversely appear as dots. The nuclei are becoming flattened by the pressure of expansion. The central mass stains irregularly and all traces of the nuclei in that region are gone.
pig embryo. Stained with haematoxylin and Congo red. The corpuscle is
 
well advanced in development. Concentric lamellze of colloid have been
 
formed. The cytoplasm between the lamellae is in an early stage of colloid
 
transformation. Colloid fibers cut transversely appear as dots. The nuclei
 
are becoming flattened by the pressure of expansion. The central mass stains
 
irregularly and all traces of the nuclei in that region are gone.
 
  
FIG. 8. Two cystic concentric corpuscles. From a 16-cm. pig embryo.
+
FIG. 8. Two cystic concentric corpuscles. From a 16-cm. pig embryo. Stained with iron-haematoxylin and Congo red. On the left, a nucleated mass of protoplasm has been separated off by the formation of a vacuole annular in section. This might be mistaken for a blood-vessel containing a nucleated red cell. In this central protoplasmic mass the nucleus is shrunken and the cytoplasm vacuolated. In the small corpuscle on the right, two large vacuoles have formed.
Stained with iron-haematoxylin and Congo red. On the left, a nucleated
 
mass of protoplasm has been separated off by the formation of a vacuole
 
annular in section. This might be mistaken for a blood-vessel containing a
 
nucleated red cell. In this central protoplasmic mass the nucleus is shrunken
 
and the cytoplasm vacuolated. In the small corpuscle on the right, two large
 
vacuoles have formed.
 
  
FIG. 9. Cystic concentric corpuscle. From a 14-cm. pig embryo. Stained
+
FIG. 9. Cystic concentric corpuscle. From a 14-cm. pig embryo. Stained with haematoxylin and Congo red. The central protoplasmic mass is pale and shrunken. The small circular body in it probably is the remains of the nucleus. Colloid lamellae are forming. Colloid fibers cut transversely appear as dots.
with haematoxylin and Congo red. The central protoplasmic mass is pale
 
and shrunken. The small circular body in it probably is the remains of
 
the nucleus. Colloid lamellae are forming. Colloid fibers cut transversely
 
appear as dots.
 
  
FIG. 10. Cystic concentric corpuscle. From a 10.5-cm. pig embryo.
+
FIG. 10. Cystic concentric corpuscle. From a 10.5-cm. pig embryo. Stained with haamatoxylin and Congo red. The center contains no colloid and seems to be softening. The nucleus is shrunken.
Stained with haamatoxylin and Congo red. The center contains no colloid
 
and seems to be softening. The nucleus is shrunken.
 
  
FIG. 11. Ordinary concentric corpuscle in a very early stage. From a
+
FIG. 11. Ordinary concentric corpuscle in a very early stage. From a 10.5-cm. pig embryo. Stained with iron-haamatoxylin and Congo red. The nucleus is enlarged and colloid is forming around it. A few colloid fibers may be seen in the cytoplasm for some distance from the central nucleus.
10.5-cm. pig embryo. Stained with iron-haamatoxylin and Congo red. The
 
nucleus is enlarged and colloid is forming around it. A few colloid fibers
 
may be seen in the cytoplasm for some distance from the central nucleus.
 
  
FIG. 12. Ordinary concentric corpuscle. Several nuclei are involved.
+
FIG. 12. Ordinary concentric corpuscle. Several nuclei are involved. From a 10.5-cm. pig embryo. Stained with hamatoxylln and Congo red. The deeply-staining colloid has completely obliterated the central nucleus (in the region 0 c), and nearly obliterated another (w). some of the colloid now stains Dale (0 c).
From a 10.5-cm. pig embryo. Stained with hamatoxylln and Congo red. The
 
deeply-staining colloid has completely obliterated the central nucleus (in the
 
region 0 c), and nearly obliterated another (w). some of the colloid now
 
stains Dale (0 c).
 
  
Fm. 13. Ordinary concentric corpuscle. From a 10.5-cm. pig. Stained
+
Fm. 13. Ordinary concentric corpuscle. From a 10.5-cm. pig. Stained with haamatoxylin and Congo red. The central nucleus is being obliterated 62 The Development of the Thymus
with haamatoxylin and Congo red. The central nucleus is being obliterated
 
62 The Development of the Thymus
 
  
by the deeply-staining colloid. The neighboring nuclei are beginning to
+
by the deeply-staining colloid. The neighboring nuclei are beginning to show the effect of the centrifugal pressure.
show the effect of the centrifugal pressure.
 
  
FIG. 14. Ordinary concentric corpuscle in an early stage. From a 10.5-cm.
+
FIG. 14. Ordinary concentric corpuscle in an early stage. From a 10.5-cm. pig. Stained with haematoxylin and Congo red. A band of deeply-staining colloid has been formed. Just outside this is colloid in formation.
pig. Stained with haematoxylin and Congo red. A band of deeply-staining
 
colloid has been formed. Just outside this is colloid in formation.
 
  
FIG. 15. Two simple concentric corpuscles which would have formed a
+
FIG. 15. Two simple concentric corpuscles which would have formed a compound concentric corpuscle. From a 10.5-cm. pig. Stained with haematoxylin and Congo red. The left corpuscle is a little more advanced than Fig. 11. The right corpuscle shows a large area of deeply-staining colloid which has pressed the nucleus into a small irregular shape.
compound concentric corpuscle. From a 10.5-cm. pig. Stained with haematoxylin and Congo red. The left corpuscle is a little more advanced than
 
Fig. 11. The right corpuscle shows a large area of deeply-staining colloid
 
which has pressed the nucleus into a small irregular shape.
 
  
 
PLATE III.
 
PLATE III.
  
Fm. 16. Compact irregular corpuscle in an early stage. From a 14-cm.
+
Fm. 16. Compact irregular corpuscle in an early stage. From a 14-cm. pig embryo. Stained with haematoxylin and Congo red. The colloid is not yet solid. The nuclei are not essentially different from those of the adjacent syncytium.
pig embryo. Stained with haematoxylin and Congo red. The colloid is not
 
yet solid. The nuclei are not essentially different from those of the adjacent
 
syncytium.
 
 
 
FIG. 17. Ordinary concentric corpuscle. From a 12-cm. pig embryo.
 
Stained with haematoxylin and Congo red. There is a large, central, deeplystaining colloid mass in which calcareous deposits (cl) have been made.
 
The neighboring nuclei show the effects of the centrifugal pressure.
 
  
FIG. 18. Epithelioid concentric corpuscle in an early stage. From a. 10.5cm. pig embryo. Stained with haematoxylin and Congo red. The outer colloid lamella marks off a nucleated mass of cytoplasm resembling a large cell.
+
FIG. 17. Ordinary concentric corpuscle. From a 12-cm. pig embryo. Stained with haematoxylin and Congo red. There is a large, central, deeplystaining colloid mass in which calcareous deposits (cl) have been made. The neighboring nuclei show the effects of the centrifugal pressure.
The nucleus is undergoing the same changes as occur in the central nucleus
 
of an ordinary concentric corpuscle.
 
  
FIG. 19. Compound concentric corpuscle. From a 10.5-cm. pig embryo.
+
FIG. 18. Epithelioid concentric corpuscle in an early stage. From a. 10.5cm. pig embryo. Stained with haematoxylin and Congo red. The outer colloid lamella marks off a nucleated mass of cytoplasm resembling a large cell. The nucleus is undergoing the same changes as occur in the central nucleus of an ordinary concentric corpuscle.
Stained with haematoxylin and Congo red. This would have soon lost all
 
evidence of its compound nature.
 
  
FIG. 20. Epithelioid concentric corpuscle. From a 10.5-cm. pig embryo.
+
FIG. 19. Compound concentric corpuscle. From a 10.5-cm. pig embryo. Stained with haematoxylin and Congo red. This would have soon lost all evidence of its compound nature.
Stained with haematoxylin and Congo red. Some colloid is forming outside
 
the circular area. Solid deeply-staining colloid is forming.
 
  
FIG. 21. Ordinary concentric corpuscle, showing a variation from the
+
FIG. 20. Epithelioid concentric corpuscle. From a 10.5-cm. pig embryo. Stained with haematoxylin and Congo red. Some colloid is forming outside the circular area. Solid deeply-staining colloid is forming.
usual type. From a 16.5-cm. pig embryo. Stained with iron-haamatoxylin
 
and Congo red. The central nucleus is ‘reddish but not enlarged. No solid
 
colloid has been formed.
 
  
FIG. 22. Compound concentric corpuscle. From a 16.5-cm. pig embryo.
+
FIG. 21. Ordinary concentric corpuscle, showing a variation from the usual type. From a 16.5-cm. pig embryo. Stained with iron-haamatoxylin and Congo red. The central nucleus is ‘reddish but not enlarged. No solid colloid has been formed.
Three centers are present. The pale colloid in the upper part is probably
 
older than the deeply-staining variety. In the lower part of the figure, a
 
young corpuscle is shown.
 
FIG. 23. Compact irregular corpuscle. From a 16-cm. pig embryo. Stained
 
with haematoxylin and Congo red. Some of the colloid is solid. No definite
 
center is present but one is beginning to form (0 s). The nuclei are not
 
markedly different from those of the adjacent syncytium.
 
  
 +
FIG. 22. Compound concentric corpuscle. From a 16.5-cm. pig embryo. Three centers are present. The pale colloid in the upper part is probably older than the deeply-staining variety. In the lower part of the figure, a young corpuscle is shown. FIG. 23. Compact irregular corpuscle. From a 16-cm. pig embryo. Stained with haematoxylin and Congo red. Some of the colloid is solid. No definite center is present but one is beginning to form (0 s). The nuclei are not markedly different from those of the adjacent syncytium.
  
  

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Bell ET. The development of the thymus. (1905) Amer. J Anat. 5: 29-62.

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The Development of the Thymus

The Development Of The Thymus.

BY

E. T. Bell B. S., M. D.

Instructor in Anatomy, University of Missouri.

WITH 3 PLATES AND 5 TEXT FIGURES.

This paper is intended mainly as a contribution to our knowledge of


the histogenesis of the thymus in mammals. Special attention is given to the origin and development of the corpuscles of Hassall, since their mode of formation has never been satisfactorily described in mammals and their significance in all forms is in dispute. An attempt is also made to show in detail the changes that occur during the transformation of the thymus from the epithelial to the lymphoid condition.

This work was begun at the suggestion of Dr. D. D. Lewis at the University of Chicago. The greater part of it has been done at the University of Missouri. Special acknowledgments are due Dr. C. M. Jackson for valuable criticism and suggestions. I wish also to thank Mr. Charles H. Miller of the University of Chicago for his kindness in sending me material.

MATERIAL AND METHODS.

As material for the greater part of my work, I have used pig embryos from 8 mm. to full term (26 cm. to 30 cm.). These are especially suitable for such work since they may be procured’ in abundance from the large packing houses at almost any stage of development. For special purposes I have studied a few specimens from human foetuses, and from the cat, rat, and guinea pig. The smaller pigs used (8 mm. to 27 mm.‘) belong to the collection in the anatomical laboratory at the University of Missouri. These were stained in bulk with alum-cochineal and mounted in serial sections. In the later stages, which were prepared specially for this work, the ventral half of the cervical and anterior thoracic regions was usually cut out and embedded from pigs from 3 cm. to 8 cm. On specimens from 8 cm. to 30 cm., I dissected out the thymus and used such parts as were desired.

’ The crown-rump measurement is used in all cases.

AMERICAN JOURNAL or A:\'A'roMY.—Vor.. V. 30 The Development of the Thymus

All pig material was fixed in Zenker’s fluid, embedded in parafiin, and mounted in serial sections from 3 ,u to 10 ,u. thick. Except those of the young stages (8 mm. to 27 mm.) the sections were stained on the slide. Most of them were stained with haematoxylin or iron-haematoxylin and counterstained with Congo red. For special purposes many other stains were used.

To demonstrate the delicate protoplasmic threads of the syncytium during the later stages of the lymphoid transformation, I stained by the iron—haematoxylin method but omitted the final decolorization in ironalum. Protoplasm is stained deep black; nuclear structure is poorly shown, but the finest cytoplasmic processes may be seen.

For the demonstration of connective tissue fibrillae in the syncytium, I found the method recommended by Jackson (13, S. 39) most satisfactory.


TEXT FIG. 1. TEXT FIG. 2.

TEXT FIGURE 1. Cranial view of third gill pouch (thymic anlage); X 33; pig embryo, 11 mm.; ec, ectoderm: Z, lumen; n t, nodulus thymicus; ph, connection to pharynx; s p, sinus praacervicalis.

TEXT FIGURE 2. Ventral view of thymic anlage; X 33; pig embryo, 15 mm.; ec, ectoderm; Z, lumen; nt, nodulus thymicus; ph, connection to pharynx; s p, sinus praecervicalis.

To determine the relation of the blood-vessels to the corpuscles of Hassall, I put a young kitten under deep anaesthesia and injected a large quantity of a strong aqueous solution of Prussian blue into the aorta through the common carotid artery. The heart continues to beat even after an amount of fluid twice as great as the total volume of the blood has been injected. An injection made in this way is under a slightly increased blood pressure and easily reaches the finest capillaries. There is therefore a thorough injection with little danger of rupturing delicate blood-vessels.

ORGANOGENESIS.

My observations on this phase of development are not sufficiently complete to warrant a full discussion. A brief survey may however prepare the Way for a better understanding of the histogenesis. The text figures show the shape in outline of the third gill pouch and thymic anlage E. T. Bell 31

from 11 mm. to 27 mm. They are graphic reconstructions. Since this pouch is nearly all converted into thymus it may be regarded as the thymic anlage from a very early stage.

At 11 mm. (Text Figure 1), the pouch is a hollow epithelial tube directed from without ventrally and mesially. The lumen (Z) is large and communicates freely with the pharynx. On the dorso—lateral aspect of the pouch is a solid epithelial mass (n t) distinctly different in structure from the rest of the pouch. This is the nodulus thymicus (Kastschenko, 14) and will be referred to by that term. This structure has been described by Stieda (26), Prenant” (22), and others as the anlage of the carotid gland.

It was evidently mistaken by Minot 3 in a 12-mm. pig for the anlage of the entire thymus. It may be seen as early as the 8 mm. stage budding off from the cranio—lateral aspect of the pouch. Immediately behind the nodulus thymicus, but not connected to it at this stage is the inner blind extremity of the sinus praecervicalis (s p). These become fused at 12 mm. or 13 mm.

At 15 mm. (Text Figure 2) the thymic anlage is more elongated. It now projects ventrally and medianwards, its free end lying immediately caudad to the median thyroid anlage and just craniad to the pericardium. Its lumen (Z) is still in communication with the pharynx. The ‘sinus praecervicalis (s p) is drawn out, its lumen being smaller and longer. It is now fused to the outer two-thirds of the posterior aspect of the nodulus thymicus (n t).

At 18 mm. (Text Figure 3) the anlage is growing rapidly in a caudal direction and just entering the thoracic cavity. It is connected to the pharynx by a delicate epithelial cord. There is still a lumen in its caudal part. The sinus praacervicalis has lost its connection to the nodulus thymicus.‘ The outer part of its lumen has disappeared and it seems about to lose its connection with the ectoderm.

At 20 mm. (Text Figure 4) the thymus extends well into the thoracic cavity. Its thoracic segment (25 h) has increased considerably in size and is united to the gland of the opposite side.

The nodulus thymicus still forms the greater part of the head. The anlage has no connection with the pharynx or the epidermis. There is

’Prenant is said to have since abandoned this idea and accepted Kestschenko’s view. (v. Ebner in K'ci11iker’s Gewebelehre des Menchen. Aufl. 6, Bd. 3, 1, S. 340.)

’Laboratory Text of Embryology, p. 191; also p. 209 and Fig. 124.

‘ On the opposite side in this specimen, these structures were fused over a. very small area. 32 The Development of the Thymus

now an elongated mass of thymic tissue extending upwards behind the nodulus thymicus, and fused with it. Its upper pointed extremity curves outwards around the hypoglossal nerve. This is the “thymus superficialis ” (t s) of Kastschenko and is regarded by him as being formed from the sinus praecervicalis. Kast-schenko describes this elongated portion as being always separate from the rest of the head, being connected only by connective tissue. In my preparations it is clearly continuous with the rest of the anlage, and seems to have formed by growing out from it. The presence of a lumen in its lower end favors Kast—

/is

/E ’_}” \ EC 310 I

]

225

1512 /f

TEXT Frc. 3. TEXT FIG. 4.

Tnxr FIGURE 3. Ventral view of thymic anlage; X 33; pig embryo, 18 mm.; 60, ectoderm; Z, lumen; nt, nodulus thymicus; ph, connection to pharynx; s 12, sinus przecervicalis.

Tnxr FIGURE 4. Ventral view of thymic anlagef X 33; pig embryo, 20 mm.; 7‘, area fused with gland of opposite side; I, lumen; nt, nodulus thymicus; th, thoracic segment; ts, thymus superflcialis.

schenko’s view, for there is no lumen in the head at 18 mm. in my preparations. On the other hand the separation from the head at the 18 mm. stage favors the idea that the sinus praecervicalis degenerates. I have not studied a sufficient number of specimens at this transition stage to enable me to decide this point, though I believe the ectoderm takes no part in the formation of the thymic anlage. Kastschenko’s results are opposed by nearly all other students of this problem, but his work should not be discarded before the development of the thymus superficialis in the pig has been accurately determined.

At 27 mm. (Text Figure 5) the thymus is much longer and extends E. T. Bell 33

well down into the thoracic cavity in relation to the base of the heart where it has fused with the gland of the opposite side. Buds are now beginning to form through the greater part of its extent. Traces of the original lumen (Z) may still be seen in several places. The “thymus superficialis ” is bent around the twelfth nerve. A delicate cord of thymic

TEXT FIG. 5.

TEXT FIGURE 5. Ventral View of thymic anlage; X 33; pig embryo, 27 min; f, area fused with gland of opposite side; I, lumen; nt, nodulus thymicus; th, thoracic segment; ts, thymus superflclalis.

tissue fused with the back of the nodulus thymicus connects the thymus

superficialis to the rest of the head. From about 3 cm. until toward the end of foetal life the thymus shows the two constrictions, described by Prenant (22) (for the sheep) as the

intermediary and the cervico-thoracic cords. These cords connect three 3 34 The Development of the Thymus

enlargements which we may call the head, the mid-cervical segment, and the thoracic segment. I will consider each part separately. The thoracic segment develops rapidly, spreading out above and in front of the heart. The glands of the two sides fuse completely in this region. The lymphoid transformation is noticeable at 3.5 cm. and well advanced at 4.5 cm. The medulla begins to form at 8 cm. The cervico—thoracic cord is at first very narrow but soon thickens and joins the c_ord of the opposite side. At full term they form a sharp constriction, 3 mm. to 4 mm. wide and 5 mm. to 6 mm. long, situated at the superior aperture of the thorax, and connecting the mid-cervica.l and thoracic segments. The histological changes take place here later than in the enlargements.

The mid-cervical segment develops like the thoracic segment but somewhat more slowly. Budding, lymphoid transformation, and formation of the medulla all begin here a little later than in the head and thoracic segment. Its caudal end is slightly in advance of its cranial end. The intermediary cord is well marked at 4 cm. It -soon becomes very attenuated, having at 6 cm. in many places a diameter of only 15 p.. Prenant suggests that this drawing out of the gland is caused by the rapid growth of the neck. Later it increases in size and at full term is noticeable only as a very slight constriction between the head and the mid-cervical segment. The histological changes are much later here than in other parts of the gland.

The greater part of the head in the early stages is formed by the nodulus thymicus. This body grows slowly, attaining a diameter in crosssection of .5 mm. at 8 cm. Its cross-sectional area at 8 cm. is about onethird that of the rest of the head of the thymus. At this stage the nodulus thymicus is a rounded body lying on the inner aspect of the head in relation to the carotid artery. A small area of its outer surface is fused with the lymphoid tissue of the thymus. Its histological structure has been fully described by Prenant (22). From the earliest stages, it consists of cords of epithelial cells separated by blood capillaries. At 8 cm. the thymus superficialis (Kastschenko) is a large body lying craniad and dorsal to the rest of the head but connected to it at its caudal extremity.

I have no observations on the head of the thymus between 8 cm. and full term, having overlooked it in collecting my material. In a full term pig (30 cm.), the cervical part of the thymus is in two distinct parts. The postero-ventral part, representing part of the head, the intermediary cord, and the mid-cervical segment, is about 3 cm. long and 1 cm. wide. It extends from the upper border of the thyroid cartilage to the thorax, and with the corresponding part of the opposite gland encloses the E. '1‘. Bell 35

trachea, thyroid, and lower part of the larynx. A slight narrowing at the junction of the upper and middle thirds indicates the position of the intermediary cord. The antero-dorsal part, the thymus superficialis, is rounded in cross-section, tapering to a point behind. Its anterior end is about '7 mm. in diameter and’ loops around the twelfth nerve as in the earliest stages. A lobule hangs over ventral to the nerve, a thin cord being dorsal to it. The posterior end of the thymus superficialis extends to the cricoid cartilage dorsal to the postero—ventral part of the gland. It is united to this part of the gland by a very delicate band of thymic tissue. I did not find the nodulus thymicus at full term. It has either moved away from the head or degenerated.

The duct of the thymus is the lumen of the third gill pouch. A glance at the Text Figures will show its development. It is broken up into segments and finally obliterated. I could find no traces of it at 3.7 cm. or later. On the theory of the exclusively endodermal origin of the thymus, I cannot explain the absence of a lumen in the head at 18 mm. and its presence at 20 mm. and 27 mm. unless it be due to individual variation in different specimens.

It appears from the foregoing that in the pig the exclusively endodermal origin of the thymus from the third gill pouch is probable, but a slight participation by the ectoderm has not been satisfactorily excluded. Kastschenko’s conclusions, however, as to the ectodermal origin of the thymus are unwarranted by his recorded observations. Prenant, after his careful work (on the sheep), was not sure that a small mass of ectoderm did not enter into the formation of the head. Practically all other investigators of this problem maintain that the ectoderm takes no part in the formation of the thymus. The epithelial body (nodulus thymieus) developing in connection with the head of the thymus from the third gill pouch does not form the carotid gland. Kastschenko’s description of the origin of the carotid gland in mammals from the adventitia of the internal carotid is now accepted by the majority of anatomists, and it therefore has nothing to do with the thymus in origin.

THE HISTOLOGY or THE FULLY-FORMED THYMUS.

Before taking up the histogenesis, I shall briefly consider the histology of the gland as shown in a 24-em. embryo. At this stage the gland may be regarded as fully formed. As is well known, the thymic lobule consists of a cortical and a medullary portion,—the medulla of all the lobules being united by the medullary cord. The cortex. consists of a delicate reticulum with its spaces well filled by cells, usually lymphocytes. The reticulum may be regarded as composed of small branched anastom36 The Development of the Thymus

osing cells, though of course no cell boundaries are distinguishable. The nuclei are poor in chromatin, rounded, and usually 4.5;; to 5.5,; in diameter. The amount of cytoplasm around the nuclei and connecting them is usually very small. At some nodes there is a greater amount of cytoplasm giving the appearance of a large reticulum cell. In connection with the blood-vessels, which are numerous in the cortex, are often found branched cells with pale nuclei and cytoplasm that stains more intensely than that of the rest of the reticulum. By a modification of Mallory’s method used by Jackson (13, S. 39), I have been able to demonstrate numerous fibrillae in the reticulum. In some cortical areas at this stage there are a great many erythroblasts. Masses of free erythrocytes are often found, usually near a comparatively large vessel, but such cells occur singly anywhere in the cortex.

In the medulla, the syncytial character of the stroma is much more pronounced. The cytoplasm is much more abundant than in the cortex, and the spaces are smaller and not so numerous. There is not so much room for lymphocytes as in the cortex, hence the lighter color of the medulla in stained preparations. The nuclei of the syncytium are either pale or dark, both types showing wide variations in size. By J ackson’s method (13, S. 39), fibrillaa may be readily demonstrated in the syncytium. In Plate I, Fig. 6, is shown the arrangement of these fibrillae (s f) in the medulla at 24 cm. They often may be traced into the areas which are forming the concentric corpuscles. In some parts of the medulla the fibrillae are very numerous; in a few places, entirely absent. In both cortex and medulla eosinophile cells are often found. These occur in groups in the interlobular tissue around the blood-vessels, around some of the corpuscles of Hassall, and singly in the reticulum. These have been described by Schaffer (24). Free erythrocytes are rarely found in the medulla. In the medulla are also found the corpuscles of Hassall. Since the structure of these bodies depends largely upon their age, it may be better understood from the consideration of their development.

THE LYMPHOID TRANSFORMATION.‘

Kolliker, in 79, first advanced the idea that the leucocytes are formed directly from the epithelial cells of the thymic anlage. According to Minot (in human embryology, p. 878), “ he records for the rabbit that between the twentieth and twenty-third days the cells of the thymus become smaller and their outlines disappear, so that the organ appears to

‘This term will be used to include those changes that occur in the thymus during its passage from an epithelial to a characteristic lymphoid structure. E. T. Bell 37

be an accumulation of small round nuclei. At about the same period blood-vessels and connective tissue grow into the epithelial anlage.”

His (12 b), 80, and Stieda (26), 81, claimed that the corpuscles of Hassall are the only remnants of the epithelial anlage, that the lymphocytes, reticulum, etc., are of mesenchymal origin.

Maurer (17 a), 86, described the leucocytes as arising directly from the cells of the epithelial anlagc in the thymus of teleosts. In the amphibian thymus (17 b), 88, he thinks that the leucocytes are probably of 1nesenchymal origin. He was unwilling to conclude that they arose from the epithelium because he could not find transition forms. In lizards (17 c), 99, he records that even before the separation of the epithelial anlage of the thymus from the pharynx, changes begin. The peripheral cells are closely crowded together and show many mitoses. There arises between the central cells, or is formed in vacuoles in their protoplasm, a fluid which accumulates until the nuclei surrounded by a thin zone of protoplasm are connected only by protoplasmic threads. A loose medulla is thus form.ed which looks like a cellular reticulum. The cortex is still solid. The lymphocytes are formed from the epithelial cells; none come from without. Later, blood-vessels and connective tissue grow in. He believes that the reticulum is of mesenchymal origin in all forms. Maurer (17 d), 02, still holds that in amphibians the lymphocytes are probably of mesenchymal origin.

Hermann et Tourneux (11), 87, find that in man and other mammals the epithelial anlage of the thymus is gradually converted into leucocytes and reticulum cells. Vacuoles appear during the transformation which seem to be formed by the absorption of large cells. In a sheep embryo of 130 mm., the clear epithelial cells have all disappeared, giving rise to small round cells and reticulum cells. Prolongations of connective tissue, each containing a blood-vessel, grow into the anlage during the transformation. They are not sure that all the thymic elements are epithelial in origin, being especially in doubt about the origin of some of the reticulum cells.

Gulland (8), 91, describes the development of the tonsil in the rabbit. Leucocytes first appear in the connective tissue around the thymus. Later they appear in the connective tissue around the tonsil. They infiltrate the tonsillar epithelium. N o leucocytes are of epithelial origin. After studying the tonsil he examined the thymus in a few specimens and concluded that the same process of leucocyte infiltration occurred there. He does not give the details of their infiltration, and did not see any of the transition forms of nuclei in the thymus at that period.

Prenant (22), 94, made a careful study of the development of the 38 The Development of the Thymus

thymus in sheep embryos. His results are as follows. At 25 mm. the gland is composed of distinct polyhedral cells with nuclei of only one kind. A few mitoses and an occasional direct division are to be seen. At 26 mm. mitoses are numerous (one nucleus in fifty). Nuclei are regularly rounded or elliptical and some small nuclei occur juxtaposed to large nuclei. At 28 mm. many mitoses are present and irregular spaces have appeared. These spaces are not blood-vessels nor parts of the thymic duct but vacuoles. Some nuclei, noticeably small and darkly colored, lie close to the large, clear nuclei and seem to be budded off from them. Some nuclei (rare) are broken into three or four chromatic bodies. Embryos of 40 mm. have undergone in great part the lymphoid transformation. All transitions are found between the large, pale elliptical nuclei of clear reticular structure and the small, deeply colored rounded nuclei whose sap is strongly stained. These last are certainly lymphocytes and constitute an immense majority of the cellular elements. Large, clear nuclei are found joined to small dark ones——nuclear couples.

At 85 mm. the medulla appears; the cortex corresponds to the entire thymic mass of preceding stages. The cortex contains a great many lymphocytes separated by islands and rows of pale nuclei- There are about thirty lymphocytes to one pale nucleus. Mitoses are numerous in the cortex. In the medulla at this stage, the large clear, and small dark, nuclei are about equal in number, and mitoses are rarer than in the cortex. In later embryonic stages avclear peripheral zone is present where cell proliferation takes place. Mitoses are now more numerous in the medulla than in the cortex. It is probable that a certain number of the epithelial cells persist as reticulum cells in the fully-formed organ.

J. Beard (3 a), 94, (3 b), 99, thinks that the function of the thymus is to form the first leucocytes. He finds that in 'the skate the epithelial cells are converted early into lymphocytes which emigrate into the blood. There are many breaks in the gland where the lymphocytes escape in masses. The thymus is the only source of leucocytes until the other lymphoid organs are formed.

Vcr Eecke (28), 99, finds that in the frog the epithelial thymus is invaded by lymphocytes and connective tissue. The epithelial cells are not destroyed but merely dispersed by the mcsenchymal elements. He calls the resulting tissue lympho-epithelial. This idea of the commingling of the two tissues had already been advanced by Retterer.

Nusbaum and Prymak (20), OI, on teleosts, agree with Maurer that the lymphocytes are of epithelial origin but disagree on the details of their formation. They find that the epithelial anlage is at first composed of cells with distinct boundaries. It is not difierent from the epithelium E. T. Bell 39

of the pharynx. Before any blood-vessels or connective tissue have invaded the organ, changes begin in the central part. These changes consist in the breaking up of the cytoplasm so that the cells become branched and connected by delicate processes. These processes finally break apart leaving a nucleus surrounded by a thin layer of protoplasm— a lymphocyte. The peripheral epithelial layer multiplies rapidly, forming nuclei somewhat smaller and darker than their own. These nuclei become gradually changed into the nuclei of lymphocytes and break away from the other cells. All transitions are present between the large, clear epithelial nuclei and the lymphocytes. Blood-vessels and connective tissue grow in from the outside.

It appears from a survey of the literature that, of those who have studied the origin of lymphocytes in the mamalian thymus, Hi-s, Stieda, and Gulland have advocated the idea that they invade the gland from without, and that the original epithelial anlage persists only as remnants, the corpuscles of Hassall. They also consider the stroma of mesenchymal origin. On the other hand, Ktilliker, Herm-ann and Toumeux, and Prenant, have described the lymphocytes as derived directly from the epithelial cells of the anlage. Hermann and Tourneux and Prenant ascribed a similar origin to part of the reticulum.

Neither His, Stieda, nor Gulland made a detailed histological study of the changes that take place in the thymus during the transformation. They did not see the vacuolization of the cytoplasm, the changes in the epithelial nuclei, etc.—processes which undoubtedly occur. Gulland made nearly all his observations on the tonsil and then from a superficial examination of the thymus concluded that the process is the same there. The conclusions of these men are therefore not to be compared on this point with those obtained by the thorough and careful work of Prenant.

On amphibians, Maurer hesitatingly agrees with His, and Ver Eecke accepts the mesenchymal origin of the leucoeytes ; while on fishes Maurer, Beard, and Nusbaum and Prymak believe in the epithelial origin of lymphocytes. Maurer’s work on reptiles is in agreement with his work on teleosts.

As to the origin of leucocytes in the lymphoid organs of the alimentary canal, opinion is divided. Retterer, v. Davidoff, Rudinger, Klaatsch, and others have described the leucoeytes as arising from epithelium and being invaded by mesenchymal elements forming adenoid tissue. Stiihr, Gulland, Tomarkin, and others describe them as penetrating the epithelium from without.

I hall now discuss my own observations on the lymphoid transformation in the thymus of the pig. From a very early stage (11 mm.), the 40 The Development of the Thymus

epithelium of the third gill pouch is a syncytium. No cell boundaries exist. The nuclei, large and irregular in shape, are embedded in a common mass of cytoplasm. In the thymus at 20 mm. I find a syncytium of dense cytoplasm embedded in which are large nuclei of irregular shape and size. No distinct types of nuclei are present yet; all stain with medium intensity. A few mitoses are to be seen.

In a section of the mid—cervical segment at 3.7 cm. (Plate I, Fig. 1), I find evidence that the lymphoid transformation has begun. ‘The syncytium is composed of coarsely reticulated cytoplasm much looser in texture than that of the preceding stage. It contains a few irregular spaces (3 s) which are evidently of the nature of vacuoles. These may be formed, as Maurer suggests, by liquefaction of the cytoplasm. There is no reason to suppose that cells degenerate and form them as Hermann and Tourneux believed. Three types of nuclei may be distinguished; large pale nuclei (Z pn) large dark nuclei (Z (In), and small dark nuclei (lymphoblasts) (lb). Transition’ forms occur between these types. The l-arge dark nuclei are intermediate forms between the pale nuclei and the lymphoblasts. A few mitoses (m) occur. No blood-vessels are present inside the anlage but they may be seen between the buds just outside. At this stage, the head and the thoracic segment have areas that are somewhat farther advanced than this. The intermediary and cervico-thoracic cords show no changes.

At a later stage than the above (Plate I, Fig. 2), in the thoracic segment of a 4.5-cm. pig, the spaces of the syncytium (s s) have increased greatly in number and size. The anlage is now a cellular reticulum. The large pale nuclei are somewhat less numerous than the dark nuclei and many have become angular, adapting themselves to the nodes of the syncytium. They contain less chromatin than in the preceding stage. Large dark nuclei and lymphoblasts are present; the lymphoblasts are much more numerous than in the preceding stage. A very few small dark nuclei are completely separated from the syncytium. These are lymphocytes. There are no lymphocytes in the connective tissue around the thymus or in the blood at this stage. I did not examine the tonsil or spleen at any stage. A few small blood-vessels are to be seen; their walls consist of endothelium only. There are more mitoses than at the preceding stage, but none happened to be present in the area shown in the figure. During mitosis, at all stages of development, except the early epithelial condition, the chromosomes are so closely packed that it is very difficult to distinguish them individually.

In a section through the mid—cervical segment of a 7-cm. pig (Plate I, Fig. 5), we see a stage somewhat later than the one shown in E. T. Bell 41

Fig. 2. In various parts of the section lymphocytes (Z) are completely formed. The great majority of the small round nuclei are inithe lymphoblast (Z b) condition, i. e.,they are not yet completely separated from the syncytium. There are a few lymphocytes outside the thymus in the interlobular tissue in this region; around the head and the thoracic segment at 7 cm. they are numerous, these parts being in a later stage of transformation. I have never seen lymphocytes outside the thymus, where there were none inside it ; but they appear outside shortly after they are formed here. Those formed next the interlobular septa seem to pass out very early. Of course the lymphoblasts, which are distinguishable from the lymphocytes only by being imbedded in the syncytium, are to be seen in the thymus long before any appear outside.

At the stage shown in Fig. 5, a great many nuclei are in mitosis. I have not seen at any stage, the amitoses and nuclear couples described by Prenant for the sheep. In some parts of the section comparatively large solid epithelial areas occur. These are found as often in the central as in the peripheral part. Many of the pale nuclei are smaller than those shown in Fig. 2. The blood-vessels are somewhat larger and more numerous than those at 4.5 cm.

It is to be noted that the epithelial anlage does not at any stage become converted entirely into small round cells as many observers have stated. Distinctly pale angular reticular nuclei can always be seen.

In the mid-cervical segment at 8.5 cm. (Plate I, Fig. 4), a great many lymphocytes (Z) are formed. These lie between the persisting epithelial cells which are now arranged in irregular cords and islands. In these epithelial masses, lymphoblasts may still be seen indicating -that the formation of lymphocytes is still in progress. Many of the pale nuclei are now small. The heavy haematoxylin stain in this case makes the nuclei darker than they would appear with an ordinary‘ stain. A few nuclei are in mitosis.

This figure shows also the first appearance of the medulla (md). The medulla is formed directly, as shown in the figure, from persisting parts of the epithelial syncytium. Certain centrally situated masses of this syncytium undergo changes of such a nature that they stain readily with cytoplasmic stains such as Congo red. In sections stained with haematoxylin and Congo red, the medulla is first recognized as a brightly colored area situated usually about the center of the lobule. These epithelial masses that give rise to the medulla seem to increase in size about the time of the change in staining capacity. The first differentiation of the medulla is chemical rather than morphological, for there'are other persisting epithelial mvasses even larger than it in the same section 42 The Development of the Thymus

that do not react in the same way with the cytoplasmic stains. The medulla appears in the head and the thoracic segment at 7.5 cm. to 8 cm. All the gland except this small central area forms the cortex. Bloodvessels now reach all parts of the gland, but are still few in number. I cannot distinguish any wall except the endothelium on those actually inside the gland.

In a 9.5-cm. pig, the medulla is larger. It contains pale nuclei of various sizes, large dark nuclei, and lymphoblasts. Its spaces are smaller than those of the cortex. The early stages in the formation of the corpuscles of Hassall appear as soon as the medulla begins to form. The epithelial cords in the cortex have become less conspicuous, but are still forming lymphocytes. A few nuclei are in mitosis. Many blood cap illaries may now be seen penetrating the gland from the periphery:

These vessels run in the epithelial masses and have a wall of large endothelial cells which gives them the appearance of radiating cords. When these vessels first appear, as at this stage, they have only an endothelial wall. The blood-vessels grow in as small c-ap-illaries which, after their entrance, increase in size and branch; they do not break in as large Vessels surrounded by mesenchymal tissue. I am fairly sure that aside from the endothelial cells few or no mesenchymal cells come into the thymus. Around the greater part of the periphery of the gland is a solid zone of syncytium two or three nuclei deep which is in transformation like the epithelial cords inside. This zone, described by Prenant as a zone of proliferation, grows rapidly, as the frequent mitoses indicate. Its inner boundary is forming lymphocytes and reticulum cells.

In a 14-cm. pig, the lymphoid transformation is practically at an end except in the medulla. The peripheral zone of proliferation has disappeared. The cortex has about the same structure as at 24 cm., as previously described. In the medulla, lymphoblasts, large pale nuclei, and the large dark intermediate types are still present. There are a few mitoses here. It is very probable therefore that the formation of lymphocytes is still in progress in the medulla. The medulla at 24 cm. shows a similar structure except that there are fewer lymphoblasts. These facts persuade me to regard the medulla as a center for lymphocyte formation at least as late as birth. Connective tissue fibrillae begin to appear in the gland along the large blood-vessels and the interlobular septa as early as 10.5 cm. They are only a little farther in at 16 cm. ; but near full term they are present in nearly all parts of the stroma. (See Plate I, Fig. 6.)

The above account may be summarized as follows: In the pig the epithelial syncytium of the thymic anlage becomes loosened up by the E. T. Bell 43

formation of vacuoles in it. These vacuoles increase in number and size, converting the anlage into a cellular reticulum. While this vacuolization is in progress, the nuclei, which at first are of one kind with a medium amount of chrom-atin, differentiate into large clear, large dark, and small dark (lymphoblast) forms. The large dark nuclei probably divide by mitosis and form the lymphoblasts. The lymphoblasts gradually break loose from the syncytium, passing into its spaces and becoming lymphocytes. Shortly after lymphocytes begin to be formed, some of them pass out of the gland into the surrounding connective tissue. The lymphoid transformation begins in embryos of 2.5 cm. to 3 cm. and continues in the cortex until 12 cm. or 13 cm. In the medulla it is not complete at birth. Since the thymus increases greatly in size during this period the epithelial syncytium must grow rapidly. Lymphocytes are constantly being formed at the expense of the growing syncytium. A peripheral zone of proliferation is present from about 8 cm. to 12 em. The medulla is formed as a chemical differentiation of certain centrally situated areas of the epithelial syncytium. The histological changes occur earlier in the head and thoracic segment than in the mid-cervical segment and very much earlier than in the cords. The reticulum of both cortex and medulla is practically all of epithelial origin. Some branched cells around the blood-vessels in the cortex may be of mesenchymal origin. ‘ T

My reasons for regarding the lymphocytes as of epithelial origin are as follows:

A. The lymphoblasts are true epithelial nuclei, because ( 1) there are numerous transition forms between them and the large dark nuclei which later cannot be regarded as invading lymphocytes; (2) they are closely embedded in the syncytium and show no evidence of having eaten their way through the protoplasm; (3) they are present from a very early stage and increase in number as development proceeds; (4) they are present before blood-vessels invade the gland and have no constant relation to blood-vessels or to the surface of the gland that indicates an invasion from either of these directions; ( 5) they are present before lymphocytes appear in the connective-tissue around the thymus.

B. Some observers admit that the small dark nuclei (lymphoblasts) are of epithelial origin but do not admit that they form lymphocytes. The considerations that lead me to believe that the lymphoblasts do form the lymphocytes are: (1) the small dark nuclei (l.ymphoblasts) show every possible relation to the syncytium from being completely embedded in it to lying free in the syncytial spaces. A comparison with later stages shows that this appearance is not due to poor fixation or to the 44 The Development of the Thymus

adherence of the nuclei to the reticulum; (2) the first free nuclei often appear in the center of the gland when there are no other free nuclei in the periphery at that level; (3) there is good evidence that lymphocytes emigrate from the thymus in large numbers. If we examine the thymus of a '7-cm. pig in serial sections we find that the lymphoid transformation is less advanced in the mid—cervical segment than in the head. In the mid-cervical segment there are a few lymphocytes in the interlobular tissue. In the lower end of the head where there are more lymphocytes inside the gland, lymphocytes pack the interlobular tissue and form a thin zone around the periphery of the gland. In the middle of the head where the transformation is far advanced, lymphocytes pack the interlobular tissue and form a thick zone around the entire gland. Indeed, in some sections, there are more lymphocytes in the zone outside than are present inside the gland. If this zone of lymphocytes be passing into the gland, it is not easy to understand why it is formed from within outwards, and why it is thickest where the greatest number of lymphocytes are already present inside. No satisfactory suggestion has yet been made as to why lymphocytes should thus suddenly pour into the thymus at a time when if present at all elsewhere they are rare. They do not come to break up the thymic epithelium, for that is already a reticulum before free cells are present (Fig. 2, Plate I). Where lymphocytes invade intestinal epithelium as in the tonsil they eat paths through it leaving spaces. The epithelial reticulum of the thymus is not formed in that way. On the other hand it is not diificult to believe that this zone of lymphocytes is formed by cells passing out the periphery of the thymus and that the gland thus contributes a great number of lymphocytes to the organism; (4) I have not been able to find lymphocytes in the connectivetissue around the thymus before they are present inside. An invasion by way of the blood-vessels may be excluded, since the thick zone of lymphocytes formed around the gland shows that these cells either enter or leave it through the preiphery.

THE CoRPUsoLEs or HASSALL.

These bodies were first mentioned by Hassall (10) in 46. He speaks of them as being composed of mother cells which enclose the newlyformed daughter cells and nuclei. He thought the central mass was formed by the outer enclosing layers. He found bodies which he regarded of the" same nature in fibrous coagulations in the heart.

Virchow (29), 51. in a discussion of endogenous cell formation, compares Hassall’s corpuscles to carcinoma pearls. He had about the same E. T. Bell 45

conception of the nature of the corpuscles as Hassall. This oft—quoted comparison was therefore not based upon a deep insight into their nature.

Giinzburg (9), 57, did not advance beyond Hassall’s conception that the central mass is formed by the peripheral layers.

Paulitzky (21), 63, described the center of the corpuscles as homogeneous or granular. They sometimes contain an elliptical nucleus, sometimes fat droplets. The larger ones have in the center several nuclei or cell-like forms. The central part is formed from masses of epithelial cells. Connective tissue cells grow around them and are transformed into epithelial cells forming the peripheral part of the corpuscle.

The term “ concentric corpuscles ” was introduced by Ecker (6), who described them as arising directly from gland cells by fatty metamorphosis. He distinguished (1) simple corpuscles, round vesicles with thick concentric hulls, containing inside a fatty opalescent mass, and (2) compound corpuscles, which consist of several vesicles with a common hull. The peripheral layers of a corpuscle consist of flattened cells.

His (12 a, 12 b), 60, 80, described the corpuscles as consisting of an outer striated shell, probably composed of nucleated cells, and containing lymphocyte-like cells inside. He supposed them to be the original cells of the epithelial anlage which become entangled in the reticulum in some way. Their rapid growth in their narrow confines causes the concentric form.

Cornil et Ranvier (5), 69, considered the corpuscles as arising from the endothelium of blood-vessels and compared them to the spheres of their “ Sarcome angiolithique.”

This suggestion of a vascular origin, made by Cornil et Ranvier, was elaborated by Afanassiew (la), 77.

Afanassiew held that the corpuscles of Hassall arise from the endothelium of the smaller veins and capillaries. The endothelial cells increase in size, become cubical, and later fill the lumen of the vessel. During the proliferation of the endothelium, the blood-vessels break up into segments which are now nearly solid cords.) These cords are at first connected to each other and to blood-vessels, but they soon break apart. The surest proof that the corpuscles are of vascular origin is that erythrocytes may be found -inside them. Vascular injections, however, do not go into a corpuscle except in a very early stage, since the lumen is soon obliterated by the endothelial plugs. The corpuscles are formed

entirely by the endothelial cells. Afanassiew worked on embryos of man, the rabbit, and the calf.

Stieda (26), 81, in sheep embryos, describes the epithelial mass of the 46 The Development of the Thymus

thymic anlage as being broken up by ingrowing adenoid tissue. From 50 mm. to 60 mm., there are no large epithelial cells; but later at 100 mm. he finds in the adenoid tissue large cells 9 ,u. to 15 a in diameter, isolated or united in groups, whose protoplasm colors light-red with carmine. These large cells have a concentric structure. Some of them are enclosed by large cells whose cytoplasm does not color with carmine, giving rise to a yellowish mass of irregular form and stratified appearance. In older embryos (250 mm.), the cellular masses are numerous but the large colored cells are rare. The yellowish masses are groups of the large cells which have undergone a transformation like that of the stratum corneum of the epidermis. Stieda considers the large colored cells which form the corpuscles as remnants of the epithelial anlage, although he admits that for a long period during development he found no trace of them. He explains the formation of the corpuscles in accordance with (Johnheim’s hypothesis that most tumors‘ arise from unused tissue remnants.

Ammann (2) 82, made most of his observations on human foetuses. He describes the corpuscles as arising from connective tissue. The corpuscles are cellular in structure and are formed of one, two, or three central cells around which -a Variable number of cells, increasing with age, are arranged like the coats of an onion. The corpuscles are formed from reticulum cells and leucocytes. Growth consists in the apposition of cells from without. The life of a corpuscle consists usually of four stages: ( 1) Stadium der Transparenz; (2) Stadium der colloiden Entartung; (3) Stadium der Verkalkung; (4) Stadium des Zerfalls. The nucleus of a reticulum cell or leucocyte increases in size at the expense of the cell body. Its increase in- size establishes the concentric form. The corpuscle undergoes colloid and usually calcareous degeneration. Fat droplets, cholesterin crystals, and colloid granules are found together in the degenerating corpuscles. Breaking up in this way makes absorption possible. No epithelial remnants are to be observed. No erythrocytes are found in the corpuscles.

In four cases of atrophic thymus gland which yet contained lymphoid tissue Ammann found corpuscles in all stages of development. He also found that the corpuscles are formed most rapidly when the thymus is at the height of its development. From these facts he concluded that they are not connected with the involution of _the thymus as Afanassiew thought. He thought that their formation is due to a physiological decrease in the intensity of growth of the medulla, due to the rapid growth of the cortex.

Watney (31), 83, agreed with Ammann that the corpuscles arise from connective tissue cells. E. T. Bell 47

Monguidi (18), 85, distinguished true and false concentric corpuscles—the latter being only sections of blood-véssels.

Hermann et Tourneux (11), 87, gave a description of the structure and formation of the concentric corpuscles about like that given by Ammann except that they regard the reticulum cells from which the corpuscles develop as of epithelial origin.

Gulland (8), 91, regarded the corpuscles as epithelial remnants and compared them to the epithelial pearls of the tonsil.

Maurer (17 c), 99, described the corpuscles as epithelial in origin. His description of their formation is however different from that of His. All the cells of the epithelial anlage at first assume a lymphoid character. Later, some of these cells reassume their epithelial nature and then form the corpuscles. His conclusions for teleosts and amphibians are similar to the above results which he obtained from the lizard.

Ver Eecke (28), 99, for the frog, describes the leucocytes and connective tissue cells as invading the thymic anlage and separating the epithelial cells. The epithelial cells, separated by the mesenchymal elements, lie at first in groups or singly. They go through a cycle of two phases, a stage of growth, and a stage of involution. In the former stage, they increase to three or four'times their original size and their cytoplasm differentiates into circular layers like the coats of an onion. The majority are monocellular. Some cells grow together making a more complex multicellular type. There are some intermediate forms, cells with a dense dark protoplasmic body, indistinct striations, and a nucleus partly or completely hidden in a precocious degeneration. In the stage of involution, which sets in early, th'e.cytoplasm degenerates by the formation of vacuoles containing a hyaline liquid. The liquefaction may be in the form of a diffuse vacuolization, a large central vacuole, or a peripheral vacuole circular in section. The nucleus loses its affinity for stains, becomes deformed, breaks up, and finally disappears. The corpuscles are finally absorbed. They never contain erythrocytes. The cells do not degenerate to form a corpuscle. The liquefaction forms an internal secretion which is forced out by the muscle tissue in the reticulum.

Entirely different results on amphibians are reported by Nusbaum and Machowski (19), 02. These investigators revive the old idea of Afanassiew, accepting his results except that they think the adventitia as well as the endothelium of the blood-vessels takes part in the formation of the corpuscles. They find erythrocytes in the corpuscles. These erythrocytes either gradually shrivel and disappear, or they are absorbed by leucocytes or endothelial cells. The leucocytes after digesting the haemoglobin of the erythrocytes become eosinophile cells which are numerous in the thymus. 48 The Development of the Thymus

Wallisch (30), 03, measured the volume of the human thymus and of the corpuscles of Hassall at various stages. He finds that the total volume of the corpuscles of a 7-mo. embryo is 4.6 mm.,’‘ and of those of a 6-mo. child, 174.6 mm? The total volume of the thymus of a 78-mm. embryo, when it has already been partly transformed into adenoid tissue is only 6.8 mm? Since there is no evidence that the cells of the corpuscles multiply, he concludes that they cannot be regarded merely as remnants of the original epithelial anlage.

Disregarding the crude observations of the earliest investigators, there remain three distinct theories of the formation of the corpuscles of Hassall.

1. The epithelial anlage of the thymus is broken up by the invading mesenchymal elements. The separated masses of epithelial cells undergo further changes mainly of a degenerative nature to form the corpuscles. This was the belief of His and Kolliker. According to this interpretation, the corpuscles are to be regarded as remnants that have nothing further to do with the gland. Stieda, Maurer, and Ver Eecke held this View in a modified form. Stieda regarded the cells forming the corpuscles as epithelial remnants but admitted that they go through a stage in Which, for a time, they lose their epithelial character. This is -substantially the same as Maurer’s view. He thinks that the cells of the epithelial anlage all become lymphoid, and that some of them afterwards reassume their epithelial nature and form the corpuscles. Ver Eecke regards the corpuseles as epithelial remnants but thinks that they are glandular in nature, not mere useless remains.

2. The corpuscles are formed from the proliferating ‘Walls of bloodvessels. This idea was suggested by Cornil and Ranvier and elaborated by Afanassievv. Nusbaum and Maehowski accept Afanassiew’s view except that they believe the adventitia of the blood-vessels as well as their endothelium takes part in the formation of a corpuscle. These investigators thought that the formation of the corpuscles is connected with the involution of the thymus.

3. The corpuscles are formed from reticulum cells of the medulla and grow by apposition of the surrounding cells. This View was advanced by Ammann. Ammann thought that the reticulum is of connective tissue origin. He also believed that leucocytes formed the central part at least of some corpuscles. Hermann -and Tourneux accepted Amma.nn’s results, except that they ascribed an epithelial origin to the reticulum. (I do not know Whether they accepted the origin from leucocytes described by Ammann.) Ammann thought that the corpuscles formed because of a physiological decrease in the rate of growth in the medulla. E. T. Bell 49

My own observations on the development of the corpuscles of Hassall in pig embryos, will now be considered. The medulla, as previously described, begins to form from the epithelial syncytium usually near the center of the lobule. It is first recognized by its more marked reaction with cytoplasmic stains such as Congo red. Shortly after the medulla begins to form, the earliest stages of the corpuscles may be observed. A few corpuscles have appeared at 9.5 cm. I did not find them earlier. They are all formed from the epithelial syncytium of the medulla.

Before beginning this discussion I will explain the use of my terms, By a corpuscle of Hassall, I mean a modified area of the epithelial syncytium of the medulla, containing at some period of its development, one or more nuclei, and whose cytoplasm has been in part or entirely transformed into colloid. The term colloid is applied to various substances probably of widely different chemical nature, but is fairly adapted to our imperfect knowledge. I shall use the term here in the restricted sense «employed by Ziegler,‘ i. e., hyaline substances of epithelial origin, that do not give the reactions of mucin.

Colloid in the corpuscles of Hassall does not usually appear as solid "masses in its early formation, but as fibers, granules, or sheets which are separated by more or less cytoplasm that is not yet changed. This stage I have called, “colloid in formation” (c f). It later assumes a more solid homogeneous appearance which I call solid colloid (c s). Often the solid colloid stains intensely with cytoplasmic stains. I call this kind solid deeply-staining colloid (c s d). In later tages, the colloid often loses its affinity for cytoplasmic stains, staining a very pale color or not staining at all. I call this variety old colloid (0 0).

According to their mode of development, the corpuscles of Hasall may be classified as follows:

A. Concentric Corpuscles. a. Simple. 1. Ordinary type. 2. Epithelioid type. 3. Cystic type. b. Compound. 13. Irregular Corpuscles. a. Compact type. b. Reticular type.

‘Gen. Pathology, 10th ed., Warth1n’s translation, p. 205.

4 50 The Development of the Thymus

A. The concentric corpuscles include those that from their earliest appearance are concentric in structure. Adopting Ecker’s classification., I distinguish simple concentric corpuscles and compound concentric corpuscles.

(a) Three types of simple concentric corpuscles are to be considered. (1) The ordinary type is far more numerous than any other. The earliest recognizable stage is shown in Plate II, Fig. 11. A nucleus (n) of the syncytium of the medulla has enlarged to perhaps twice its ordinary volume and has lost the ability to stain in the characteristic way with haematoxylin. Its sap is clear and a few reddish stained granules represent its chromatin. Around it in the cytoplasm is an indistinct uneven layer of colloid (c f). The colloid is not yet solid and is being formed in concentric fibers or sheets. A slightly later stage is shown in Plate II, Fig. 14 and Fig. 15 (left side of figure). Some of the colloid (c s) next to the nucleus is now solid. The next stage is shown in Plate II, Fig. 15 (right side of figure). These corpuscles show a thick layer of colloid (c s d) that stains intensely with Congo red. Just outside the deeply staining colloid, colloid in formation may be seen. The nuclei are clear, and have become smaller and irregular in outline. The colloid seems to be pressing upon them and obliterating them. The colloid transformation gradually involves the adjacent cytoplasm of the syncytium until other nuclei are involved. The corpuscle has now reached the condition shown in Plate II, Fig. 12. The central area (0 c) is solid, the nucleus having disappeared entirely. Another (72!) is nearly obliterated by the colloid. Part of the central area (0 c) no longer stains intensely, and it is breaking loose by the formation of a concentric space. Several nuclei are surrounded by colloid in formation. Their long axes are nearly in a tangential direction. These nuclei are clear but only

moderately swollen.

In the further development of the corpuscle (Plate III, Fig. 17 and Plate II, Fig. 7), the central area (c s (1) increases in size. The nuclei involved in this area become obliterated probably by the pressure of the colloid and are no longer distinguishable. This central area usually splits off and may break up into many smaller masses. The peripheral part of the corpuscle increases by extension of the colloid formation into the adjacent part of the syncytium. This extension takes place in- the early stages by direct progressive involvement of the immediately adjacent cytoplasm; in later stages (Fig. 7), by the formation of concentric lamellae which cut ofi’ unchanged areas of cytoplasm. The lamellm increase in size and number, the cytoplasm _included between them is changed into colloid. They finally become closely packed, giving the characteristic and THE DEVELOPMENT OF THE THYMUS. PLATE I.

E. 1'. BELL.

AMIRIOAN JOURNAL OF ANATOMY--VOL V. 5- 7- BEL‘-y DEL NELIOTVPE 00., IOITON. THE DEVELOPMENT OF THE THYMUS. PLATEII.

E. T. BELL.

AMERICAN aounnu. or ANATOMY--VOL v. 5- 7- “Lt: “Et HELIOTVPE 00., BOSTON‘ THE DEVELOPMENT OF THE THYMUS. PLATEIII.

E. T. BELL.

AMERICAN JOURNAL OF ANATOMY--VOL V.

E. T. BELL, DEL. E. T. Bell 51

Well-known onion-like structure found in the fully-formed corpuscle. The nuclei that are enclosed between the lamellae gradually lose their chromatin and become flattened out. They do not swell and are not obliterated. It seems that swelling occurs only in nuclei that are surrounded by deeply staining colloid, and that this change is preparatory to their obliteration by or transformation into colloid. The amount of the corpuscle that breaks up to form the softer center i-s very variable. The size of the center usually seems to increase with the age of the corpuscle.

Plate III, Fig. 21, shows a variation from the ordinary concentric type. The central nucleus (n) stains reddish but is not enlarged. Most of the other nuclei are unchanged. All the colloid (c f) is in the early fibrous and granular stage.

From 20 cm. to full term many corpuscles show masses of calcareous material in or near the center. This material rarely appears in younger corpuscles (Plate III, Fig. 1?’, cl). It stains a violet blue with Delafield’s haematoxylin.

The majority of the corpuscles of Hassall belong to the ordinary type of simple concentric corpuscles described above. It is very clear that they have nothing to do with blood—vessels. ‘They never contain erythrocytes nor anything resembling them. Rarely a lymphoblast or leuco— cyte is found inside the corpuscle. These seem to be usually involved in the corpuscle like ordinary stroma nuclei during the formation of the lamellae. (Their occurrence in other types will be discussed later.) It is also clear that these corpuscles arise from the syncytium of the medulla. They are epithelial in origin, since the entire stroma of the gland is derived from epithelium, but they are certainly not remnants of the original epithelial anlage. Neither are they formed from lymphoidlike elements that reassume their epithelial nature as Maurer described for the lizard.

Some of Ammann’s observations are in accord with my results. The swelling of the nucleus was noted by Ammann as the first step toward the formation of the corpuscle. It should be noted, however, that rarely a corpuscle begins to form as a mass of colloid out in the cytoplasm and involves nuclei secondarily. I cannot distinguish his “Stadium der Transparenz ” for I cannot be sure that a corpuscle is beginning to form until some colloid is present. The formation of the colloid is associated with the swelling of the nucleus. His other three stages, “ Stadium der colloiden Entartung,” “ Stadium der Verkalkung,” and “Stadium des Zerfalls ” are easily seen. I have never seen corpuscles begin in leucocytes as Ammann described. His statement that the corpuscle grows by apposition of reticulum cells is true in a modified sense. He thought that 52 The Development of the Thymus

the outer part of a corpuscle is formed of reticulum cells that have moved up and flattened themselves out around it. The description just given shows that the corpuseles are never composed of distinct cells, and that the increase in size is due to an extension outward of the colloid formation and not to a moving in of the adjacent tissue.

The concentric form of this type of corpuscle is due at first to its being formed around a spherical or ellipsoidal nucleus. The swelling of this nucleus creates a centrifugal pressure in the adjacent cytoplasm. Before or during its transformation into colloid, the cytoplasm also increases in quantity. That the cytoplasm increases in quantity is shown by the fact that the nuclei are fewer in the corpuscle than in any adjacent area of the syncytium of equal size. This centrifugal pressure presses the newly formed colloid into concentric lamellae. It at first /turns the long axes of the nuclei tangentially, and later flattens them and makes them concave toward the center.

2. The epithetliovld type of corpuscle is characterized by large areas of cytoplasm so marked off by colloid lamellae as to give the appearance of a mass of large epithelial cells. They may contain only one nucleus embedded in a well-defined area of cytoplasm (Plate III, Figs. 18 and 20). These correspond to the monocellular corpuseles that have been described for lizards and amphibians. They are rare in the pig. I have not been able to trace these very far, as they soon become indistinguishable from other forms. The only difference I have noted is that the outer colloid lamellae begin to form early, causing the peculiar appearance of a large epithelial cell. Again the epithelioid type may present an appearance such as shown in Plate I, Fig. 3. These do not seem to be formed around any special nucleus. The outer colloid lamellaa form before any center has been established, marking off large cytoplasmic areas that may look like large cells. The centrifugal pressure of expansion caused by the great increase of cytoplasm in this area determines the concentric form in these corpuseles. Pure epithelioid corpuseles are very rare, but epithelioid areas in other corpuseles are not uncommon. The occurrence of epithelioid areas in corpuseles of the ordinary type shows that it is due to variations in a fundamentally similar process.

3. In the cystic type of corpuscle, the central part, instead of becoming transformed into colloid, undergoes early liquefaction, forming vacuoles. The central nucleus does not increase in size as in the ordinary type, but shrivels up and disappears. The corpuscle begins by the formation of outer colloid lamellae—the central mass is not changed into colloid. In Plate II, Fig. 10, the central area (p m) is undergoing a diffuse liquefaction. The nucleus (n) is colorless and shrunken. In Plate II. E. T. Bell 53

Fig. 8 (right side of figure), the central area has formed two large vacuoles (zz). On the left side of the same figure, a concentric vacuole (12) has formed, separating off a central spherical nucleated mass of protoplasm. The nucleus of this mass of protoplasm is shrunken and the cytoplasm shows many small vacuoles. The corpuscle shown in Plate II, Fig. 9, is probably a later stage of the form just described. The central protoplasmic mass has become converted into an ellipsoidal pale body (p m). The small circular body in this shriveled mass is probably the nucleus. Some corpuscles like the one shown in Fig. 9 are found in which the central mass has entirely disappeared. The further growth of corpuscles of this type seems to be by formation of colloid lamellae as in the ordinary type. They soon become indistinguishable from other forms.

The cystic type of corpuscle is rare in the pig. This evidently corresponds to the form in amphibia that misled Nusbaum and Machowski into reviving Afanassiew’s theory. The central masses, in Figs. 8 and 9, might readily be mistaken for red corpuscles in animals in which these cells are nucleated. But the red cells of the blood of the pig are not nucleated at this stage. I have traced a number of these corpuscles (as well as those of other types) in serial sections and have never seen any indications of a connection to blood-vessels. Nusbaum and Machowski (19), (Fig. 1, c, S. 116) show a corpuscle which is similar to my Fig. 9. It will be noted that the central space in neither of these figures is lined by endothelium. The early form of corpuscle shown by Nusbaum and Machowski (Fig. 1, d, S. 116) is very probably a normal bloodvessel with cubical endothelium. I have often found such vessels with cubical endothelium in the interlobular tissue of the pig’s thymus at 10 cm. to 12 cm. They probably may be found at other stages also. In the thymus of a kitten, injected by the inrtraavitam Prussian blue method previously described, the majority of the corpuscles were found to be in early stages. The injection did not penetrate any corpuscle. I had a somewhat better opportunity to study the relations of the corpuscles to the blood-vessels in a 14 cm. human embryo. Here the vessels of the thymus were all very much distended with blood and the corpuscles were in early stages. No blood cells were found in the corpuscles.

(b) C'o~mpo/und concentric corpuscles are formed whenever two or more simple concentric corpuscles begin to form so close together that they come in contact during their later growth. An early stage of such a corpuscle is shown in Plate II, Fig. 15. The colloid lamellae are formed around each center until they come in contact; they are then formed around both centers. In Plate III, Fig. 22, a compound concentric cor54 The Development of the Thymus

puscle is shown. There are three simple concentric corpuscles in it— one of them (the lowest in the figure.) in a very early stage. Several l-amellae are common to the older corpuscles, and one is common to all three. This arrangement of the lamellae is a mechanical effect of the tension in the cytoplasm, due to the centrifugal pressure from the two centers. The size of the separate centers in a compound corpuscle depends upon the stage they have reached when they come in contact. If a compound corpuscle be formed by the union of two simple corpuscles in an early stage, as in Plate III, Fig. 19, all indications of its coinpound nature are soon lost. A corpuscle originally compound may, then, in later stages, become indistinguishable from simple corpuscles. The simple corpuscles uniting to form a compound concentric corpuscle may be of any of the types previously described.

B. IRREGULAR CORPUSCLES.

This group includes those corpuscles which are not at first concentric. Concentric areas may appear later. According to the classification previously given, I distinguish a compact type and la reticular type.

(a) The compact type (Plate III, Fig. 16) first appears as a compact area of syncytium of irregular shape. It is recognizable by the colloid it contains. The nuclei are not noticeably increased in size and have no regular arrangement. Their chromatin still stains dark with nuclear stains. The colloid (c f) is not yet solid. The corpuscle has no distinct center. These corpuscles grow by direct colloid transformation of the adjacent syncytium. No distinct lamellae are formed. The colloid may remain in the fibrous condition shown in the figure (cf) or it may become solid, but it never reaches the deeply staining condition unless a concentric area be established.

A later stage of this type is -shown in Plate III, Fig. 23. The cor-v

puscle is sharply marked off from the syncytium. Some of its colloid is solid. A concentric area (as) is beginning to form. The nuclei are not markedly different from those of the adjacent syncytium. These corpuscles may become large and branched. Often one or more concentric areas are developed after the corpuscle has attained considerable size. By the growth of these concentric areas, irregular corpuscles may become converted into concentric corpuscles.

(b) The reticular type is produced by colloid formation in the ordinary reticulum of the medulla. In the types previously described, the spaces of the reticulum are usually obliterated as the colloid form-ation advances; but in this form the spaces persist as a part of the corpuscle. Pure reticular corpuscles vary greatly in size, sometimes involving only E. T. Bell 55

one node of the syncytium. They are never concentric, and never form lamellae. Reticular areas often occur in other forms of corpuscles. In this way leucocytes are often involved in the eorpusele, since they lie in the spaces of the reticulum. Lymphocytes often get into a eorpusele in the lymphoblast condition, being cut off by the formation of lamellze outside them (Plate III, Fig. 22). The leucocytes shut in the corpuscle in this way during development may not degenerate. '1‘hey probably persist and help to remove the eorpusele in its final stages of degeneration.

The amount of expansion of the eytopla-sm before or during the colloid transformation is probably small in the irregular reticular corpuscles, since it does not obliterate the spaces of the syncytium. In the compact type, the spaces of the syncytium are obliterated and there is evidence of some expansive force (note the arrangement of the nuclei in the upper part of Fig. 2-3, Plate III). In the figure referred to, the number of nuclei in any part of the eorpusele is less than in an equal area of the adjacent reticulum. These facts indicate that there is an expansion of the cytoplasm. That this expansive force does not produce a concentric form is due primarily to the fact that there is no expansion of a nucleus and distinct center of formation as is present in concentric corpuscles of the ordinary type. The absence of the onion-like structure in irregular corpuscles is due to the fact that the colloid is not laid ‘down in lamellze.

Significance of the corpuscles of I-Iassall. It has been shown in the preceding pages that the corpuscles of Hassall in the pig are not epithelial remnants, and also that they are not_ formed from blood-vessels. There is no evidence connecting their development with the involution of the thymus, for they begin to form before the lymphoid transformation is complete a.nd are most numerous when the thymus is at the height of its development. I have not been able to see the decrease in the rate of growth of the medulla described by Ammann, and even if such dirl occur it is difilcult to understand how it could cause the formation of 9. eorpusele.

The above theories are, therefore, inconsistent with the facts of development in the pig. It seems to me that the formation of a eorpusele is not to be regarded as a process of degeneration. The fact that the formation of colloid is an essential feature in the development of every eorpusele is a strong argument that it is a form of secretion such as occurs in its neighboring branchial derivative, the thyroid. The "fact that the corpuscles differentiate in an apparently uniform syiicytium is further evidence against a theory of degeneration. Since the lymphocyte-forming function of the thymus is probably secondary, it is not 56 The Development of the Thymus

unreasonable to suppose that its primitive function was the formation of a colloid secretion such as occurs in the thyroid, and that ‘the corpuscles are abortive expressions of this primitive function.’

GIANT CELLS.

Polykaryocytes may often be seen in the medulla. These bodies develop from the syncytium of the medulla. They are first noticeable as groups of small spherical nuclei in a solid area of the syncytium. These nuclei stain with medium intensity and are all very similar in size.and color. The area containing this group of nuclei becomes a well-defined node of the reticulum and persists as such. 'A polykaryocyte is, therefore, a large node of the reticulum containing a number of small nuclei very similar in appearance. These cells often occur in groups. They are entirely distinct from the corpuscles. They are evidently similar to the polykaryocytes found in bone marrow and other lymphoid tissues.

SUMMARY.

The following is a resume of the development of the thymus in the P18 3

The thymus of the pig is probably developed entirely from the endoderm of the third gill pouch.

By a gradual process of vacuolization and liquefaction of the cytoplasm, the epithelial syncytium of the thymic anlage is converted into a cellular reticulum.

From the first appyarance of vacuolization, three types of nuclei are present: large pale nuclei; small dark nuclei (lymphoblasts), and large dark intermediate forms.

The lymphobla.sts gradually break loose from the cellular reticulum, moving into its spaces and forming lymphocytes. Mitoses are most numerous at the period of the most rapid formation of lymphocytes. The medulla continues to form lymphocytes at least as late as birth.

Lymphocytes appear in the connective tissue around the thymus shortly after they are formed; and lymphoblasts, which are distinguishable from lymphocytes only by being embedded in the syncytium, are present in the thymus a long period before lymphocytes are found anywhere in the neighborhood of the thymus.

The cellular reticulum of the earlier stages persists in a modified form as the reticulum of both cortex and medulla. It retains more cytoplasm

‘Ver Eecke (28) believes that the corpuscles in amphibians are of a. glandular nature. E. T. Bell 5'?’

in the medulla. Practically all the reticulum of both cortex and medulla., as well as the lymphocytes, are, therefore, of epithelial origin.

The corpuscles of Hassall develop from the syncytium and are, there-. fore, epithelial in origin. They are, however, not to be considered as remnants of the original epithelial anlage.’

In development various types of corpuscles are distinguished. The ordinary type of concentric corpuscles first appears as an enlarged clear nucleus around which colloid is being formed. Before or during the formation of colloid, the cytoplasm increases in quantity, filling the spaces of the reticulum and producing a centrifugal pressure which shapes the newly—formed colloid into concentric lamellae and flattens the neighboring nuclei, making them concave toward the center. The central nuclei usually become obliterated.

The epithelioid type is distinguished by its resemblance to large epithelial cells, this appearance being due to the formation of colloid lamellae around large areas of clear cytoplasm. The central part of the corpuscle usually remains unchanged until after some of the colloid lamellae are formed.

The cystic type differs from the ordinary type only in that the central part undergoes vacuolization instead of colloid transformation. Those with concentric vacuoles may simulate blood-vessels containing nucleated red cells. Corpuscles never contain erythrocytes; neither can they be injected at any stage of development. Serial sections also show that there is no connection to blood-vessels at any stage.

Compound concentric corpuscles are formed by the union of two or more simple concentric corpuscles during development.

Irregular corpuscles are not concentric in arrangement, and are formed in the syncytium in an irregular manner. In the compact type of irregular corpuscles, concentric areas may form.

The formation of colloid is an essential feature in the development of every corpuscle, and is not to be considered as a process of degeneration.

Since the conclusion of my work and after my manuscript was given to the publishers, two articles dealing with the thymus have appeared.

Ph. Stohr (Ueber die Thymus, Sitzungsberichte der phys.-med. Gesellschaft zu Wiirzburg, June 8, 1905) believes that the thymus first epithelial in nature becomes converted entirely into small cells of lymphoid appearance. Later the large reticulum cells are formed from these by enlargement. The corpuscles of I-Iassall.are formed by the massing together and enlargement of these lymphoid—like cells. The small round cells of the gland are epithelial in origin but are to be regarded not as lymphocytes but as epithelial cells. The thymus is not a. source of lymphocytes. 58 The Development of the Thymus

The author apparently believes that none of the small round cells leave the gland though he admits that lymphocytes enter. But as mentioned above the zone of connective tissue immediately around the head at 7 cm. may contain even more lymphocytes than are present inside the gland at that time If these are all entering the gland then it is probable that most of the small round cells are really lymphocytes. This conception then does not simplify the problem but is only a theoretical compromise between the two views as to the origin of the lymphocytes.

J. Aug. Hammar (Zur Histogenese und Involution der Thymusdriise, Anat. Anz. Bd. XXVII, June 17, 1905) regards the reticulum as formed from the epithelial anlage but thinks the evidence at hand insufiicient to decide the question as to the origin of the lymphocytes. He finds lymphocytes outside the thymus in many animals (man, cat, chick, frog) before any are present inside the gland. The corpuscles of Hassall develop from the epithelial reticulum and undergo hyaline (colloid?) degeneration.

My description of the formation of the corpuscles of Hassall differs essentially from Hammar’s, in that I believe the formation of the corpuscle consists in the expansion of the cytoplasm of the syncytium and its conversion into colloid. Hammar did not recognize “colloid in formation,” though he speaks of the coarse fibrillar structure of the protoplasm. He did not describe such corpuscles as are shown in Fig. 7, Plate II.

The considerations presented above in favor of the epithelial origin of the lymphocytes seem to me much stronger than those given by Hammar. His statements as to the presence of lymphocytes around the thymus before they are present inside are to be taken with some reservation inasmuch as he mentions small round cells separate from the syncytium earlier, but regards them as degenerating epithelial cells (S. 65). His figure from the human foetus (Fig. 18, S. 66) does not seem to be strong support for his statement. Certainly many lymphocytes are present in the pig thymus when the reticulum is broken up as much as shown in the figure referred to. It is also to be borne in mind that the different parts of the thymus undergo the lymphoid transformation at different times and that a single section may therefore be

misleading. LITERATURE.

la. AFANASSIEW, B.—Ueber die concentrischen Korper der Thymus. Archiv f. mlkr. Anat., Bd. XIV, 1877.

Weitere Untersuchungen iiber den Bau und die Entwickelung der Thymus und der Wintcrschlafdriise der Siiugethiere. Archiv f. mikr. Anat., Bd. XIV, 1877.

2. AMMANN, A.—Beitrage zur Anatomie der Thymusdriise. Basel, 1882.

3a. BEARD.—The development and probable function of the thymus. Anat. Anz., Bd. IX, 1894.

The true function of the thymus. Lancet, 1899.

4. BORN, G.—Ueber die Derivate der embryonalen Schlundbogen und Schlundspalten bel Saugethleren. Archiv f. mikr. Anat., Bd. XXII, 1883.

5. CORNIL et RANVIER.—MaI1l181 d’histologie pathologique. Paris, 1869, p. 135 (cited from Ammann).

lb. 10.

11.

129.. 1.2b. 13.

14.

15.

16.

17a.

17b.

17c.

17d.

18. 19.

20.

21.

22.

23.

E. T. Bell 59

EcKER.—Art. “ Blutgeffissdriisen,” Wa.gner’s Handw. der Phys., III (cited from Ammann). FRIEDLEBEN, A.—Die Physiol. der Thymusdriise. Frankfurt, 1858.

G-ULLANl).—The Development of adenoid tissue with special reference to the tonsil and thymus. Laboratory Reports issued by the Royal College Phys., Edinburgh, Vol. III, 1891.

GtiNzsURe.—Ueber die geschichteten Kiirper der Thymus. Zeitschr. f. klin. Med., Bd. VI, 1857, S. 456 (cited from Henle und Meissner. Berlcht iiber die Fortschritte der Anat. u. Physiol.).

HASSALL.—-The microscropical anatomy of the human body in health and disease. London, 1846 (cited from Ammann).

HERMANN et ToUnNEUx.—-Article thymus, Dict. encycl. des Sciences Médicales. Troisieme Série, 17, 1887.

His, W.—Zeitschrift f. wiss. Zoologie, Bd. X, S. 348. Leipzig, 1860.

Anatomie menschlicher Embryonen. Leipzig, 1880, S. 56.

JACKSON, 0. M.—-Zur I-Iistologie und Histogenese des Knochenmarkes. Archiv f. Anat. und Physio1., Anat. Abth., 1904.

KASTSCI-IENKO.—-Das Schicksal der embryonalen Schlundspalten bei Siugethieren. Archiv f. mikr. Anat., Bd. XXX, 1887.

KL:«:1N.—-—Neuere Arbeiten iiber die Glandula Thymus. Centralbl. f. allg. Pathol. u. pathol. Anat., 1898.

LANGERKANS und SAvEL1r.w.—Beitr§.ge zur Physiologie der Brustdriise. Virchow’s Archiv,»Bd. 134, 1893.

MAUREn.—Schi1ddriise und Thymus der Teleostier. Morph. Jahrb., Bd. XI, 1886.

Schilddriise, Thymus, und Kiemenreste bei Amphibien. Morph.

Jahrb., Bd. XIII, 1888. 4

Schilddriise, Thymus, und andere Schlundspaltenderlvate bei der Eidechse. Morph. Jahrb., Bd. XXVII, 1899.

——— In I-Iertwig’s Handbuch der Entwickelungslehre der Wirbelthiere, Lief. 6-8, S. 131 111., 1902.

MoNoUmI.——-Sulla glandula timo. P,arma, 1885 (cited from Prenant-).

NUSBAUM, J., und MACHOWSKI.-—Die Bildung der concentrischen K6rperchen und die phagocytotischen Vorgiinge bei der Involution der Amphlbienthymus, etc. Anat. Anz., Bd. XXI, 1902.

NUSBAUM, J., und PRYMAK, T.-— Zur Entwickelungsgeschichte der lymphoiden Elemente der Thymu bei den Knochenflschen. Anat. Anz., Bd. XIX, 1901.

PAULITzKY.—DiSqulS. de stratis glandulae thymi corpusculi. Habi1itationsschr., Halis, 1863 (cited from Henle und 'Meissner’s Bericht fiber die Fottschritte der Anat. und Physiol.).

P3ENAN:r.—Développement organique et histologique du thymus, de la glande thyroide, et de la glande carotidienne. La Cellule, Tome X, 1894.

PRYMAK, T.—Beitria'.ge zur Kenntnis des feineren Baues und der Involution der Thymusdriise bei den Teleostieren. Anat. Anz., Bd. XXI, 1902. 60 The Development of the Thymus

24. SCHAFFER, J.—Ueber den feineren Bau der Thymus und deren Beziehung zur Blutbildung. Sitzungsber. d. K. Acad. d. Wissensch. Math.naturw. Kl. Wien., Bd. CII, Abt. III, 1893.

25. SCHEDEL, J.——Zellvermehrung in der Thymusdriise. Archiv f. mikr. Anat., Bd. XXIV.

26. STIEDA, L.—Untersuchungen iiber die Entwickelung der Glandula_Thymus, Glandula. Thyroidea, und Glandula Carotica. Leipzig, 1881 (cited from Hermann et Tourneux).

27. SUL'rAN.—Beitrag zur Involution der Thymusdriise. Virchow’s Archiv, Bd. 144, 1896.

28. VER EEcK1~:.—Structure et modifications fonctionelles du thymus de la. grenouille. Bulletin de l’Académie royale de Medicine de Belgique,

1899. 29. Vmcnow, R.—Kritisches fiber den oberschlesischen Typhus. Archiv,Bd. 3, 1851, S. 222.

30. VVALLISCI-I.—-Z111‘ Bedeutung der Ha.ssail’schen Kdrperchen. Archiv f. mikr. Anat., 1903.

31. WATNEY.—The minute anatomy of the thymus. Philos. Transact. of the Royal Society of London, Vol. 173, Part III, 1883 (cited from Pre nant).

EXPLANATION OF PLATES.

All the figures were drawn with Leitz obj. 1/12, 0c. 4, and camera lucida. The magnification after the reduction of the plates is about 1060 diameters. All drawings were made from transverse sections of the mid-cervical segment of ‘the thymus unless they are otherwise indicated.

The following abbreviations designate the structures indicated in all the figures:

c f—colloid in formation. 11; n——large pale nucleus. cl-calcareous deposit. m—nucleus in mitosis. c s——solid colloid. md—beginning of medulla. csd—solid colloid that stains n—nucleus. deeply. 0 c—old colloid. e—erythrocyte. pm—protoplasmic mass. end—endothelial nucleus. sf—-fibril in syncytium. I-—-lymphocyte. " ss~—-space in syncytium. lb—iymphoblast. 12-—va.cuole.

ldn——large dark nucleus.

PLATE I.

FIG. 1. From a 3.7~cm. pig embryo. Stained with iron-haematoxylin and Congo red. Vacuolization of the cytoplasm and differentiation of the nuclei have begun.

FIG. 2. From the thoracic segment of a 4.5-cm. pig embryo. Stained with iron-haematoxylin and Congo red. A cellular reticulum is now formed. Large pale nuclei, lymphoblats, and the large dark intermediate forms are present.

FIG. 3. Epithelioid type of concentric corpuscle. From a 16-cm. pig embryo. Stained with haematoxylin and Congo red. Colloid lamellae (c s d)


separate large areas of clear cytoplasm, causing the appearance of large epithelial cells. Colloid is being formed between the lamellee and around several nuclei.

FIG. 4. From a 8.5-cm. pig embryo. Stained with iron-haamatoxylin (not decolorized). The medulla has appeared. Lymphocytes are present between the epithelial cords.

FIG. 5. From a 7-cm. pig embryo. Stained with iron-haematoxylin and Congo red. A few lymphocytes have been formed. In the cellular reticulum are large pale nuclei, lymphoblasts, and large dark intermediate nuclei. The nuclei in mitosis are very compact.

FIG. 6. From the medulla of a 24-cm. pig embryo. Stained with Jackson’s modification of Mallory’s method (ref. in text). Many fibrillae are seen in the syncytium.

PLATE II.

FIG. 7. Ordinary type of simple concentric corpuscle. From a 16.5-cm. pig embryo. Stained with haematoxylin and Congo red. The corpuscle is well advanced in development. Concentric lamellze of colloid have been formed. The cytoplasm between the lamellae is in an early stage of colloid transformation. Colloid fibers cut transversely appear as dots. The nuclei are becoming flattened by the pressure of expansion. The central mass stains irregularly and all traces of the nuclei in that region are gone.

FIG. 8. Two cystic concentric corpuscles. From a 16-cm. pig embryo. Stained with iron-haematoxylin and Congo red. On the left, a nucleated mass of protoplasm has been separated off by the formation of a vacuole annular in section. This might be mistaken for a blood-vessel containing a nucleated red cell. In this central protoplasmic mass the nucleus is shrunken and the cytoplasm vacuolated. In the small corpuscle on the right, two large vacuoles have formed.

FIG. 9. Cystic concentric corpuscle. From a 14-cm. pig embryo. Stained with haematoxylin and Congo red. The central protoplasmic mass is pale and shrunken. The small circular body in it probably is the remains of the nucleus. Colloid lamellae are forming. Colloid fibers cut transversely appear as dots.

FIG. 10. Cystic concentric corpuscle. From a 10.5-cm. pig embryo. Stained with haamatoxylin and Congo red. The center contains no colloid and seems to be softening. The nucleus is shrunken.

FIG. 11. Ordinary concentric corpuscle in a very early stage. From a 10.5-cm. pig embryo. Stained with iron-haamatoxylin and Congo red. The nucleus is enlarged and colloid is forming around it. A few colloid fibers may be seen in the cytoplasm for some distance from the central nucleus.

FIG. 12. Ordinary concentric corpuscle. Several nuclei are involved. From a 10.5-cm. pig embryo. Stained with hamatoxylln and Congo red. The deeply-staining colloid has completely obliterated the central nucleus (in the region 0 c), and nearly obliterated another (w). some of the colloid now stains Dale (0 c).

Fm. 13. Ordinary concentric corpuscle. From a 10.5-cm. pig. Stained with haamatoxylin and Congo red. The central nucleus is being obliterated 62 The Development of the Thymus

by the deeply-staining colloid. The neighboring nuclei are beginning to show the effect of the centrifugal pressure.

FIG. 14. Ordinary concentric corpuscle in an early stage. From a 10.5-cm. pig. Stained with haematoxylin and Congo red. A band of deeply-staining colloid has been formed. Just outside this is colloid in formation.

FIG. 15. Two simple concentric corpuscles which would have formed a compound concentric corpuscle. From a 10.5-cm. pig. Stained with haematoxylin and Congo red. The left corpuscle is a little more advanced than Fig. 11. The right corpuscle shows a large area of deeply-staining colloid which has pressed the nucleus into a small irregular shape.

PLATE III.

Fm. 16. Compact irregular corpuscle in an early stage. From a 14-cm. pig embryo. Stained with haematoxylin and Congo red. The colloid is not yet solid. The nuclei are not essentially different from those of the adjacent syncytium.

FIG. 17. Ordinary concentric corpuscle. From a 12-cm. pig embryo. Stained with haematoxylin and Congo red. There is a large, central, deeplystaining colloid mass in which calcareous deposits (cl) have been made. The neighboring nuclei show the effects of the centrifugal pressure.

FIG. 18. Epithelioid concentric corpuscle in an early stage. From a. 10.5cm. pig embryo. Stained with haematoxylin and Congo red. The outer colloid lamella marks off a nucleated mass of cytoplasm resembling a large cell. The nucleus is undergoing the same changes as occur in the central nucleus of an ordinary concentric corpuscle.

FIG. 19. Compound concentric corpuscle. From a 10.5-cm. pig embryo. Stained with haematoxylin and Congo red. This would have soon lost all evidence of its compound nature.

FIG. 20. Epithelioid concentric corpuscle. From a 10.5-cm. pig embryo. Stained with haematoxylin and Congo red. Some colloid is forming outside the circular area. Solid deeply-staining colloid is forming.

FIG. 21. Ordinary concentric corpuscle, showing a variation from the usual type. From a 16.5-cm. pig embryo. Stained with iron-haamatoxylin and Congo red. The central nucleus is ‘reddish but not enlarged. No solid colloid has been formed.

FIG. 22. Compound concentric corpuscle. From a 16.5-cm. pig embryo. Three centers are present. The pale colloid in the upper part is probably older than the deeply-staining variety. In the lower part of the figure, a young corpuscle is shown. FIG. 23. Compact irregular corpuscle. From a 16-cm. pig embryo. Stained with haematoxylin and Congo red. Some of the colloid is solid. No definite center is present but one is beginning to form (0 s). The nuclei are not markedly different from those of the adjacent syncytium.



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