Difference between revisions of "Book - Contributions to Embryology Carnegie Institution No.20 part 5"

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==DEVELOPMENT OF THE PERICHONDRIUM==
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{{Carnegie No.20 Header}}
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==Development of the Perichondrium==
  
 
In the description of the development of the periotic reticulum we have seen  
 
In the description of the development of the periotic reticulum we have seen  
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seen how the im-asion or spread of the reticulum into the surrounding area of precartilage is brought about, at least in the later stages, by a dedifferentiation of the  
 
seen how the im-asion or spread of the reticulum into the surrounding area of precartilage is brought about, at least in the later stages, by a dedifferentiation of the  
 
latter into the former.  
 
latter into the former.  
 +
  
 
Furthermore, along with this latter process, the inner margin of cartilage surrounding the duct is dedifferentiated into precartilage, so that a new area of precartilage becomes established as the old area disappears. The conversion of precartilage into reticulum in the later stages, however, is more rapid than the conversion  
 
Furthermore, along with this latter process, the inner margin of cartilage surrounding the duct is dedifferentiated into precartilage, so that a new area of precartilage becomes established as the old area disappears. The conversion of precartilage into reticulum in the later stages, however, is more rapid than the conversion  
 
of cartilage into precartilage, and consefjuently there comes a time when the precartilage has nearly all disappeared. In such specimens the reticuhnn extends  
 
of cartilage into precartilage, and consefjuently there comes a time when the precartilage has nearly all disappeared. In such specimens the reticuhnn extends  
practicall\' from the epithelial duct to the margin of the cartilaginous canal. The  
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practically from the epithelial duct to the margin of the cartilaginous canal. The  
 
(|ualifying term "practically" is used because the inner and outer margins of the  
 
(|ualifying term "practically" is used because the inner and outer margins of the  
 
reticulum are modified in a special manner. The inner margin becomes condensed  
 
reticulum are modified in a special manner. The inner margin becomes condensed  
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that result in the formation of the jjerichondrium.  
 
that result in the formation of the jjerichondrium.  
  
In di.scussing the lu'richondrium it is important to kcej) in imnd the active  
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alterations in the tissue along the margin of the cartilage that accomj^any the  
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In discussing the lu'richondrium it is important to kcej) in imnd the active  
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alterations in the tissue along the margin of the cartilage that accompany the  
 
growth of the labyrinth. It has been seen how the enlargement of the cartilaginous canals and their alterations in form and position is obtained partly by excavation of cartilage and partly by the laying down of new cartilage, the excavation  
 
growth of the labyrinth. It has been seen how the enlargement of the cartilaginous canals and their alterations in form and position is obtained partly by excavation of cartilage and partly by the laying down of new cartilage, the excavation  
being accomplished by its dedifferentiation into ])recartilage and reticulum, and the  
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being accomplished by its dedifferentiation into precartilage and reticulum, and the  
new cartilage being l>uilt up through a i)recartilage stage from the periotic reticular tissue. Throughout the entire period of growth of the cartilaginous canals  
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new cartilage being built up through a precartilage stage from the periotic reticular tissue. Throughout the entire period of growth of the cartilaginous canals  
 
the elements of this continual transformation exist along their margin. The margin  
 
the elements of this continual transformation exist along their margin. The margin  
during this period is in a state of temporarj' eciuilibrium and is cai)able of advancing or receding as the conditions determine.  
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during this period is in a state of temporary eciuilibrium and is capable of advancing or receding as the conditions determine.  
 +
 
  
 
The first and relatively the major part of the hollowing-out of the cartilaginous  
 
The first and relatively the major part of the hollowing-out of the cartilaginous  
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through the lateral canal of a rabbit embryo (fig. 457, page 735), in which this  
 
through the lateral canal of a rabbit embryo (fig. 457, page 735), in which this  
 
zone of precartilage is labeled as periosteum of the future bone.  
 
zone of precartilage is labeled as periosteum of the future bone.  
 +
  
 
The real perichondrium does not make its appearance until the fetus reaches a  
 
The real perichondrium does not make its appearance until the fetus reaches a  
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form. It does not abut directly against the cartilage, but is separated from it by  
 
form. It does not abut directly against the cartilage, but is separated from it by  
 
a thin layer of transition tissue that is in process of dedifferentiation from precartilage into reticulum.  
 
a thin layer of transition tissue that is in process of dedifferentiation from precartilage into reticulum.  
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Passing inward from the cartilage, the transitions are rapid from cartilage to  
 
Passing inward from the cartilage, the transitions are rapid from cartilage to  
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fetus examined, 130 mm. crown-rump length, there is still found a distinct though narrow precartilage-reticular transitional zone between the cartilage and the perichondrium. Presumably this indicates that the margin is still in an unstable  
 
fetus examined, 130 mm. crown-rump length, there is still found a distinct though narrow precartilage-reticular transitional zone between the cartilage and the perichondrium. Presumably this indicates that the margin is still in an unstable  
 
condition.  
 
condition.  
 +
  
 
After the perichondrium has  
 
After the perichondrium has  
 
made its first appearance it rapidly becomes thicker and more  
 
made its first appearance it rapidly becomes thicker and more  
conspicuous. In a fetus 80 mmcrown-rump length (Carnegie Collection, No. 172) it is found as  
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conspicuous. In a fetus 80 mm crown-rump length (Carnegie Collection, No. 172) it is found as  
 
quite a dense fibrous coat, more  
 
quite a dense fibrous coat, more  
 
than twice as thick as that shown  
 
than twice as thick as that shown  
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cartilage and precartilage by a  
 
cartilage and precartilage by a  
 
narrow zone of reticular tissue.  
 
narrow zone of reticular tissue.  
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The character of the perichondrium as existing in slightly  
 
The character of the perichondrium as existing in slightly  
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coat to tho epithelial duct.  
 
coat to tho epithelial duct.  
  
\\'hen one examines the cartilaginous semicircular canals in fetuses 130 mm.
 
long there can no longer be any ([uestion as to the identitj' of the perichondrium.
 
A specimen showing the superior semicircular canal at this stage is represented in
 
figure 19, which is taken from a fetus 130 mm. crown-rump length (Carnegie Collection, No. 1018). The blood-vessels are injected with India ink. The main cartilaginous mass in this specimen is (luite mature; the capsules are well defined and
 
the cartilage cells now possess a considerable amount of granular bodj'-protoplasm.
 
  
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When one examines the cartilaginous semicircular canals in fetuses 130 mm. long there can no longer be any ([uestion as to the identity of the perichondrium.
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A specimen showing the superior semicircular canal at this stage is represented in figure 19, which is taken from a fetus 130 mm. crown-rump length (Carnegie Collection, No. 1018). The blood-vessels are injected with India ink. The main cartilaginous mass in this specimen is quite mature; the capsules are well defined and the cartilage cells now possess a considerable amount of granular body-protoplasm.
  
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<div id="Fig004"></div>
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[[File:Streeter004.jpg]]
  
Cartilage
 
  
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'''Fig. 4. Detail of the posterior canal in a human fetus 73 mm. long'''
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:(Carnegie Collection, No. 1373, slide 9, row 3. section 1) The section is 10 microns thick and is enlarged 370 diameters (in original printed version). It shows how the inner margin of the reticulum becomes condensed into the membrane propria of the epithelial duct and the outer margin into the perichondrium. The perichondrium does not lie in direct contact with the cartilage, but is separated by a narrow zone of tissue which consists of precartilage, into which the cartilage is still being dedifferentiated.
  
  
Flu. 4. — Detail of the posterior canal in a human fetus 73 iniii. long
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In many instances capsules are found containing more than one cartilage cell, showing the tendency to cell columns.  
(Carnegie Collection, No. 1373, slide 0, row 3. section 1)'
 
The section is lOiU thick and is enlarged 370 diameters. It
 
shows how the inner margin ()f< the reticulum becomes con(hm.sed into the niembnina projiria of the epithelial duct and
 
the outer margin into I he iicricliondrium. The perichondrium
 
does not lie in direct cdiitact with the cartilage, hut is separated by a narrow zone of tissue which consists of precartilage,
 
into which the cartilage is still being dedifferentiated.  
 
  
In many instances capsules are found containing more than one cartilage cell,
 
showing the tendency to cell columns.
 
  
A casual glance at a section under lower powers might indicate that the inner  
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A casual glance at a section under lower powers might indicate that the inner margin of the cartilage is in direct contact with the perichondrium. Examination under higher magnification, however, shows that between the thick perichondrium and the cartilage there is a narrow zone of dedifferentiated cartilage. In it the matrix has largely disappeared and the capsules have collapsed and are flattened out, allowing the elongated endoplasm of adjacent cartilage cells to come in contact, separated only by the remnants of the capsular margins. Dyes that stain endoplasm red cause this zone to appear as a deep-red line. This zone represents a state of transition between cartilage and precartilage and its presence doubtless indicates that the margin of the cartilage is still in an unstable condition. The narrowness of the zone and the abruptness of the transition are characteristic of later stages, where the process is more gradual and relatively small in amount.  
maigin of the cartilage is in direct contact with the perichondrium. Examination  
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The transition from this zone to the perichondrium is likewise abrupt. The perichondrium consists of a dense protoplasmic stratum thickly studded with nuclei, and has all the appearance of late embryonic fibrous connective tissue. It is of about the same tliickness around the whole margin of the canal. At the outer margin (toward the right) it fuses wdth the membrana propria of the epithelial duct, therebyforming an attachment which is regarded as a suspensory ligament for the support of the membranous labyrmth. The trabeculae of the reticulum  
under higher magnification, however, shows that between the thick perichondrium  
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extending between the membrana propria and the perichondrium are just beginning to break apart, allowing the adjacent spaces of the reticulum, as they are seen in section, to coalesce in the formation of larger spaces.  
and the cartilage there is a narrow zone of dedifferentiated cartilage. In it the  
 
matrix has largely disappeared and the capsules have collapsed and are flattened  
 
out, allowing the elongated endoplasm of adjacent cartilage cells to come in contact, separated only by the remnants of the capsular margins. Dyes that stain  
 
endoplasm red cause this zone to appear as a deep-red line. This zone represents  
 
a state of transition between cartilage and precartilage and its presence doubtless  
 
indicates that the margin of the cartilage is still in an unstable condition. The  
 
narrowness of the zone and the abruptness of the transition are characteristic of  
 
later stages, where the process is more gradual and relatively small in amount.  
 
The transition from this zone to the perichondrium is likewise abrupt. The perichondrium consists of a dense protoplasmic stratum thickly studded with nuclei,  
 
and has all the appearance of late embryonic fibrous connective tissue. It is of  
 
about the same tliickness around the whole margin of the canal. At the outer  
 
margin (toward the right) it fuses wdth the membrana propria of the epithelial  
 
duct, therebj' forming an attachment which is regarded as a suspensory ligament  
 
for the sujjport of the membranous labyrmth. The trabeculae of the reticulum  
 
extending between the membrana propria and the perichondrium are just beginning to break apart, allowing the adjacent spaces of the reticulum, as they are seen  
 
in section, to coalesce in the formation of larger spaces.  
 
  
Having completed the review of the early history of the reticulum and its
 
formative relations to the adjacent tissues, we are now in a position to consider
 
the development and the fate of these larger spaces in the reticulum, which ha\'e
 
hitherto been generally known by the misleading term "perilymphatic spaces."
 
  
==DEVELOPMENT OF PERIOTIC TISSUE— SPACES==
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Having completed the review of the early history of the reticulum and its formative relations to the adjacent tissues, we are now in a position to consider the development and the fate of these larger spaces in the reticulum, which have hitherto been generally known by the misleading term "perilymphatic spaces."
  
In the }3receding pages of this article the main features of the development of  
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==Development of Periotic Tissue Spaces==
 +
 
 +
 
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In the preceding pages of this article the main features of the development of  
 
the cartilaginous capsule that incloses the membranous labyrinth have been  
 
the cartilaginous capsule that incloses the membranous labyrinth have been  
 
described. We have traced the process step by step from the first condensation  
 
described. We have traced the process step by step from the first condensation  
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as they will be referred to in this paper, the development of which will now he  
 
as they will be referred to in this paper, the development of which will now he  
 
outlined.  
 
outlined.  
 +
  
 
Thus far attention has been directed primarily to regions included in typical  
 
Thus far attention has been directed primarily to regions included in typical  
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there, and even in their completed form they are not so well defined and highly  
 
there, and even in their completed form they are not so well defined and highly  
 
differentiated as those in the region of the vestibule and cochlea.  
 
differentiated as those in the region of the vestibule and cochlea.  
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The earliest evidence of a periotic space makes its appearance opposite the  
 
The earliest evidence of a periotic space makes its appearance opposite the  
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reticulum becomes very extensive, in embryos between 30 and 40 mm. long, it can  
 
reticulum becomes very extensive, in embryos between 30 and 40 mm. long, it can  
 
be seen that its meshes are more irregular and more open in this region than elsewhere. This is the rudimentary form of the periotic vestibular cistern, which is  
 
be seen that its meshes are more irregular and more open in this region than elsewhere. This is the rudimentary form of the periotic vestibular cistern, which is  
the first space to become established.  
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the first space to become established.
 +
 
 +
 
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==Development of the Periotic Cistern of the Vestibule==
  
==DEVELOPMENT OF THE PERIOTIC CISTERN OF THE VESTIBULE==
 
  
 
Aside from the scala vestibuli and the scala tympani, the largest of the periotic  
 
Aside from the scala vestibuli and the scala tympani, the largest of the periotic  
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perilymphatica (Retzius). In order to eliminate the word lymphatic from the  
 
perilymphatica (Retzius). In order to eliminate the word lymphatic from the  
 
terminology it will be designated here as the cisterna periotica vestibuli, or less  
 
terminology it will be designated here as the cisterna periotica vestibuli, or less  
formally the j^eriotic cistern. In this manner the descriptive term introduced by  
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formally the periotic cistern. In this manner the descriptive term introduced by  
 
Retzius is retained.  
 
Retzius is retained.  
 +
  
 
Before there is any trace of the scalse the initial steps in the formation of the  
 
Before there is any trace of the scalse the initial steps in the formation of the  
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the size of the mesh seems to be attained by the detachment and retraction of its  
 
the size of the mesh seems to be attained by the detachment and retraction of its  
 
constituent i)rotoplasmic bridges, thereby allowing adjacent spaces to unite in the  
 
constituent i)rotoplasmic bridges, thereby allowing adjacent spaces to unite in the  
formation of (•om])osite large spaces. Thus in the above section a few irregular  
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formation of composite large spaces. Thus in the above section a few irregular  
protopla.smic free-ends are seen still jjrojecting into the newly enlarged spaces.  
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protopla.smic free-ends are seen still projecting into the newly enlarged spaces.  
This interesting histogenetic process will l)e taken up again later in connection with the development of the two scalae. The area of this rudimentary periotic cistern  
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This interesting histogenetic process will be taken up again later in connection with the development of the two scalae. The area of this rudimentary periotic cistern  
 
is as yet very small and merges indefiniteh' into the adjoining reticulum. It is  
 
is as yet very small and merges indefiniteh' into the adjoining reticulum. It is  
 
not until we come to fetuses about 40 mm. long that it develops spaces of any considerable size, and it is not until we come to fetuses about 50 mm. long that we find  
 
not until we come to fetuses about 40 mm. long that it develops spaces of any considerable size, and it is not until we come to fetuses about 50 mm. long that we find  
a single large space with walls that are definitely outlined, so that it can be satisfactorilj' modeled.  
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a single large space with walls that are definitely outlined, so that it can be satisfactorily modeled.  
 +
 
  
 
In a fetus 43 mm. long (Carnegie Collection, No. 886), which is cut in a coronal  
 
In a fetus 43 mm. long (Carnegie Collection, No. 886), which is cut in a coronal  
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now just opposite the stapes one space which is much larger than the adjoining  
 
now just opposite the stapes one space which is much larger than the adjoining  
 
spaces. On part of its margin the protoplasmic bridges are stretched along so as to  
 
spaces. On part of its margin the protoplasmic bridges are stretched along so as to  
form a smoothly curved continuous boundarj-, which is defective in some portions,  
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form a smoothly curved continuous boundary, which is defective in some portions,  
 
and at such places the space merges with the adjoining secondary spaces. Within  
 
and at such places the space merges with the adjoining secondary spaces. Within  
 
the space are some fainth' refractive branching threads of coagulated plasma. The  
 
the space are some fainth' refractive branching threads of coagulated plasma. The  
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fenestra cochleae (rotunda), along the basal border of the first turn of the cochlear  
 
fenestra cochleae (rotunda), along the basal border of the first turn of the cochlear  
 
duct.  
 
duct.  
 +
  
 
In fetuses 50 mm. long the outlines of the cistern become very distinct, due to  
 
In fetuses 50 mm. long the outlines of the cistern become very distinct, due to  
 
the marked increase in the size of its main cavity and to the more definite membrane  
 
the marked increase in the size of its main cavity and to the more definite membrane  
 
at its junction with the rest of the reticulum. Its form and relations are shown in  
 
at its junction with the rest of the reticulum. Its form and relations are shown in  
figures 26 and 27. Thej- represent a median and a lateral view of a wax-plate  
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figures 26 and 27. They represent a median and a lateral view of a wax-plate  
 
reconstruction of this region in a human fetus 50 mm. long (Carnegie Collection,  
 
reconstruction of this region in a human fetus 50 mm. long (Carnegie Collection,  
Xo. 84). Onlj^ the main cavity is shown in the model. At certain places around  
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Xo. 84). Only the main cavity is shown in the model. At certain places around  
 
its borders the meshes of the reticulum are uniting into larger spaces and these in  
 
its borders the meshes of the reticulum are uniting into larger spaces and these in  
 
turn are taken up by the main cavity as it advances into the new territory. These  
 
turn are taken up by the main cavity as it advances into the new territory. These  
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border should be traced on the plates and included in the model. This rule was  
 
border should be traced on the plates and included in the model. This rule was  
 
adhered to in all the models of this series.  
 
adhered to in all the models of this series.  
 +
  
 
Figures 26 and 27 show that the periotic cistern in 50-mm. embryos consists  
 
Figures 26 and 27 show that the periotic cistern in 50-mm. embryos consists  
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the stapes to the cistern. The scala tymi)ani is already well started at this time,  
 
the stapes to the cistern. The scala tymi)ani is already well started at this time,  
 
but its development is quite independent of the cistern. Within the cistern can  
 
but its development is quite independent of the cistern. Within the cistern can  
be seen scattered clumi:)s of faintly refractive granular threads of what seems to be  
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be seen scattered clumps of faintly refractive granular threads of what seems to be  
 
a coagulated constituent of the plasma.  
 
a coagulated constituent of the plasma.  
 +
  
 
The subsequent growth of the cistern is shown in figures 28 to 31. Figures 28  
 
The subsequent growth of the cistern is shown in figures 28 to 31. Figures 28  
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inner side of the jjostcrior end of the lateral semicircular duct. Ventrally it communicates freely with the scala vestibuli, which now extends well down along the  
 
inner side of the jjostcrior end of the lateral semicircular duct. Ventrally it communicates freely with the scala vestibuli, which now extends well down along the  
 
cochlear duct.  
 
cochlear duct.  
 +
  
 
The oldest stage studied is shown in figures 30 and 31. These show two views  
 
The oldest stage studied is shown in figures 30 and 31. These show two views  
 
of a wax-plate reconstruction of these structures in a human fetus 130 mm. long  
 
of a wax-plate reconstruction of these structures in a human fetus 130 mm. long  
 
(Carnegie Collection, No. 1018). At this time the periotic cistern has spread over  
 
(Carnegie Collection, No. 1018). At this time the periotic cistern has spread over  
the vestibular part of the meml>ranous labyrinth, covering it nearly eveiywhere  
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the vestibular part of the membranous labyrinth, covering it nearly eveiywhere  
 
excepting at the macular jiortions where the nerves terminate. In figure 31 it can  
 
excepting at the macular jiortions where the nerves terminate. In figure 31 it can  
 
be seen that the mesial surface of the saccule is not covered ; this lies close against  
 
be seen that the mesial surface of the saccule is not covered ; this lies close against  
 
the wall of the cartilaginous vestibule. The uppermost division of the cistern,  
 
the wall of the cartilaginous vestibule. The uppermost division of the cistern,  
 
situated between the crus commune and the ampulla of the posterior semicircular  
 
situated between the crus commune and the ampulla of the posterior semicircular  
duct, does not yet open into the general cavit3^ It has formed separately and owing  
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duct, does not yet open into the general cavity It has formed separately and owing  
 
to the i)osition in which it lies its coalescence with the other parts of the cistern is  
 
to the i)osition in which it lies its coalescence with the other parts of the cistern is  
retarded ; otherwise, free communication exists between all divisions of the cistern.  
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retarded ; otherwise, free communication exists between all divisions of the cistern.
  
==DEVELOPMENT OF THE PERIOTIC SPACES OF THE SEMICIRCULAR DUCTS==
 
  
From the descriptions given of the adult the reticulum along the ducts never
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==Development of the Periotic Spaces of the Semicircular Ducts==
develops a single continuous wide periotic space like that of the cistern and the
 
two scala?. There always remain a few trabecular, such as are seen in the cistern
 
and scala? in their earlier stages, and these constitute partitions which traverse the
 
space and give it in sections the a])pearance of a series of sej^arate spaces extending
 
along the inner margins of the semicircular ducts. Although these spaces along
 
the ducts are inc()mi)lete as compaicd with the cistern and scahc, they are, however,
 
entirely analogous with them in their formation.
 
  
The space along the lateral semicircular duct is the largest. Its posterior end
 
exists as a continuation of the cistern. This can be seen in the lateral view of the
 
model shown in figure 30, where the cistern extends for a considerable distance along the inner border of the lateral duct. Along the other two ducts of the same
 
specimen (130 mm. crown-rump length) the reticulum has commenced the process
 
of space-formation, but complete channels are not yet established. A typical
 
section through one of the semicircular ducts in a fetus of this size, and this is the
 
oldest fetus studied, is shown in figure 23. As compared with the scalse in the
 
same fetus, as shown in figure 20, the space-formation along the ducts is very much
 
retarded.
 
  
==DEVELOPMENT OF SCALA TYMPANI AND SCALA VESTIBULI==
+
From the descriptions given of the adult the reticulum along the ducts never develops a single continuous wide periotic space like that of the cistern and the two scala?. There always remain a few trabecular, such as are seen in the cistern and scala? in their earlier stages, and these constitute partitions which traverse the space and give it in sections the a])pearance of a series of separate spaces extending along the inner margins of the semicircular ducts. Although these spaces along the ducts are inc()mi)lete as compaicd with the cistern and scahc, they are, however, entirely analogous with them in their formation.
  
The scala vestibuli may be regarded as an extension of the cistern downward
 
into the region of the cochlea and as such its growth starts from a focus opposite
 
the fenestra vestibuli (ovahs) . The scala tympani in a similar way makes its first
 
appearance opposite the fenestra cochleae. From these two foci the scalae extend
 
gradually downward along the cochlear duct as two separate spaces which do not
 
communicate with each other until they reach the tip of the duct, where there is
 
finallj^ developed a free opening between them known as the helicotrema.
 
  
In their formation they go through a series of histogenetic changes essentially
+
The space along the lateral semicircular duct is the largest. Its posterior end exists as a continuation of the cistern. This can be seen in the lateral view of the  
in the same manner that has been followed in the case of the formation of the cistern ; this (as we shall see) consists of the enlargement of the spaces of the periotic
+
model shown in figure 30, where the cistern extends for a considerable distance along the inner border of the lateral duct. Along the other two ducts of the same specimen (130 mm. crown-rump length) the reticulum has commenced the process of space-formation, but complete channels are not yet established. A typical section through one of the semicircular ducts in a fetus of this size, and this is the oldest fetus studied, is shown in figure 23. As compared with the scalse in the same fetus, as shown in figure 20, the space-formation along the ducts is very much retarded.
reticulum that originally occupied this region, the enlargement being a result of  
 
the disappearance of the protoplasmic bridges of the reticulum, whereby adjacent
 
spaces unite in the formation of composite larger spaces. This process continues
 
until there is a single continuous space extending down along the cochlear duct
 
representing each scala and at the margins of each of them there is developed a
 
membranous arrangement of the reticular cells which completely walls ofif the space  
 
from the surrounding tissue. In these alterations in the reticular mesh and in the
 
formation of the surrounding membrane there is an active change in the form of
 
the reticular cells, which repeatedly adapt themselves to the new conditions. There
 
is no evidence to indicate that smy other cells take part in the formation of the scalae.  
 
  
The first evidence of the formation, of scalae is found in fetuses about 40 mm.
 
long, which stage is a little later than the first appearance of the cistern. In a fetus
 
43 nmi. crown-rump length (Cargnegie Collection, No. 886), along the proximal
 
part of the cochlear duct on its basal surface there is a distinct widening of the
 
meshes of the periotic reticulum. This is the beginning of the scala tj^mpani.
 
On the opposite side of the cochlear duct, where one would look for the scala vestibuh, the periotic reticulum retains its primitive appearance characterized by a
 
narrow and rather uniform mesh. Thus the scala tjTnpani makes its appearance
 
slightly in advance of the scala yestibuli — that is, if we regard the latter as distinct
 
from the cistern.
 
  
In fetuses 50 mm. long both the scala tympani and the scala vestibuli can be
+
==Development of Scala Tympani and Scala Vestibuli==
plainly identified, although they are still very incomplete. A wax-plate reconstruction of them, rejiresenting their form and their relation to the membranous
 
labyrinth in a human fetus 50 mm. crown-rump length (Carnegie Collection, No.
 
84), is shown in figures 26 and 27, being a median and a lateral view respectively.
 
  
In preparing this and tlie models shown in figures 28 to 31, it is to be remembered
 
that only those periotic spaces are included that were outlined by a membranelike margin. In the adjacent reticulum there are spaces that are actively coalescing and gradually uniting with the main cavity. No attempt, however, was
 
made to show such spaces in the models. From figures 26 and 27 it will be seen
 
that the scala tympani is larger and more advanced in its development than the
 
scala vestibuli. The hitter is in its earliest stage and consists of hardly more than
 
a row of enlarged reticular spaces which extend downward from the cistern along
 
the dorsal and apical surface of the cochlear duct. A section through the scala
 
vestibuli in another fetus of about the same age (Carnegie Collection, No. 448) is
 
roughly shown in figure 21, the spaces of the scala being situated between the
 
cistern and the cochlear duct.
 
  
The scala tympani consists of an elongated oval space lying along the basal
+
The scala vestibuli may be regarded as an extension of the cistern downward into the region of the cochlea and as such its growth starts from a focus opposite the fenestra vestibuli (ovalis) . The scala tympani in a similar way makes its first appearance opposite the fenestra cochleae. From these two foci the scalae extend gradually downward along the cochlear duct as two separate spaces which do not communicate with each other until they reach the tip of the duct, where there is finally developed a free opening between them known as the helicotrema.  
surface of the proximal jiart of the cochlear duct, about corresponding to the proximal half of the first turn of the duct. In the main part it is a single space with a
 
distinct margin separating it from the general periotic reticulum. In the more apical
 
portion it tapers off into multiple incompletely united smaller spaces which actively
 
coalesce as the process advances into the new territory along the duct. It is of  
 
interest to note that the most mature and the largest part of this scala, representing
 
the focus at which it first appeared, is opposite the fenestra cochleae (rotunda),
 
just as the cistern forms opposite the stapes and the fenestra vestibuli. The scala
 
tympani alw-aj's begins at the same place and extends downward along the cochlear  
 
duct, at first a Uttle in advance of the scala vestibuli, but subsequenth^ the latter
 
catches up with it and the two reach the tip of the duct at about the same time.  
 
  
It is well known that the proximal portions of the cochlear duct mature sooner
 
than the distal portions. One might expect that the accompanying periotic spaces
 
would correspond in their development to the maturity of the duct and therefore
 
the proximal parts of the scalse would differentiate first. In other words, the
 
maturation of the cochlea proceeds as a wave from the proximal end to its tip,
 
involving all of its constituent structures as it passes along, including mesenchymal
 
j)arts as well as epithelial.
 
  
This conception might exi)lain the direction of development of the scalae, but can
+
In their formation they go through a series of histogenetic changes essentially in the same manner that has been followed in the case of the formation of the cistern ; this (as we shall see) consists of the enlargement of the spaces of the periotic reticulum that originally occupied this region, the enlargement being a result of the disappearance of the protoplasmic bridges of the reticulum, whereby adjacent spaces unite in the formation of composite larger spaces. This process continues until there is a single continuous space extending down along the cochlear duct representing each scala and at the margins of each of them there is developed a membranous arrangement of the reticular cells which completely walls ofif the space from the surrounding tissue. In these alterations in the reticular mesh and in the formation of the surrounding membrane there is an active change in the form of the reticular cells, which repeatedly adapt themselves to the new conditions. There is no evidence to indicate that smy other cells take part in the formation of the scalae.  
hardly be ai:)plied to the cistern, the vestibular repres(>ntative of the scala vestibuli.
 
One can not say that those portions of the membranous labyrinth lying opi^osite the focus of development of the cistern (that is, the lateral walls of the saccule
 
and utricle) mature in advance of the rest of the labyrinth. There is no indication
 
that a wave of differentiation passes through the epithelial elements of the labyrinth in the same direction and sj'nchronously with the extension of the cistern
 
as it advances from its j)rimary focus u])()n the roof of the utricle and over on its
 
median surface. In the case of the ci.stern it seems much more likely that the point
 
at which it first apjiears is determined by the position of the stai)es, which is doubtless an expression of the physical relation that subsequently exists between the two.  
 
Hy analogy this would yield additional significance to the relation existing between
 
the fenestra cochlea; and the point of beginning development of the scala tympani.  
 
  
In dealing with the cistern and also with the scalse one should not consider
 
them as insignificant accessories that merely fill in the waste intervals between the
 
membranous labyrinth and the surrounding cartilage. From stud3dng their development it becomes apparent that they have a morphological individuality in many
 
respects as definite as that of the ossicles themselves. They make their appearance at a definite time and at definite places, they spread in a definite manner, and
 
eventually they attain a form and structure that are adapted to a definite function.
 
This becomes more and more evident as we examine older stages.
 
  
The form and relations of the scalie in fetuses between 12 and 13 weeks old are
+
The first evidence of the formation, of scalae is found in fetuses about 40 mm. long, which stage is a little later than the first appearance of the cistern. In a fetus 43 mm. crown-rump length (Carnegie Collection, No. 886), along the proximal part of the cochlear duct on its basal surface there is a distinct widening of the meshes of the periotic reticulum. This is the beginning of the scala tympani. On the opposite side of the cochlear duct, where one would look for the scala vestibule, the periotic reticulum retains its primitive appearance characterized by a narrow and rather uniform mesh. Thus the scala tympani makes its appearance slightly in advance of the scala yestibuli that is, if we regard the latter as distinct
shown in figures 28 and 29. These figures show median and lateral views of a waxplate reconstruction of the membranous labyrinth and the surrounding periotic
+
from the cistern.  
spaces in a human fetus 85 mm. crown-rump length (Carnegie Collection, Xo.  
 
1400-30). Attention has already been directed to these figures in the description
 
previously given of the cistern. The scala vestibuli can be seen in figure 28. Above,  
 
it opens freely into the cistern and extends downward along the apical side of the  
 
duct as a single main space, possessing a rather uniform diameter. It extends
 
along the first two turns of the duct, gradually tapering off and showing a less
 
mature character in its distal portions. Along the second turn of the duct the
 
spaces are incompletely fused and the contour becomes correspondingly irregular.  
 
As a rule the peripheral margin of the scala is less mature and more irregular than
 
the central margin. The scala, vestibuli does not connect with the scala tjTiipani
 
at any point as yet. The two are separated in the first place by the cochlear duct
 
and then more centrally b}- a framework of connective tissue in which are the
 
radiating bundles of the cochlear nerve with the nodes of ganglion cells that form
 
the spinal gangUon. These latter structures are not shown in the model; they
 
occupy, however, the V-shaped groove seen between the two scalae.  
 
  
The scala tj^mpani, as can be seen in figure 29, extends downward on the basal
 
side of the cochlear duct along its first two turns. This corresponds to about the
 
same linear dimension as that of the scala vestibuli. In its proximal portion it
 
shows a greater area in cross-section than the latter, but further toward the apical
 
region it is of about the same size and in some places it is even smaller. The peripheral margin of the scala tympani is distinctly more irregular than the central
 
margin. The irregularity is due to spaces along this margin that are actively
 
coalescing with the main sjiace, but in which the fusion is not yet complete. The
 
irregularity of this margin is thus an indication of the direction of the expansion of
 
the scala. As the diameter of the whole cochlear mass increases, it is evident that the
 
main growth of the scala must radiate outward in a peripheral direction. This is
 
accomplished by the continual assimilation of new reticular spaces along this margin.
 
At the proximal end of the scala tympani can be seen an oval depression which
 
corresponds to the fenestra cochleie (rotunda) and with which it stands in intimate
 
relation.
 
  
In fetuses about 16 weeks old the form and relations of the scalae have nearly
+
In fetuses 50 mm. long both the scala tympani and the scala vestibuli can be plainly identified, although they are still very incomplete. A wax-plate reconstruction of them, representing their form and their relation to the membranous labyrinth in a human fetus 50 mm. crown-rump length (Carnegie Collection, No. 84), is shown in figures 26 and 27, being a median and a lateral view respectively.
attained the adult conditions, and this represents the oldest stage studied in connection with the present paper. The conditions found at this time are shown in figures 30 and 31, which present nuMhan and lateral views of a wax-plate model
 
of a human fetus 130 mm. crown-rump length (Carnegie Collection, No. 1018).
 
On comparing the scala tympani and scala vestibuli as seen in these figures with
 
those in figures 28 and 29, it will be seen that they are larger in cross-section and
 
more nearlj- cover the cochlear duct. Furthermore, they now extend to the extreme
 
tip of the duct and communicate with each other across its central margin, thus
 
forming a helicotrema. A section through this point can be seen in figure 25, in
 
which these structures are shown as seen under low magnification. It will be
 
noted that now, even as far as the tip of the cochlea, each of the scalse consists of
 
a continuous principal space, though both are more mature and larger in their
 
proximal portions. Along the first turn of the cochlear duct they are walled off
 
by a smooth membranous margin which separates them from the adjacent reticular
 
tissue. The spaces of the latter do not seem to be taking anj^ further part in the
 
I)rocess of enlargement of the scala". Along the second turn of the cochlear duct,  
 
a section of which is shown in figure 20, the coalescence of reticular spaces w'ith
 
each other and with the scalse is still in active operation. This produces a greater
 
irregularity of the scalar than is shown in the model. The subsidiary spaces are
 
shown as a solid mass; the slender clefts separating them are not represented. The
 
nearer one a})proaches the tip of the duct the more immature are the scalse, until
 
the condition is reached that is shown in figure 25, where the membrane-like margin
 
is {[uite incomplete and the spaces merge irregularly with the surrounding reticulum.
 
Thus a single specimen, if studied in its different parts, shows several stages in this
 
interesting process of the formation and growth of the scalaj.  
 
  
The figures grouped on plate 3 illustrate some of the histological features
 
of this process. An early stage in space-formation is shown in figure 23. This
 
is a section through the canal region where the changes in the reticulum are late
 
in making their appearance. In fact, the periotic spaces never reach the same
 
degree of differentiation here that occurs in the case of the cistern and scalse. The
 
initial steps, how-evcr, are the same, and this figure presents very w^ell the appearance of the periotic reticulum as it begins to open up into larger spaces. Unmodified reticulum is characterized by a rather uniform narrow mesh. The essential
 
change in space-formation consists in the disappearance of some of the trabeculse
 
of the mesh, with the consequent coalescence of the corresponding adjacent spaces.
 
The trabecuhe consist of the protoplasmic processes of the constituent cells of the
 
reticulum and their disappearance is to be explained in either of two ways: It is
 
I)ossiblc that owing to some property of the fiuid element of the tissue the protoplasmic strands are dissolved or liquefied; this would account for their complete
 
disajipearance. On the other hand, the same result could be accomj)lished by an
 
alt(.'ration in the form of the cell processes. A given trabecula could separate at
 
either end, or at some jioint along its line, and the free ends of protoplasm could
 
then retract and reshape themselves and become a part of the remaining framework. Whether we are dealing with a licjuefaction of tissue or with active motility
 
of the cell, protoplasm involviiig detachment and retraction of the trabeculae can not be definitel}^ determined by observations of fixed tissue; but the appearance
 
of sections where the process is in active operation seems to the writer to indicate
 
the latter.
 
  
In the above paragraph and elsewhere in this paper reference is made to  
+
In preparing this and tlie models shown in figures 28 to 31, it is to be remembered that only those periotic spaces are included that were outlined by a membranelike margin. In the adjacent reticulum there are spaces that are actively coalescing and gradually uniting with the main cavity. No attempt, however, was made to show such spaces in the models. From figures 26 and 27 it will be seen that the scala tympani is larger and more advanced in its development than the scala vestibuli. The hitter is in its earliest stage and consists of hardly more than a row of enlarged reticular spaces which extend downward from the cistern along the dorsal and apical surface of the cochlear duct. A section through the scala vestibuli in another fetus of about the same age (Carnegie Collection, No. 448) is roughly shown in figure 21, the spaces of the scala being situated between the cistern and the cochlear duct.  
trabeculse ser^dng as "partitions" between "spaces" and the disappearance of
 
trabeculse resulting in the "coalescence of adjacent spaces." In making this use
 
of the term "space" it should be explained that it is done in a descriptive sense, in
 
application to the appearance of the tissue as seen in sections in which form human
 
embryological material is mainly available. In thin sections of a reticular tissue
 
one sees trabecule as partitions separating adjacent spaces. The same tissue in
 
a mass would show that the spaces everywhere communicate freely with each other,  
 
hke the spaces in a sponge, and that the trabeculse are thread-like strands which
 
at the best are very incomplete partitions. Instead of a meshwork containing
 
manj' small spaces, one could perhaps equalh' well describe reticular tissue as a
 
single large space traversed by mam^ trabeculse. If the latter practice were adopted,
 
one would describe the development of the tissue-spaces with which we are concerned as a process of gradual decrease in the number of traversing trabeculse,  
 
with the result that the mesh thereby becomes coarser. For descriptive purposes,
 
however, it is convenient to refer to the intervals between the strands of the mesh
 
as spaces, at the same time not granting them the significance that is attached to
 
such membrane-Uned tissue-spaces as are represented by the vestibular cistern  
 
and the two scalse, though the latter are in reaUty derived from them.  
 
  
In figure 23 the free detached ends of the trabeculse will be noted everywhere,
 
as is characteristic of this stage of development. It is a necessarj^ step in the coalescence of adjacent spaces. The detached trabeculse seem to be gradually retracting
 
and adapting themselves to the formation of larger spaces. Their constituent
 
protoplasm reshapes itself as a smooth border or as a part of other trabeculse.
 
Larger spaces necessitate longer trabeculse, and as trabecuUe become longer they
 
also tend to become heavier. These phenomena are all in evidence in the spreading
 
and enlargement of the scalse.
 
  
Figure 20 shows a characteristic view of the scalse as seen under low magnification. It will be noted that the scala vestibuli is relatively mature at this
+
The scala tympani consists of an elongated oval space lying along the basal surface of the proximal part of the cochlear duct, about corresponding to the proximal half of the first turn of the duct. In the main part it is a single space with a distinct margin separating it from the general periotic reticulum. In the more apical portion it tapers off into multiple incompletely united smaller spaces which actively coalesce as the process advances into the new territory along the duct. It is of interest to note that the most mature and the largest part of this scala, representing the focus at which it first appeared, is opposite the fenestra cochleae (rotunda), just as the cistern forms opposite the stapes and the fenestra vestibuli. The scala tympani always begins at the same place and extends downward along the cochlear duct, at first a little in advance of the scala vestibuli, but subsequently the latter catches up with it and the two reach the tip of the duct at about the same time.  
point; the scala tympani, however, is in the act of spreading peripherally, so as to
 
underUe, as it eventually will do, the future basilar membrane. The scala tympani
 
finally reaches the peripheral margin of the cochlear duct, and it does this by the  
 
coalescence of the enlarging reticular spaces, which become incorporated with the
 
main cavity of the scala. This can be observed better in figure 22, which shows a  
 
detail of the same section as seen under higher magnification. By comparing this
 
figure with figure 20 the exact location can be readily made out. A portion of the  
 
main cavity of the scala is indicated and to the right of this are a few enlarged
 
reticular spaces that are uniting with each other and will in the end become part  
 
of the main space. In adcUtion to the enlarged reticular spaces there is a certain
 
amount of residual undifferentiated reticulum. It is this tissue that will play the  
 
part of an adventitial coat to the completed scala. The trabecula? that separate
 
the enlarged spaces seem to be under tension and about ready to snap apart. In
 
fact, in most sections one can see the fragmentary ends of trabeculse where this
 
interruption of continuity has apparently occurred.  
 
  
The differentiation of the margin of the scalse constitutes the final feature in
 
their maturation. During the i)eriod in which the enlargement of an individual
 
scala is being brought about by the coalescence f)f enlarging reticular spaces, the
 
margins of the main cavity can be seen to consist of smooth, delicate strands of
 
nucleated protoplasm that resembles in all essentials that of the trabeculae between
 
the large reticular spaces. These linear margins are interrupted here and there
 
by openings into adjacent spaces, but they tend to form a continuous line that
 
definitely marks off the space from the adjacent reticulum. An early stage in the
 
formation of such a margin is shown in figure 25, where the margin is indicated at
 
a few places, but for the most i)art the space abuts against the surrounding ragged
 
reticulum. The margin of the space is more complete in the scala tympani shown
 
in figure 22, but it is still thin and delicate and can be easil}^ opened up to allow the
 
taking in of new spaces. If we examine the borders of more mature spaces we find
 
them inclosed by a firmer membrane, which finally reaches a state that will probably
 
not admit of any further oi)ening up for the coalesence of additional spaces. Any
 
further growth must thereafter be limited to simple distention of the wall of the
 
space with the consequent adjustment of its constituent cells. Such a condition
 
is represented in figure 24. This shows a more mature section of the wall of the
 
scala vestibuli, being a detail of the same section shown in figure 20. The only
 
difference between such a membrane, as we must now call it, and the corresponding
 
structure in younger stages is its density; it is wider and its protoplasm perhaps
 
more opaque, or in other words, more protoplasm is accumulated there.
 
  
If figures 24, 22, 25, and 23 are comi^aied and followed in that order, it
+
It is well known that the proximal portions of the cochlear duct mature sooner than the distal portions. One might expect that the accompanying periotic spaces would correspond in their development to the maturity of the duct and therefore the proximal parts of the scalse would differentiate first. In other words, the maturation of the cochlea proceeds as a wave from the proximal end to its tip, involving all of its constituent structures as it passes along, including mesenchymal parts as well as epithelial.  
will be seen that the lining m('ml)rane of the scala* can be traced backward, step
 
by step, to the ordinary trabeculie of the periotic reticulum. There is no histological evidence that any new cells enter into its formation. It seems to be simply
 
a product of the proliferation and adaptive reshaping of the cells already there.  
 
In its final form the margin of the space r(\seml)les an endothelial membrane. One
 
could describe, as inunediately lining the s])ace, a thin membrane with flattened
 
nuclei, which is sui)i)orted underneath by a thin coat of nucleated protoplasm thai,
 
has the form of fibrous connective tissue. The former, judging only from its final
 
aj)pearance, one might designate as endothelium and thus make a distinction between it and the underl^ying tissue. In its histogenesis, however, it differs in no way
 
from the rest of the wall and the difference that exists later seems to be merely the
 
result of its adaptation to the existing physical conditions. Its early l)ehavior is
 
entirely different from that of vasculiu- endothcliuiu. Thus if its final ajipearance
 
is stres.sed and the term endothelium is used for its designation, il must be done
 
with a considerable amount of reservation. It is preeminently a i)lace where the
 
term mesothelium could be used with great advantage.  
 
  
==COMMUNICATION OF PERIOTIC SPACES WITH ARACHNOID SPACES==
 
  
The relation of the scala tj^mpani and scala vestibuli to the subarachnoid
+
This conception might explain the direction of development of the scalae, but can hardly be applied to the cistern, the vestibular representative of the scala vestibuli. One can not say that those portions of the membranous labyrinth lying opposite the focus of development of the cistern (that is, the lateral walls of the saccule and utricle) mature in advance of the rest of the labyrinth. There is no indication
spaces surrounding the hind-brain is of considerable interest, both on account of  
+
that a wave of differentiation passes through the epithelial elements of the labyrinth in the same direction and synchronously with the extension of the cistern as it advances from its primary focus upon the roof of the utricle and over on its median surface. In the case of the cistern it seems much more likely that the point at which it first apjiears is determined by the position of the stapes, which is doubtless an expression of the physical relation that subsequently exists between the two. By analogy this would yield additional significance to the relation existing between the fenestra cochlea; and the point of beginning development of the scala tympani.  
the possibiUty of their functional relationship and on account of the similarity that  
 
exists in their development. For a satisfactory investigation of the establishment
 
and the character of the communications that are formed between these two allied
 
systems of tissue-spaces, one should resort to other methods than those used in
 
the present study, and, furthermore, one should examine older fetuses than those
 
described here. In fact, a problem lies here that would be well worth careful study.  
 
  
Certain observations, however, were made in the course of the above investigation that bear relation to these matters, and they will be briefly outlined here.
 
In the first place, the histological picture of the periotic reticulum is essentially the
 
same as that of the early stages of the pia-arachnoidal tissue investing the central
 
nervous system. The enlargement of the meshes of the latter and the formation
 
of the subarachnoid spaces and the arachnoid cistern, as has been recently described
 
by Weed (1917), correspond exactly with the appearance seen in the histogenesis
 
of the periotic spaces in the ear. The periotic spaces are not, however, extensions
 
of the arachnoid spaces that have invaded the cavity of the cartilaginous labyrinth.
 
If this were so we should find them first appearing among the rootlets of the vestibular and cochlear nerves, along which the .subarachnoid space extends for some
 
little distance. Instead, they begin at points where there can be no connection
 
with the arachnoid tissue and their direction of growth is quite independent of it.
 
The periotic spaces maj' be analogous to the arachnoid spaces, but they are not
 
identical with them, nor are they an extension of them.
 
  
According to the descriptions of the adult anatomy of the ear, a communication becomes established between the scala tympani and the subarachnoid space
+
In dealing with the cistern and also with the scalse one should not consider them as insignificant accessories that merely fill in the waste intervals between the membranous labyrinth and the surrounding cartilage. From studying their development it becomes apparent that they have a morphological individuality in many respects as definite as that of the ossicles themselves. They make their appearance at a definite time and at definite places, they spread in a definite manner, and eventually they attain a form and structure that are adapted to a definite function. This becomes more and more evident as we examine older stages.  
near the fenestra cochleae, the so-called aquaeductus cochleae. Vague and conflicting statements are also made concerning a communication through the internal
 
auditory meatus connecting the arachnoid spaces with the scalae. Such communication must be estabhshed quite late. In the oldest fetus examined, 130 mm.
 
crown-rump length, they did not yet exist. As to the latter communication, it
 
can be seen that the arachnoid spaces extend peri])herally through the internal
 
auditory meatus along the trunk of the acoustic nerve-comjilex, and slender pockets
 
and clefts from them extend along the larger bundles of the cochlear nerve; they  
 
terminate, however, before reaching the margins of the scalae, and there is no evidence at this stage that there is ever to be a conununication between them and the
 
scalae. As to the aquaeductus cochleae, in the 130 mm. fetus it can be plainly seen
 
that it is already forming as a deri\'ative of the arachnoid spaces, although the communication with the scala tympani is not yet established. The arachnoid spaces
 
invest the glossojiharyngeal nerve and extend down along its trunk and pa.ss directly
 
V)y the region of the fenestra cochleae (rotunda). A thin-walled tubular pouch
 
projects from these spaces, leaving the nerve trunk and extending obliquely toward
 
the scala tympani in a direction that would meet it just distal to the fenestral impression on its basal surface. This fundament of the aqua?ductus cochleae is
 
present in fetuses 85 mm. crown-rump length, but is longer in the 130 mm. fetus,
 
where it nearly reaches the scala tympani. The communication must be established soon after this.  
 
  
  
 +
The form and relations of the scalie in fetuses between 12 and 13 weeks old are shown in figures 28 and 29. These figures show median and lateral views of a waxplate reconstruction of the membranous labyrinth and the surrounding periotic spaces in a human fetus 85 mm. crown-rump length (Carnegie Collection, No. 1400-30). Attention has already been directed to these figures in the description previously given of the cistern. The scala vestibuli can be seen in figure 28. Above, it opens freely into the cistern and extends downward along the apical side of the duct as a single main space, possessing a rather uniform diameter. It extends along the first two turns of the duct, gradually tapering off and showing a less mature character in its distal portions. Along the second turn of the duct the spaces are incompletely fused and the contour becomes correspondingly irregular. As a rule the peripheral margin of the scala is less mature and more irregular than the central margin. The scala, vestibuli does not connect with the scala tympani at any point as yet. The two are separated in the first place by the cochlear duct and then more centrally by a framework of connective tissue in which are the radiating bundles of the cochlear nerve with the nodes of ganglion cells that form the spinal ganglion. These latter structures are not shown in the model; they occupy, however, the V-shaped groove seen between the two scalae.
  
==SUMMARY==
 
  
The changes in size and form which the cartilaginous capsule of the ear undergoes during its development in the human embryo are accomplished in part by a
+
The scala tympani, as can be seen in figure 29, extends downward on the basal side of the cochlear duct along its first two turns. This corresponds to about the same linear dimension as that of the scala vestibuli. In its proximal portion it shows a greater area in cross-section than the latter, but further toward the apical region it is of about the same size and in some places it is even smaller. The peripheral margin of the scala tympani is distinctly more irregular than the central margin. The irregularity is due to spaces along this margin that are actively coalescing with the main sjiace, but in which the fusion is not yet complete. The irregularity of this margin is thus an indication of the direction of the expansion of the scala. As the diameter of the whole cochlear mass increases, it is evident that the main growth of the scala must radiate outward in a peripheral direction. This is accomplished by the continual assimilation of new reticular spaces along this margin. At the proximal end of the scala tympani can be seen an oval depression which corresponds to the fenestra cochleie (rotunda) and with which it stands in intimate relation.  
progressive and in part by a retrogressive differentiation of its constituent tissues.  
 
Throughout the entire period of growth, as far as material was available for study,
 
it was foimd that the margins of the cartilaginous cavities undergo a process of
 
continual transformation. They exhibit a state of unstable equilibrium in respect
 
to the opposing tendencies toward a deposit of new cartilage on the one hand and  
 
toward the excavation of the old on the other. The margins therebj^ are always
 
either advancing or receding, and it is in this way that the progressive alterations in  
 
the size, shape, and position of the cavities are produced, due to which a suitable
 
suite of chambers is always provided for the enlarging membranous labyrinth.  
 
  
The general tissue mass of the otic capsule, during the period represented by
 
embryos from 4 to 30 mm. long, passes through three consecutive histogenetic
 
periods, nameh', the stage of mesenchymal syncj'tium, the stage of precartilage,
 
and the stage of true cartilage. In the subsequent growth of the capsule it is found
 
that in areas where new cartilage is being deposited the tissues of the areas concerned follow a definite and progressive order of development. In areas, however,
 
where excavation occurs, where cartilage previously laid down is being removed,
 
it is found that the process is reversed. The tissue in such areas returns to an
 
earlier embryonic state — that is, it undergoes dedifferentiation. Tissue that has
 
accjuired all the histological characteristics of true cartilage can thus be traced in
 
its reversion to precartilage and from jirecartilage in turn to a mesenchymal syncytium. In the latter form it redifferentiates into a more specialized tissue, in
 
this case for the most part into a vascular reticulum.
 
  
The formation of the periotic reticulum is first indicated by a cluster of deeply
+
In fetuses about 16 weeks old the form and relations of the scalae have nearly attained the adult conditions, and this represents the oldest stage studied in connection with the present paper. The conditions found at this time are shown in figures 30 and 31, which present nuMhan and lateral views of a wax-plate model of a human fetus 130 mm. crown-rump length (Carnegie Collection, No. 1018). On comparing the scala tympani and scala vestibuli as seen in these figures with those in figures 28 and 29, it will be seen that they are larger in cross-section and more nearly cover the cochlear duct. Furthermore, they now extend to the extreme
staining nuclei that can be seen along the central edge of the semicircular ducts in
+
tip of the duct and communicate with each other across its central margin, thus forming a helicotrema. A section through this point can be seen in figure 25, in which these structures are shown as seen under low magnification. It will be noted that now, even as far as the tip of the cochlea, each of the scalse consists of a continuous principal space, though both are more mature and larger in their proximal portions. Along the first turn of the cochlear duct they are walled off by a smooth membranous margin which separates them from the adjacent reticular tissue. The spaces of the latter do not seem to be taking any further part in the process of enlargement of the scala. Along the second turn of the cochlear duct, a section of which is shown in figure 20, the coalescence of reticular spaces with each other and with the scalse is still in active operation. This produces a greater irregularity of the scalar than is shown in the model. The subsidiary spaces are shown as a solid mass; the slender clefts separating them are not represented. The nearer one approaches the tip of the duct the more immature are the scalse, until the condition is reached that is shown in figure 25, where the membrane-like margin is quite incomplete and the spaces merge irregularly with the surrounding reticulum.
embryos soon after the ducts are formed, and at about the time the otic capsule
+
Thus a single specimen, if studied in its different parts, shows several stages in this interesting process of the formation and growth of the scalaj.  
begins to change from condensed mesenchyme into precartilage. These nuclei
 
constitute a focus at which the development of the reticulum and its blood-vessels
 
takes origin. Here the tissue of the otic capsule takes on an appearance that is
 
less like that of a cartilage-forming tissue and more like that of an embryonic connective tissue. Spreading from this focus, a narrow area is established which soon
 
encircles the semicircular ducts and becomes the open-meshed vascular reticulum
 
which, in embryos 30 mm. long, everywhere bridges the space existing lietween the  
 
epithelial lal)yrinth and the surrounding cartilage. In the earlier stages it could
 
not be definitely shown that the primordium of the i^eriotic reticular tissue is not
 
derived from a few ])rcdestined mesenchyme cells which become inclosed, along with  
 
the otic vesicle, by the condensed tissue of the capsule and after a certain latent period undergo proliferation and occup}' the space vacated by the receding precartilage. In the later stages, however, it is cjuite evident that precartilage tissue
 
is actually' converted into a reticulum, and that the replacement of precartilage
 
by a reticular connective tissue is brought about through a process of dedifferentiation.  
 
  
The perichondrium is a derivative of the periotic reticulum and forms an outei
 
limiting membrane along its cartilaginous margin. During the fetal period the
 
perichondrium does not rest directly against the true cartilage, but is separated from
 
it by a zone of transitional tissue consisting partly of precartilage and partly of
 
reticulum. This transitional zone intervening between the perichondrium and the
 
surrounding cartilage was ob-served in all of the specimens that were studied, which
 
includes fetuses up to 130 mm. crown-rump length. Owing to the fact that the
 
perichondrium is late in making its appearance, being first seen in fetuses about
 
70 mm. long, it can take no part in the early changes in the cartilaginous capsule,
 
either as regards the deposit of new cartilage or the excavation of cartilage that
 
had been previously laid down.
 
  
The periotic tissue-spaces are formed by a modification of the meshes of the  
+
The figures grouped on plate 3 illustrate some of the histological features of this process. An early stage in space-formation is shown in figure 23. This is a section through the canal region where the changes in the reticulum are late in making their appearance. In fact, the periotic spaces never reach the same degree of differentiation here that occurs in the case of the cistern and scalse. The initial steps, however, are the same, and this figure presents very well the appearance of the periotic reticulum as it begins to open up into larger spaces. Unmodified reticulum is characterized by a rather uniform narrow mesh. The essential change in space-formation consists in the disappearance of some of the trabecule of the mesh, with the consequent coalescence of the corresponding adjacent spaces. The trabecuhe consist of the protoplasmic processes of the constituent cells of the reticulum and their disappearance is to be explained in either of two ways: It is possible that owing to some property of the fiuid element of the tissue the protoplasmic strands are dissolved or liquefied; this would account for their complete disappearance. On the other hand, the same result could be accomplished by an alteration in the form of the cell processes. A given trabecula could separate at either end, or at some jioint along its line, and the free ends of protoplasm could then retract and reshape themselves and become a part of the remaining framework. Whether we are dealing with a licjuefaction of tissue or with active motility of the cell, protoplasm involving detachment and retraction of the trabeculae can not be definitely determined by observations of fixed tissue; but the appearance of sections where the process is in active operation seems to the writer to indicate the latter.  
periotic reticulum. The latter consists originally of a rather uniform narrow mesh.  
 
The essential change which it undergoes in the process of space-formation consists  
 
in the gradual disappearance of the traversing trabeculse. The trabeculae consist  
 
of the protoplasmic processes of the constituent cells of the reticulum, and their  
 
disappearance is apparently due, not to a dissolution or Uquefaction of these cellprocesses, but to an alteration in their form. It apparently is the result of an active  
 
motility of the cell protoplasm involving the successive detachment and retraction  
 
of the trabeculse. Wlien a trabecula becomes detached it retracts and adapts
 
itself to the formation of the enlarging space, reshaping itself either as a smooth
 
border or as a constituent part of another trabecula.  
 
  
The differentiation of the margin of the periotic spaces constitutes the final
 
feature in their maturation. During the period in which the enlargement of an
 
individual space is activeh' going on,. the margins of the main cavity consist of
 
smooth, delicate strands of nucleated protoplasm that resemble the trabeculse
 
between the large reticular spaces. These linear margins are interrupted here and
 
there by openings into adjacent spaces. They tend, however, to form a continuous
 
hne that definitely marks ofT the space from the adjacent reticulum. As the space
 
becomes more mature, the membrane-like border becomes thicker until it reaches
 
a state that will probably not admit of any further opening-up for the coalescence
 
of additional spaces. Any further growth is thereafter limited to a simple distention
 
of the wall of the space, with the consequent adjustment of its constituent cells.
 
In its final form the margin of the space constitutes a mesothehal membrane.
 
Immediately lining the space is a thin membrane with flattened nuclei which is
 
supported underneath by a thin coat of nucleated protoplasm having the form of
 
fibrous connective tissue. The former in its histogenesis differs in no way from
 
the rest of the wall and the difference that exists later seems to be merely the
 
result of its adaptation to the existing physical conditions.
 
  
'J'hc oarlie.st histological evidence of the formation of the periotic spaces occurs
+
In the above paragraph and elsewhere in this paper reference is made to trabeculae ser^dng as "partitions" between "spaces" and the disappearance of trabeculse resulting in the "coalescence of adjacent spaces." In making this use of the term "space" it should be explained that it is done in a descriptive sense, in application to the appearance of the tissue as seen in sections in which form human embryological material is mainly available. In thin sections of a reticular tissue one sees trabecule as partitions separating adjacent spaces. The same tissue in a mass would show that the spaces everywhere communicate freely with each other, like the spaces in a sponge, and that the trabeculse are thread-like strands which at the best are very incomplete partitions. Instead of a meshwork containing many small spaces, one could perhaps equally well describe reticular tissue as a single large space traversed by many trabeculse. If the latter practice were adopted, one would describe the development of the tissue-spaces with which we are concerned as a process of gradual decrease in the number of traversing trabeculae, with the result that the mesh thereby becomes coarser. For descriptive purposes, however, it is convenient to refer to the intervals between the strands of the mesh as spaces, at the same time not granting them the significance that is attached to such membrane-lined tissue-spaces as are represented by the vestibular cistern and the two scalse, though the latter are in reality derived from them.  
near the stapes, in the reticulum that bridges the interval l)etween the sacculus
 
and the fenestra vestibuh. In embrj'os between 30 mm. and 40 mm. long, it can
 
be seen that the meshes in this region are becoming irregular and larger, due to the  
 
disapjiearance of some of the trabeculae and a consequent coalescence of the intertrabecular si)aces. The widening of the mesh at this point constitutes the primordium of the vestibular cistern. It makes its appearance before there is any trace
 
of the scalse, but it is not until the fetus reaches a length of about 50 mm. that the  
 
cistern becomes definitely outlined and clearly differentiated from the adjoining
 
reticulum.  
 
  
Following the appearance of the cistern, the scala tympani is the next space to
 
become established. It can be recognized as a moderate widening of the meshes
 
of the reticulum in the region of the fenestra cochleae in fetuses 43 mm. long, along
 
the basal border of the first turn of the cochlear duct. The scala vestibuli, as can
 
be seen in fetuses 50 mm. long, develops as an extension downward of the cistern
 
along the apical border of the cochlear duct. Starting from these definite foci,
 
these three spaces spread into their destined territory, absorbing as they go the
 
enlarging reticular spaces of the invaded region by a process of space-coalescence,
 
or, in other words, the progressive formation of areas that are free of trabeculse. In
 
fetuses 85 mm. long the two scalse extend downward along the cochlear duct to
 
its last turn, as two separate spaces which do not communicate with each other.
 
When they reach the tip of the duct, which occurs in fetuses about 130 mm.
 
crown-rump length, a free opening is developed between them which represents
 
the hehcotrema. After being completely established along the whole length of
 
the cochlear duct, the scalae continue to enlarge by further coalescence of tissue along
 
their peripheral border, in which the trabecular disappear.
 
  
The periotic sjxices are analcjgous in their development to the pia-arachnoidal
+
In figure 23 the free detached ends of the trabeculse will be noted everywhere, as is characteristic of this stage of development. It is a necessary step in the coalescence of adjacent spaces. The detached trabeculse seem to be gradually retracting and adapting themselves to the formation of larger spaces. Their constituent protoplasm reshapes itself as a smooth border or as a part of other trabeculae. Larger spaces necessitate longer trabeculae, and as trabeculae become longer they also tend to become heavier. These phenomena are all in evidence in the spreading and enlargement of the scalse.  
spaces; they are not, however, extensions of them that have invaded the cavity of  
 
the cartilaginous labyrinth. They begin at points where there can be no connection with the arachnoidal tissue and their direction of growth is quite independent
 
of it. The communication that is found in the adult between the scala tympani
 
and the subarachnoid sjjace in the neighborhood of the fenestra cochleae, the socalled af|ua'ductus cochleae, is established (juite late. In fetuses 85 mm. crownrump length it exists as a tubular pouch projecting from the subarachnoid s])aces
 
along the glossopharyngeal nerve toward the scala tj^mpani. In the 13()-mm. fetus,
 
the (jldest examined, this j^ouch is longer and nearly reaches the scala. The communication must be established soon after this.  
 
  
Similar iirojections from the subarachnoid spaces at the internal auditory
 
meatus extend as perineural clefts along the trunk and branches of the acoustic
 
nerve. No actual cf)mnnuiications, however, were seen between these spaces and
 
the two scalae.
 
  
 +
Figure 20 shows a characteristic view of the scalse as seen under low magnification. It will be noted that the scala vestibuli is relatively mature at this point; the scala tympani, however, is in the act of spreading peripherally, so as to
 +
underlie, as it eventually will do, the future basilar membrane. The scala tympani
 +
finally reaches the peripheral margin of the cochlear duct, and it does this by the
 +
coalescence of the enlarging reticular spaces, which become incorporated with the
 +
main cavity of the scala. This can be observed better in figure 22, which shows a
 +
detail of the same section as seen under higher magnification. By comparing this
 +
figure with figure 20 the exact location can be readily made out. A portion of the
 +
main cavity of the scala is indicated and to the right of this are a few enlarged
 +
reticular spaces that are uniting with each other and will in the end become part
 +
of the main space. In addition to the enlarged reticular spaces there is a certain
 +
amount of residual undifferentiated reticulum. It is this tissue that will play the
 +
part of an adventitial coat to the completed scala. The trabecular that separate
 +
the enlarged spaces seem to be under tension and about ready to snap apart. In
 +
fact, in most sections one can see the fragmentary ends of trabeculse where this
 +
interruption of continuity has apparently occurred.
  
  
==BIBLIOGRAPHY==
+
The differentiation of the margin of the scalae constitutes the final feature in their maturation. During the period in which the enlargement of an individual scala is being brought about by the coalescence of enlarging reticular spaces, the margins of the main cavity can be seen to consist of smooth, delicate strands of nucleated protoplasm that resembles in all essentials that of the trabeculae between the large reticular spaces. These linear margins are interrupted here and there by openings into adjacent spaces, but they tend to form a continuous line that definitely marks off the space from the adjacent reticulum. An early stage in the formation of such a margin is shown in figure 25, where the margin is indicated at a few places, but for the most part the space abuts against the surrounding ragged reticulum. The margin of the space is more complete in the scala tympani shown in figure 22, but it is still thin and delicate and can be easily opened up to allow the taking in of new spaces. If we examine the borders of more mature spaces we find them inclosed by a firmer membrane, which finally reaches a state that will probably not admit of any further opening up for the coalesence of additional spaces. Any further growth must thereafter be limited to simple distention of the wall of the space with the consequent adjustment of its constituent cells. Such a condition is represented in figure 24. This shows a more mature section of the wall of the scala vestibuli, being a detail of the same section shown in figure 20. The only difference between such a membrane, as we must now call it, and the corresponding structure in younger stages is its density; it is wider and its protoplasm perhaps more opaque, or in other words, more protoplasm is accumulated there.  
 
 
 
 
 
 
Balfocr, F. M., 1885. A treatise on comparative em
 
brj'ologj'. 2 ed., vol. 2. London.
 
Bardeen, C. R., 1910. Die Entwicklung des Skeletts
 
und des Bindegewebe.s. Keibel-Mall, Handbuch
 
 
 
der Entwicklungsgeschichte, i. 296-456.
 
BoETTCHER, A., 1869. Ueber Entwiokelung und Bau des
 
Gehorlabyrinths nach Untersuchungen an Saugethieren. Dresden. (Quoted from Retzius, 1884.)
 
, 1872. Kritische Bemerkungen und neue Beitrage ziir Literatur des Gehorlabyrinths. Dorpat.
 
(Quoted from Retzius, 1884.)
 
Breschet, G., 1836. Recherches anatomiques et physi
 
ologiques sur I'organe de I'ouie et sur I'audition
 
 
 
dans I'homme et les animau-x vertebres. 2 ed.
 
 
 
Paris.
 
Cassebohm, J. F., 1735. Tractatus anatomici de auie
 
 
 
humana, etc. Halae, Magdebiu-gica?. (Quoted
 
 
 
from Breschet, 1836.)
 
Child, C. M., 1915. Senescence and rejuvenescence.
 
 
 
University of Chicago Press.
 
CoRTi, A., 1851. Recherches sur I'organe de I'ouie des
 
 
 
mammiferes. Premiere partie. Zeitschr. f. wiss.
 
 
 
Zool.. Ill, 109-169.
 
CoTUNNn, D., 1760. De aqujeductibus auris humaris
 
 
 
intemae anatomica dissertatio. NapoU. (Quoted
 
 
 
from Breschet. 1836.)
 
vox Ebner, v., 1902. Kolliker's Handbuch der Gewe
 
belehre. Bd. in. 2, 6 .\ufl.
 
FiLATOFF, D., 1906. Zur Frage iiber die Anlage des
 
 
 
Knorpelschadels bei einigen Wirbeltieren. ."Vnat.
 
 
 
Anz., XXIX, 623-633.
 
Gaupp, E., 1906. Die Entwickelung des Kopfskelettes.
 
 
 
Hertwig's Handbuch der Entwick . d. Wirbeltiere.
 
H.\sse, C, 1873. Die Lymphbahnen des inneren Ohres
 
 
 
der Wirbelthiere. In his Anatomische Studiem.
 
 
 
Leipzig., I, Heft 4, Xo. 19, 766-816.
 
, 1881. Bemerkungen iiber die Lymphbahnen des
 
 
 
inneren ohres. Arch. f. Ohrenh., xvii, 188-194.
 
Hensen, v., 1863. Zur Morphologic der Schnecke des
 
 
 
Menschen und der Saugethiere. Ztschr. f. wiss.
 
 
 
Zool., XIII, 481-512.
 
His, W., 1903. Die Haute und Hohlen des Korpers.
 
 
 
Wiederabdruck eines akademischen Programmes
 
 
 
vom Jahre 1865. Arch. f. Anat. u. Physiol., Anat.
 
 
 
Abth., 368-404.
 
Huschke, E., 1831 . Erste Bildungsgeschichte des Auges
 
 
 
und Ohres. (Vereammlung Naturforscher u.'
 
 
 
Aerzte zu Hamburg.) Isis von Oken, 950.
 
Key, E. a. H., and G. Retzius, 1875-1876. Studien in
 
 
 
der Anatomic des Xer\-ensy stems und des Binde
 
gewebes. Stockholm.
 
KoLLiKER, A., 1850-1854. Mikroskopische Anatomic
 
 
 
oder Gewebelehre des Menschen. Leipzig.
 
, 1861. Der embrj-onale Schneckenkanal und
 
 
 
seine Beziehungen zu den Theilen der fertigen
 
 
 
Cochlea. Wiirzburger natiirwiss. Zeitschr., ii, 1-9.
 
, 1861. Entwicklungsgeschichte des Menschen
 
 
 
luid der hoheren Thiere. Leipzig.
 
, 1879. Entwicklungsgeschichte des Menschen und
 
 
 
der hoheren Thiere. 2 ed., Leipzig.
 
, 1884. Grundriss der Entwicklungsgeschichte des
 
 
 
Menschen und der hoheren Thiere. 2 ed., Leipzig.
 
Krause, R., 1906. Entwickelungsgeschichte des Gehor
 
organes. Handbuch d. vergl. u. expcr. Entwick
 
lungslehre der Wirbeltiere. HeravLsg. v. O. Hert
 
wig. Jena.
 
Levi, G., 1900. Beitrag zuin Studium der Entwiekelung
 
 
 
des Knorpehgen Primordial craniums des Menschen. Archiv f. mikrosk. Anat., Bd. 55
 
 
 
 
 
 
 
Lewis, W. H., 1907. On the origin and differentiation of  
 
the otic vesicle in amphibian embryos. Anat.
 
Rec, I, 141.
 
 
 
, 1915. The use of guide planes and plaster of
 
 
 
paris for reconstruction from serial sections : Some
 
points on reconstruction. Anat. Rec, ix, 719.
 
 
 
Macklin', C. C, 1914. The skull of a human fetus of
 
40 mm. .A.nier. Jour. .\nat., vol. 16.
 
 
 
Mall, F. P., 1902. On the development of the connective tissues from the connective tissue syncytium.
 
Amer. Jour. Anat., i, 329-365.
 
 
 
Morgagni, J. B., 1740. Epist. anatom., xii, p. 469.
 
Venetiis. (Quoted from Breschet, 1836.)
 
 
 
Odenius, M. ^'., 1867. Ueber das Epithel der Maculae
 
acusticae beim Menschen. .Arch. f. mikr. Anat.,
 
Ill, 11.5-134.
 
 
 
Reichert, C. B., 1852. Bericht iiber die Abhandl. d.
 
Herm Dr. Reissner "De auris interns formatione." Bull. d. la Cl. phys.-math. d I'acad. imp.
 
d. sci. d. St. Pgtersbourg, x, 86-96.
 
 
 
, 1864. Beitrag zur feineren Anatomie der Gehor
 
schnecke des Menschen und der Saugethiere.
 
.\bhandl. d. Kgl. .\kad. d. WLss., Berlin, (Phvs.)
 
Xo. 1, 1-60.
 
 
 
Reissner, E., 1851. De auris internseformationc. Inaug.
 
diss., Dorpat. (See Reichert, 1852.)
 
 
 
, 1854. Zur Kenntniss der Schnecke im Gehor
 
organ der Saugethiere und des Menschen. J. Midler's Arch. f.*.\nat., Physiol, u. wiss. Med.420-427.
 
 
 
Retterer, E., 1900. Evolution du cprtilage transitoire.
 
Jour, de I'Anat. et de la Physiol., xxxvi, 467-565.
 
 
 
Retzius, G., 1884. Das Gehororgan der Wirbelthiere.
 
Abtheilung ii. Stockholm.
 
 
 
Sabin, F. R., 1917. Origin and development of the
 
primitive vessels of the chick and of the pig. Contributions to Embrj-ologj', vol. 6, No. 15. Carnegie Inst. Wash. Pub. No. 226.
 
 
 
Scarpa, A., 1789. Anatomicae disquisitiones de auditu
 
et olfactu. Ticini. (Quoted from Retzius, 1884.)
 
 
 
ScHWALBE, G., 1869. Der Arachnoidalraum ein Lymphraum und sein Zusammenohang mit dem Perichorioidalraum. Centralbl. f. d. med. Wiss., No.
 
30, 465-467.
 
 
 
SoLGER, B., 1889. Ueber Knorpelwachstum. Verb. d.
 
Anat. Gesellsch. BerUn, in, 67-71. (Erganzungsheft Anat. Anz., rv.)
 
 
 
Streeter, G. L., 1917. The factors involved in the  
 
excavation of the cavities in the cartihginous capsule of the ear in the human embiyo. Amer.
 
Jour. Anat., xxii, 1-25.
 
 
 
, 1917. The development of the scala tympani,
 
 
 
scala vestibuU, and perioticular ci.stern in the  
 
human embryo. .\mer. Jour. .Anat., xxi, 299-320.
 
 
 
Terry, R. J., 1917. The primordial cranium of the cat.
 
Jour. Morph., vol. 29.
 
 
 
Valsalva, A. M., 1707. De aure humana tractatus, etc.
 
Imolensi, Trajecti ad Rhenum. (Quoted from
 
Breschet, 1836.)
 
 
 
Vieussens, R., 1714. Traite nouveau de la structure de
 
I'oreille. Toulouse. (Quoted from Breschet, 18'6.)
 
 
 
Weber-Liel, 1879. Der Aquteductus cochlese des Menschen. Monatschr. f. Ohrenh., xiii, 33-39.
 
 
 
Weber, F. E., 1869. Ueber den Zasammenhaug des
 
Arachnoidalraumes mit dem LabjTiiith. Monatschr. f. Ohrenh., in, 10.5-107.
 
 
 
Weed, L. H., 1917. The development of the cerebrospinal spaces in pig and in man. Contributions to
 
Embrj'olog}', vol. 5, No. 14. Carnegie Inst. Wash.
 
Pub. No. 225.
 
 
 
 
 
 
 
==EXPLANATION OF PLATES==
 
 
 
Plate 1.
 
 
 
The figures on Plates I aiui II represent a series of photographs of the ear region inhuman embryos varying from 4 mm.
 
to i;{0 mm. long. The photographs were taken at a magnification of 1()0 diameters and as far as possible
 
at similar positions, so that a eomparison of them would indicate the actual increase in size and the relative
 
amount and form of the individual tissue-masses. In the reproduction tliey were reduced to about 90
 
diameters. The different figures include the principal stages in the development of the cartilaginous
 
capsule of the car and show the gross features of the histogenesis of the periotic reticulum. Figures 5
 
to 7 cover the period during which the mesenchyme becomes condensed around the otic vesicle. Figures
 
8 to 10 show the otic capsule in its precartilage stage and the manner in which the precartilage becomes
 
difTerentiated into relatively i)ermanent and temporary zones. The latter encircle the epithelial ducts and
 
correspond to the future cartilaginous canals. In figures 1 1 to 13 the main capsular mass has become true
 
cartilage, whereius the temporary zone of precartilage surrounding the canal is on the point of dedifferentiating into periotic reticulum. A focal area of vascularized reticulum is already established at the inner
 
margin of the epitheUal duct.
 
 
 
Fig. 5. Frontal section through the region of the ear in a human embryo 4 mm. long (Carnegie Collection, No. 588,
 
slide 6, row 6, section 6). The section is 1.5^ thick and is enlarged 90 diameters. It shows part of the
 
brain-wall and t he ot ic vesicle with the surrounding mesenchyme. The nuclei of the latter are more numerous
 
in the neighborhood of the vesicle, indicating the beginning of the capsular condensation.
 
 
 
Fig. 6. Horizontal section through the region of the ear in a human embryo 9 mm. long (Carnegie Collection, No. 721,
 
slide ,"), row 2, section 1). The section is l.'i/i thick and is enlarged 90 diameters. It shows a distinct condensation of the mesenchyme around the otic vesicle, particularly on its lateral surface (above) where it
 
extends frotn the surface of the vesicle to about half the distance from the vesicle to the ectoderm.
 
 
 
Fig. 7. Frontal section through the labyrinth in a human embryo 11 mm. long (Carnegie Collection, No. 353, shde 16,
 
row 3, section 4). The section is 10ft thick and is enlarged 90 diameters. It shows the vestibular part of
 
the labyrinth with the appendage opening out of it and passes transversely through the pouches whose
 
margins are to form the superior and lateral semicircular ducts. There is now a very complete capsule of
 
condensed mesenchyme surrounding every part of the labyrinth, with the exception of the appendage and the
 
regions of the interna! auditory meatus and the fenestra cochle«.
 
 
 
Fig. 8. Horizontal sec'tioii through the otic capsule in a human embryo 15 mm. long (Carnegie Collection, No. 719,
 
slide 3, row 2, section 3). The .section is 40^ thick and is enlarged 90 diameters. It shows a portion of the
 
utricle below and the superior semicircular duct above. Surrounding these is a definite capsule of precartilage
 
tissue.
 
 
 
Fig. 9. Sagittal section through the otic capsule in a human embryo 18 mm. long (Carnegie Collection, No. 144, slide
 
4, row 1, section 3). The section is 40fi thick and is enlarged 90 diameters. Above is the posterior semicircular duct, and just below the center is the lateral semicircular duct. The otic capsule is now differentiated
 
into relatively permanent are:is of prccarlilago and other are;is that are more temporary. The latter surround the epithelial ducts and indicate the future cartilaginous canals.
 
 
 
Fig. 10. Frontal .section through the otic capsule in a human embryo 27 mm. crown-rump length (Carnegie Collection,  
 
No. 756a, slide 47, section 2). The section is .")()/i thi<-k and is enlarged 90 diameters. It passes transversely
 
through the lateral .semicircular canal. The epithelial duct is surrounded by a zone of temporary precartilage
 
corresponding to the future cartilaginous canal. Just median to the duct (below it in the photograph) is a  
 
group of nuclei that forms the focus of the future fp-owth of reticulum.  
 
 
 
Fio. 1 1 . Section through the lateral semicircular canal in a human fetus 30 mm. crown-rump (Carnegie Collection, No.
 
86, slide 46, section 2). The section is HOit thick and is enlargetl 90 diameters. The main capsular mass is
 
now differentiated into true cartilage. The zoik- of temporary precartilage is beginning to recede from the
 
epithelial <luct, leaving a reticular area in the interval, which is more pronounced on the median side of the
 
duct (below it in the photograph).
 
 
 
Fig. 12. .Section through the lateral semicircular canal in a human fetus 37 mm. crown-rump length (Carnegie Collection, No. 972, slide 20, section 1). The section is 50m thick and is enlarged 90 diameters. The nuclei of
 
the zone of temporary precartilage form a dark field that corresponds to the future cartiliiginous canal.
 
Along the inner margin of this zone are seen large blood-vessels that belong to the periotic reticulum.
 
 
 
Fig. 13. Section through the lateral semicircular canal in a human fetus 35 mm. crown-rump length (Carnegie Collection, No. 199, .slide .58, section 2). The section is 50;u thick and is enlarged 90 diameters. It is stained
 
deeply with hematoxylin, showing the matrix of the cartilage but not the zone of precartilage that is to become
 
the cartilaginous canal.
 
 
 
Plate 2.
 
 
 
The figures on Plate II are in continuation of those on Plate I and .show the final establishment of the periotic reticular
 
tissue. They also show, on being comp.arerl with younger stages, the manner in which the cartilage becomes
 
excavated in order lo yield room for the enlarging duct and also to allow for its changing position. The  
 
excavation is brought about by the dedilTerentiation of cartilage into reticuku- tissue. Throughout this
 
period the margin of the cartilaginous canal continues in an unstable condition and is gradually either
 
 
 
52
 
 
 
 
 
 
 
EXPLANATION OF PLATES. 53
 
 
 
receding or advancing, through the processes of dedifferentiation, into precartilage or differentiation from
 
precartilage respectively. The periotic reticulum in its later stages develops fibrous membranes at its inner
 
and outer borders. The one at the inner border forms the membrana propria for the epithelial duct, and the
 
one at the outer border becomes the perichondrium.
 
 
 
Fig. 14. Section through the lateral semicircular canal in a human fetu.s 43 mm. crown-rump length (Carnegie Collection, No. 886, slide 42, section 3). The section is 100m thick and is enlarged 90 diameters. The zone of
 
precartilage is expaniling around its peripheral margin by dedifferentiation of the surrounding cartilage and  
 
on its central margin the precartilage is giving way before the advancing reticulum. A crescentic area of
 
periotic reticulum is established on the median side (to the left) of the epithelial duct, about 8 mm. deep in  
 
the photograph.
 
 
 
Fig, 1.5. Section through the lateral semicircular canal in a human fetus 46 mm. crown-rump length (Carnegie Collection, No. 9.5, slide 72, section 1). The section i.s 100m thick and is enlarged 90 diameters. The original area
 
of precartilage is now all dedifferentiated into reticulum, and a new area of precartilage has formed outside
 
of this at the expense of the smioundiiig cartilage. The new area of precartilage is about OS cm. deep in the
 
photograph. Everj-thing between this and the epithelium is reticulum, the peripheral part of which is not
 
yet completely vascularized.
 
 
 
Fig. 16. Section thiough the posterior semicircular canal in a human fetus 50 mm. crown-rump length (Carnegie
 
Collection, No. 184, shde 23). The .section is 50^ thick and is enlarged 90 diameters. The dedifferentiation
 
of precartilage into reticulum is nearly complete, there being left ordy a narrow hne of it along the margin of
 
the cartilage. The vascularization of the reticulum is not yet completed. The small diameter and the thick
 
wall of the epithelial duct in this figure and in figure 15 result from contraction. If they were distended in
 
the process of fixation they would doubtless be as large as those in figures 14 and 17.
 
 
 
Fig. 17. Section through the posterior semicircular canal in a human fetus 52 mm. crown-rump length (Carnegie
 
Collection, No. 96, shde 12, section 2). The section is 100m thick and is enlarged 90 diameters. It differs
 
from figui'e 16 in having a more mature periotic reticulum.
 
 
 
Fig. is. Section through the posterior semicircular canal in a human fetus, So mm. crown-rump length (Carnegie
 
Collection, No. 1400-30, slide 43, section 2). The section is 100m thick and is enlarged 90 diameters. At
 
the inner margin of the reticulum can now be seen the membrana propria .supporting the semicircular duct
 
and at the outer margin is the thick peri.'hondrium, between which and the cartilage there is a narrow open
 
space that is better seen on the left part of the photograph. The sharp dark line along the margin of the
 
cartilage on the right is an appearance due to the excavation of cartilage at that point. It consists of an
 
intermediate zone in which the cartilage is being dedifferentiated into precartilage and that in turn into
 
reticular tissue.
 
 
 
Fig. 19. Section through the superior semicircular canal in a human fetus 130 mm. crown-rump length (Carnegie Collection, No. 1018, slide 30, section 1). The section is 50m thick and is enlarged 90 diameters. It shows a
 
rather mature perichondrium closely attached to the cartilage, separated from it, however, by a narrow
 
intermediate zone that is not seen in the photograph. ThLs zone is connected with the further enlargement
 
of the cartilaginous canal, the growth of which is not yet completed. In the outer part of the canal the
 
perichondrium fuses with the membrana propria of the semicircular duct. The periotic reticulum is beginning to break up in the formation of larger spaces, which it does by the retraction of its trabecute, thereby
 
allowing adjacent spaces to coalesce. The blood-vessels in this specimen were injected with India ink.
 
 
 
Plate 3.
 
 
 
The figures on Plate III show the histological appearance of the periotic tissue-spaces and the manner in which they
 
are formed from the periotic reticulum. This is accomplished by the disappearance of the trabeculae and the
 
consequent repeated coalescence of adjoining spaces.
 
 
 
Fig. 20. Section through the second turn of the cochlea in a human fetus 130 mm. crown-rump length (Carnegie
 
Collection, No. 1018, slide 32, section 2), enlarged 57 diameters. This section shows the topograph}' of the
 
cochlear duct and the general character of the periotic spaces that are developing along its inner margins.
 
Details of this same section as seen under higher magnification are shown in figures 22 and 24.
 
 
 
Fig. 22. Detail of the section shown in figure 20, enlarged 278 diameters. This figure shows the part of the coclilear
 
duct that is to form the organ of Corti with the adjacent tissue that becomes incorporated in the basilar membrane. Below is the periotic reticulum, whose spaces are in the process of enlarging. By repeated coalescence these spaces finally unite with the large space which constitutes the scala tympani. This figure shows
 
the histological appearance of the reticulum where the formation of ti.ssue-spaces is in active operation.
 
 
 
Fig. 24. Detail of the section shown in figure 20, enlarged 300 diameters. It shows the character of the margin of the
 
scala vestibuli in a fairly mature condition. The scala vestibuli is inclosed by a membrane consisting of the
 
cells that had previously constituted the reticulum occupying this area and which have been modified in form
 
in adaptation to the formation of this large tissue-.space, closing it off from the surrounding tissue.
 
 
 
Fig. 21 . Section through the vestibular portion of the labyrinth in a human fetus 52 mm. crown-iump length (Carnegie
 
Collection, No. 448, slide 154, section 2), enlarged 31 diameters. This section shows the general character
 
of the periotic spaces and their relation to the different parts of the membranous labyrinth and the surrounding cartilaginous capsule. The first space to develop and the largest shown in this figure is the vestibular
 
cistern, situated between the utricle and the cartilaginous stapes. The smaller spaces, belowthe cistern
 
and extending downward along the cochlear duct, represent the scala vestibuli in an early form. The
 
arteries in this specimen were injected with Imlia ink and are shown in black.  
 
 
 
  
  
54 EXPLANATION OF PLATES.  
+
If figures 24, 22, 25, and 23 are compared and followed in that order, it will be seen that the lining membrane of the scala can be traced backward, step by step, to the ordinary trabeculie of the periotic reticulum. There is no histological evidence that any new cells enter into its formation. It seems to be simply a product of the proliferation and adaptive reshaping of the cells already there. In its final form the margin of the space resembles an endothelial membrane. One could describe, as immediately lining the space, a thin membrane with flattened nuclei, which is supported underneath by a thin coat of nucleated protoplasm thai,
 +
has the form of fibrous connective tissue. The former, judging only from its final appearance, one might designate as endothelium and thus make a distinction between it and the underlying tissue. In its histogenesis, however, it differs in no way from the rest of the wall and the difference that exists later seems to be merely the result of its adaptation to the existing physical conditions. Its early behaviour is entirely different from that of vascular- endothcliuiu. Thus if its final appearance is stressed and the term endothelium is used for its designation, it must be done with a considerable amount of reservation. It is preeminently a place where the
 +
term mesothelium could be used with great advantage.
  
Fir,. iV Section tlirough the superior semicircular canal in a human fetus 130 mm. crown-rump length (Carnegie
+
==Communication of Periotic Spaces with Arachnoid Spaces==
Collection, \o. 1018, slide 29, section 2), enlarged 90 diameters. The periotic reticulum Ls undergoing the
 
alterations characteristic of the earlj' st^es of the formation of tissue-spaces. Along the margins of the
 
cartilage the reticular tissue is condensed and (^oiLstitutes the fibrous pcrichoiulrium. .\round the epithelial
 
canal there is developed a layer of supporting tissue h hicli forms the iiieinbrana propria. This layer fuses
 
with the perichondrium along the jx-ripheral margin of thi- ciiiial :in<l thereby constitutes a ligament that
 
attaches each membranous duct throughout its whole length to the cartilaginous .space in which it is suspended.
 
  
Fin. 2.'i. Section through the apex of the cochlea of a human fetus 130 mm. crown-rump length (Carnegie Collection,
 
No. 1018, slide 32, section 2), enlarged 57 diameters. This section shows the tip of the cochlear duct and
 
the character of the communication that develops between the two scate forming the helicotrenia. It
 
will be seen that the margins of the periotic spaces are not so mature here as in the proximal parts of the
 
cochlea of the same fetua, on comparing tliis figure with figure 20.
 
  
Plate 4.  
+
The relation of the scala tympani and scala vestibuli to the subarachnoid spaces surrounding the hind-brain is of considerable interest, both on account of the possibiUty of their functional relationship and on account of the similarity that exists in their development. For a satisfactory investigation of the establishment and the character of the communications that are formed between these two allied systems of tissue-spaces, one should resort to other methods than those used in the present study, and, furthermore, one should examine older fetuses than those described here. In fact, a problem lies here that would be well worth careful study.  
  
The figures shown on this plate represent a series of median and lateral views of wax-plate reconstructions of the
 
membranous labyrinth and the surrounding periotic tissue-spaces. They illustrate under the same scale of
 
enlargement three typical stages in the development of these spaces. Abbreviations: C. s. 1., ductus
 
semicircuiaris lateralis; C. s. p., ductus semicircularis posterior; C. s. s., ductus semicircularis superior;
 
Duct, coch., ductus cochlearis; Impressio rotund., area opposite the fenestra cochleae; Impressio staped.,
 
area in contact with base of stapes; Saccus endol., saccus endolymphaticus; Scala tymp., scala tynipani;
 
Scala vestib., scala vestibuH.
 
  
Fio. 26. Lateral view of a model reconstructed from a human fetus 50 mm. crown-rump length (Carnegie Collection,
+
Certain observations, however, were made in the course of the above investigation that bear relation to these matters, and they will be briefly outlined here. In the first place, the histological picture of the periotic reticulum is essentially the same as that of the early stages of the pia-arachnoidal tissue investing the central nervous system. The enlargement of the meshes of the latter and the formation of the subarachnoid spaces and the arachnoid cistern, as has been recently described by Weed (1917), correspond exactly with the appearance seen in the histogenesis of the periotic spaces in the ear. The periotic spaces are not, however, extensions of the arachnoid spaces that have invaded the cavity of the cartilaginous labyrinth. If this were so we should find them first appearing among the rootlets of the vestibular and cochlear nerves, along which the subarachnoid space extends for some little distance. Instead, they begin at points where there can be no connection with the arachnoid tissue and their direction of growth is quite independent of it. The periotic spaces maj' be analogous to the arachnoid spaces, but they are not identical with them, nor are they an extension of them.  
No. 84). The cistern and the scala vestibuli are shown in green and the scala tympani is shown in orange.  
 
The scala vestibuU is in the first stage of its development and consists of a row of large reticular spaces which
 
extend from the ventral margin of the cistern downward along the apical surface of the cochlear duct. The
 
scala tympani is more advanced and shows more complete coalescence of its constituent spaces. Enlart;eii
 
1 1 .4 diameters.  
 
  
Fin. 27. Median view of the same model .shown in figure 26. This view shows the tojjography of the scala tympani.
 
Its large proximal end lies opposite the fenestra cochleje (rotunda) and corresponds to the focus at which its
 
development originates. Distally it tapers off rapidly where the spaces are smaller and their coalescence less
 
complete. Enlarged 11.4 diameters.
 
  
Fi(i. 28. Lateral view of wax-plate reconstruction of the left membranous labyrinth and the periotic spaces in a human
+
According to the descriptions of the adult anatomy of the ear, a communication becomes established between the scala tympani and the subarachnoid space near the fenestra cochleae, the so-called aquaeductus cochleae. Vague and conflicting statements are also made concerning a communication through the internal auditory meatus connecting the arachnoid spaces with the scalae. Such communication must be estabhshed quite late. In the oldest fetus examined, 130 mm. crown-rump length, they did not yet exist. As to the latter communication, it can be seen that the arachnoid spaces extend peripherally through the internal
fetus 85 mm. crown-rump length (Carnegie Collection, No. 1400-30), enlarged 11.4 diameters. The cistern
+
auditory meatus along the trunk of the acoustic nerve-complex, and slender pockets and clefts from them extend along the larger bundles of the cochlear nerve; they terminate, however, before reaching the margins of the scalae, and there is no evidence at this stage that there is ever to be a conununication between them and the scalae. As to the aquaeductus cochleae, in the 130 mm. fetus it can be plainly seen that it is already forming as a derivative of the arachnoid spaces, although the communication with the scala tympani is not yet established. The arachnoid spaces invest the glossopharyngeal nerve and extend down along its trunk and pass directly by the region of the fenestra cochleae (rotunda). A thin-walled tubular pouch projects from these spaces, leaving the nerve trunk and extending obliquely toward the scala tympani in a direction that would meet it just distal to the fenestral impression on its basal surface. This fundament of the aqua?ductus cochleae is present in fetuses 85 mm. crown-rump length, but is longer in the 130 mm. fetus, where it nearly reaches the scala tympani. The communication must be established soon after this.
and the connecting scala vestibuli are shown in green. Although the greater jiart of the cistern abuts against
 
the stapes, it will be noted that it is also begiiming to spread over the liorsal surface of the utricle and along  
 
the inner border of the lateral semicircular duct. The scala vestibuh communicates freely with the cistern
 
and extends downward alotig the apical surface of the cochlear duct, throughout nearly two turns, showing
 
the characteristic sacculated appearance near its tip, w here the coalescence of the spaces is less complete.  
 
  
Fi<;. 29. Median view of same model shown in figure 28, enlarged 11.4 diameters. The scala tympani is shown in
 
orange. The oval indentation in its proximal end corresponds to the fenestra cochlea (rotunda). This
 
space extends along the cochlear duct about the same distance as the scala vestibuli, but the two do not comMUinicate yet at any place. The peripheral border of the scala tympani is characterized by sacculations
 
corresponding to spaces that are coalescing with the main s|)a(c. The grow th of the scala is due to a coalescence of new spaces along its peripheral border rather than along its cential border.
 
  
Fio. 30. Lateral view of a wax-plate reconstruction of the li'ft membnmoiis labyrinth and the periotic spaces in a
 
human fetus 130 mm. crown-rump length (Carnegie Collection, No. 1018), enlarged 11.4 diameters. The
 
cistern and scala vestibuli are shown in green and the scala tympani is shown in orange, as in the previous
 
figures. The cartilaginous stapes was removed from this model and the oval impression that it makes on the
 
cistern can be plaiidy seen. The cistern has spread over the top of the utricle and part way along the lateral
 
semicircular duct. The scala vestibuli extends to the ti|) of the cochlear duct, where it communicates with
 
the McaLi tympani, thas forming the helicotrema.
 
  
Fki. 31 . Mcnlian view of .same model shown in figure 30, enlarginl 1 1.4 diameters. The oval impression on the proximal i-nd of the scala tympani corresponds to the fenestra cochle;c (rotunda). .\s yet there is no conmiunication at this point between the scala tympani and subarachnoid spaces, such :is is found in the adult and
+
{{Template:Carnegie No.20 Footer}}
known as the aqua-ductus cochlete. The spaces making up the <ist<'rn cover almost the whole of th<' utricle
 
and saccule except the places at whicli t he nervi-s enter and a small part of the medial surface near the attachment of tlie appendage.
 

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A personal message from Dr Mark Hill (May 2020)  
Mark Hill.jpg
I have decided to take early retirement in September 2020. During the many years online I have received wonderful feedback from many readers, researchers and students interested in human embryology. I especially thank my research collaborators and contributors to the site. The good news is Embryology will remain online and I will continue my association with UNSW Australia. I look forward to updating and including the many exciting new discoveries in Embryology!

Streeter GL. The histogenesis and growth of the otic capsule and its contained periotic tissue-spaces in the human embryo. (1918) Contrib. Embryol., Carnegie Inst. Wash. 8: 5-54.

Online Editor  
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If like me you are interested in human development, then this historic embryology monograph by Dr. George L. Streeter has a wonderful detail and interpretation of the otic capsule formation as available (at that given point in time) using a significant resource of human material from the Carnegie Institute. Further historic material is available on the page Contributions to Embryology series. The magnifications stated in the figure and plate legends refer to the original published images, not those available online. My thanks to the Internet Archive for making the original scanned book available. Those interested in current hearing and skull development should read the online notes on Hearing and Balance Development and Skull Development



  Streeter Links: George Streeter | 1905 Cranial and Spinal Nerves | 1906 Membranous Labyrinth | 1908 Peripheral Nervous System 10mm Human | 1908 Cranial Nerves 10mm Human | 1912 Nervous System | 1917 Scala Tympani Scala Vestibuli and Perioticular Cistern | 1917 Ear Cartilaginous Capsule | 1918 Otic Capsule | 1919 Filum Terminale | 1920 Presomite Embryo | 1920 Human Embryo Growth | 1921 Brain Vascular | 1938 Early Primate Stages | 1941 Macaque embryo | 1945 Stage 13-14 | 1948 Stages 15-18 | 1949 Cartilage and Bone | 1951 Stages 19-23 | Contributions to Embryology | Historic Embryology Papers | Carnegie Stages | Category:George Streeter George Linius Streeter (1873-1948)


Modern Notes:

Hearing Links: Introduction | inner ear | middle ear | outer ear | balance | placode | hearing neural | Science Lecture | Lecture Movie | Medicine Lecture | Stage 22 | hearing abnormalities | hearing test | sensory | Student project

  Categories: Hearing | Outer Ear | Middle Ear | Inner Ear | Balance

Historic Embryology - Hearing 
Historic Embryology: 1880 Platypus cochlea | 1892 Vertebrate Ear | 1902 Development of Hearing | 1906 Membranous Labyrinth | 1910 Auditory Nerve | 1913 Tectorial Membrane | 1918 Human Embryo Otic Capsule | 1918 Cochlea | 1918 Grays Anatomy | 1922 Human Auricle | 1922 Otic Primordia | 1931 Internal Ear Scalae | 1932 Otic Capsule 1 | 1933 Otic Capsule 2 | 1936 Otic Capsule 3 | 1933 Endolymphatic Sac | 1934 Otic Vesicle | 1934 Membranous Labyrinth | 1934 External Ear | 1938 Stapes - 7 to 21 weeks | 1938 Stapes - Term to Adult | 1940 Stapes | 1942 Stapes - Embryo 6.7 to 50 mm | 1943 Stapes - Fetus 75 to 150 mm | 1946 Aquaductus cochleae and periotic (perilymphatic) duct | 1946 aquaeductus cochleae | 1948 Fissula ante fenestram | 1948 Stapes - Fetus 160 mm to term | 1959 Auditory Ossicles | 1963 Human Otocyst | Historic Disclaimer


Development of the Perichondrium

In the description of the development of the periotic reticulum we have seen how it begins as a small focus along the central border of the epithelial semicircular duct and spreads at the expense of the temporary precartilage, forming as it does so a crescentic-shaped area of reticulum inclosing the duct. We have also seen how the im-asion or spread of the reticulum into the surrounding area of precartilage is brought about, at least in the later stages, by a dedifferentiation of the latter into the former.


Furthermore, along with this latter process, the inner margin of cartilage surrounding the duct is dedifferentiated into precartilage, so that a new area of precartilage becomes established as the old area disappears. The conversion of precartilage into reticulum in the later stages, however, is more rapid than the conversion of cartilage into precartilage, and consefjuently there comes a time when the precartilage has nearly all disappeared. In such specimens the reticuhnn extends practically from the epithelial duct to the margin of the cartilaginous canal. The (|ualifying term "practically" is used because the inner and outer margins of the reticulum are modified in a special manner. The inner margin becomes condensed into a membrane-like coat of fibrous tissue that constitutes the membrana ])ropria of the membranous canal. The outer margin at about this time undergoes changes that result in the formation of the jjerichondrium.


In discussing the lu'richondrium it is important to kcej) in imnd the active alterations in the tissue along the margin of the cartilage that accompany the growth of the labyrinth. It has been seen how the enlargement of the cartilaginous canals and their alterations in form and position is obtained partly by excavation of cartilage and partly by the laying down of new cartilage, the excavation being accomplished by its dedifferentiation into precartilage and reticulum, and the new cartilage being built up through a precartilage stage from the periotic reticular tissue. Throughout the entire period of growth of the cartilaginous canals the elements of this continual transformation exist along their margin. The margin during this period is in a state of temporary eciuilibrium and is capable of advancing or receding as the conditions determine.


The first and relatively the major part of the hollowing-out of the cartilaginous canals is complete before the perichondrium makes its appearance. This is illustrated, for instance, by the fetus of 52 mm. crown-rump length, in figure 17, where there is as yet no indication of it shown. In fetuses between 40 and 50 mm. long the zone of precartilage surrounding the margins of the canals, as seen in figures 14 and 15. might be mistaken for perichondrium. This area, however, in fetuses sUghtly older is converted almost entirely into reticulum. Kolliker (1879), in the second edition of his text-book on embryologj', pictures a transverse section through the lateral canal of a rabbit embryo (fig. 457, page 735), in which this zone of precartilage is labeled as periosteum of the future bone.


The real perichondrium does not make its appearance until the fetus reaches a a length of about 70 mm. A specimen of this age is represented in te.xt-figure 4, which shows a segment of the posterior semicircular canal in a fetus 73 mm. crownrump length (Carnegie Collection, No. 1373). On examination of this specimen it is found that there is a distinct condensation of the reticulum along its inner margin, so that it forms a membrana propria for the epithehal duct with which it is in contact. This area has largely lost its reticular character and now resembles embryonic fibrous connective tissue. Along the outer margin of the reticulum a similar condensation of its trabeculse has taken place, forming a thin fibrous lamina or membrane near the margin of the cartilage. This is the perichondrium in its early form. It does not abut directly against the cartilage, but is separated from it by a thin layer of transition tissue that is in process of dedifferentiation from precartilage into reticulum.


Passing inward from the cartilage, the transitions are rapid from cartilage to precartilage, from precartilage to the tissue that is in transition to the reticulum and then to the perichondrium. These are found as narrow zones that merge quickly from one into the other. One should remember that the cartilaginous canal has not reached its full size yet, and that the margin of the canal is still in an unstable condition. However, as the canal becomes larger and the tissues more mature, it is found that the transitions between the different zones become more abrupt and in this process the precartilage zone becomes relatively much narrower. This can be seen by comparing text-figures 3 and 4. The width of the reticulum in these two figures can not be compared, because the.v represent diflferent canals, lateral and posterior, and no attempt was made to take them from the same relative positions. The fact that the reticulum is narrower in figure 4 has no significance in the question of growth. The wide precartilage zone in figure 3 as compared with that in figure 4, on the contrary, has a direct bearing on the relative age of the two specimens. A relatively wide zone of precartilage is characteristic of younger stages. After fetuses become 70 mm. long the precartilage zone becomes quite narrow, so that the transition from cartilage to perichondrium is relatively abrupt. In older si^ecimens one might easily obtain the impression that the perichondrium rested directly against the cartilage, as doubtless it does in the adult. In the oldest fetus examined, 130 mm. crown-rump length, there is still found a distinct though narrow precartilage-reticular transitional zone between the cartilage and the perichondrium. Presumably this indicates that the margin is still in an unstable condition.


After the perichondrium has made its first appearance it rapidly becomes thicker and more conspicuous. In a fetus 80 mm crown-rump length (Carnegie Collection, No. 172) it is found as quite a dense fibrous coat, more than twice as thick as that shown in the 73 mm. embryo in figure 4. It is clearly separated from the cartilage and precartilage by a narrow zone of reticular tissue.


The character of the perichondrium as existing in slightly older fetuses is shown in figure 18, which represents a section through the posterior semicircular canal of a fetus 85 mm. crown-rump length (Carnegie Collection, No. 140030). Here the perichondrium consists of a relatively broad zone of enibrj'onic fibrous connective tissue, which in the photograph is about 5 mm. wide, encircling the whole canal. It can be seen on the median side (to the left) that it is sejxirated from the cartilage and adjacent transforming precartilage zone by a narrow, lighter area, which under higher magnification is found to consist of reticular tissue. The membrana propria at the inner margin of the reticulum is fairly well developed and it can be seen how it forms a supporting coat to tho epithelial duct.


When one examines the cartilaginous semicircular canals in fetuses 130 mm. long there can no longer be any ([uestion as to the identity of the perichondrium. A specimen showing the superior semicircular canal at this stage is represented in figure 19, which is taken from a fetus 130 mm. crown-rump length (Carnegie Collection, No. 1018). The blood-vessels are injected with India ink. The main cartilaginous mass in this specimen is quite mature; the capsules are well defined and the cartilage cells now possess a considerable amount of granular body-protoplasm.

Streeter004.jpg


Fig. 4. Detail of the posterior canal in a human fetus 73 mm. long

(Carnegie Collection, No. 1373, slide 9, row 3. section 1) The section is 10 microns thick and is enlarged 370 diameters (in original printed version). It shows how the inner margin of the reticulum becomes condensed into the membrane propria of the epithelial duct and the outer margin into the perichondrium. The perichondrium does not lie in direct contact with the cartilage, but is separated by a narrow zone of tissue which consists of precartilage, into which the cartilage is still being dedifferentiated.


In many instances capsules are found containing more than one cartilage cell, showing the tendency to cell columns.


A casual glance at a section under lower powers might indicate that the inner margin of the cartilage is in direct contact with the perichondrium. Examination under higher magnification, however, shows that between the thick perichondrium and the cartilage there is a narrow zone of dedifferentiated cartilage. In it the matrix has largely disappeared and the capsules have collapsed and are flattened out, allowing the elongated endoplasm of adjacent cartilage cells to come in contact, separated only by the remnants of the capsular margins. Dyes that stain endoplasm red cause this zone to appear as a deep-red line. This zone represents a state of transition between cartilage and precartilage and its presence doubtless indicates that the margin of the cartilage is still in an unstable condition. The narrowness of the zone and the abruptness of the transition are characteristic of later stages, where the process is more gradual and relatively small in amount. The transition from this zone to the perichondrium is likewise abrupt. The perichondrium consists of a dense protoplasmic stratum thickly studded with nuclei, and has all the appearance of late embryonic fibrous connective tissue. It is of about the same tliickness around the whole margin of the canal. At the outer margin (toward the right) it fuses wdth the membrana propria of the epithelial duct, therebyforming an attachment which is regarded as a suspensory ligament for the support of the membranous labyrmth. The trabeculae of the reticulum extending between the membrana propria and the perichondrium are just beginning to break apart, allowing the adjacent spaces of the reticulum, as they are seen in section, to coalesce in the formation of larger spaces.


Having completed the review of the early history of the reticulum and its formative relations to the adjacent tissues, we are now in a position to consider the development and the fate of these larger spaces in the reticulum, which have hitherto been generally known by the misleading term "perilymphatic spaces."

Development of Periotic Tissue Spaces

In the preceding pages of this article the main features of the development of the cartilaginous capsule that incloses the membranous labyrinth have been described. We have traced the process step by step from the first condensation of the mesenchjone around the otic vesicle, through its differentiation into a precartilaginous mass and the maturation of the latter into true cartilage, with the formation through dedifferentiation of cartilaginous chambers in wliich the membranous labyrinth is suspended. It has been shown how these spaces within the cartilaginous capsule are modified in adaptation to the continued growth of the membranous labyrinth and how they finally come to be fiUed with an open-meshed reticulum which everjT\ here bridges the space existing between the membranous labyrinth and the surrounding cartilage. It has further been shown that the membrana propria supporting the epithehal part of the labyrinth on the one hand and the perichondrium on the other are derived from and serve as the hmiting membranes of this reticulum. It is a modification in the meshes of this same reticulum that results in the formation of the so-called perilymphatic si)aces, or periotic spaces as they will be referred to in this paper, the development of which will now he outlined.


Thus far attention has been directed primarily to regions included in typical transverse sections through the semicircular canals. This was done for the purpose of uniformity and simplicity and because of the- ease with which successive stages could be compared with one another. For studying the periotic spaces, however, the region of the canals is not so favorable, because the spaces are late in developing there, and even in their completed form they are not so well defined and highly differentiated as those in the region of the vestibule and cochlea.


The earliest evidence of a periotic space makes its appearance opposite the stapes. It is developed in the reticulum that fills the interval situated between the saccule, utricle, and the cartilaginous stapes. Even before the general periotic reticulum becomes very extensive, in embryos between 30 and 40 mm. long, it can be seen that its meshes are more irregular and more open in this region than elsewhere. This is the rudimentary form of the periotic vestibular cistern, which is the first space to become established.


Development of the Periotic Cistern of the Vestibule

Aside from the scala vestibuli and the scala tympani, the largest of the periotic spaces is the large reservoir situated between the tympanic wall of the bony vestibule with its articulated stapes and the vestibular chambers of the membranous labyrinth. This is the spatium perilymphaticum vestibuli (BNA) or the cisterna perilymphatica (Retzius). In order to eliminate the word lymphatic from the terminology it will be designated here as the cisterna periotica vestibuli, or less formally the periotic cistern. In this manner the descriptive term introduced by Retzius is retained.


Before there is any trace of the scalse the initial steps in the formation of the cistern can be seen. This is well illustrated in an embryo 35 mm. long (Carnegie Collection, No. 199). This particular embryo is cut in a sagittal series and the sections on slides 53 and 54 show the periotic cistern in its most rudimentary form. It consists of an area of reticulum bounded by the utricle, saccule, ductus reuniens, the proximal end of the cochlear duct, and the ampulla of the jiosterior semicircular duct. The greater part of the periotic reticulum at this time (35-mm embryo) is characterized by a narrow and uniform mesh that is interrupted only by numerous cajjillaries branching through it; in the area mentioned, however, the spaces are larger and are more irregular both in shape and in size. They i)resent the appearance seen along the semicircular ducts in considerably older embryos, for instance, in the 52-mm. embryo, as is shown in figure 17. From the very first the increase in the size of the mesh seems to be attained by the detachment and retraction of its constituent i)rotoplasmic bridges, thereby allowing adjacent spaces to unite in the formation of composite large spaces. Thus in the above section a few irregular protopla.smic free-ends are seen still projecting into the newly enlarged spaces. This interesting histogenetic process will be taken up again later in connection with the development of the two scalae. The area of this rudimentary periotic cistern is as yet very small and merges indefiniteh' into the adjoining reticulum. It is not until we come to fetuses about 40 mm. long that it develops spaces of any considerable size, and it is not until we come to fetuses about 50 mm. long that we find a single large space with walls that are definitely outlined, so that it can be satisfactorily modeled.


In a fetus 43 mm. long (Carnegie Collection, No. 886), which is cut in a coronal series, the spaces forming the rudimentary cistern stand out much more definitely than is the case in the 3o-mm. embryo that has just been referred to. There is now just opposite the stapes one space which is much larger than the adjoining spaces. On part of its margin the protoplasmic bridges are stretched along so as to form a smoothly curved continuous boundary, which is defective in some portions, and at such places the space merges with the adjoining secondary spaces. Within the space are some fainth' refractive branching threads of coagulated plasma. The scala vestibuli is not yet laid down and the scala tympani is only represented b}' a moderate widening of the meshes of the reticulum in the neighborliood of the fenestra cochleae (rotunda), along the basal border of the first turn of the cochlear duct.


In fetuses 50 mm. long the outlines of the cistern become very distinct, due to the marked increase in the size of its main cavity and to the more definite membrane at its junction with the rest of the reticulum. Its form and relations are shown in figures 26 and 27. They represent a median and a lateral view of a wax-plate reconstruction of this region in a human fetus 50 mm. long (Carnegie Collection, Xo. 84). Only the main cavity is shown in the model. At certain places around its borders the meshes of the reticulum are uniting into larger spaces and these in turn are taken up by the main cavity as it advances into the new territory. These smaller incomplete spaces were omitted in constructing the plates of the model. The rule was adopted that only the spaces that were outlined by a membrane-like border should be traced on the plates and included in the model. This rule was adhered to in all the models of this series.


Figures 26 and 27 show that the periotic cistern in 50-mm. embryos consists of a flattened, rounded, bursa-like cavity intervening between the stapes and the lateral surface of the saccule and adjoining utricle. It extends forward to the ijmpuUa of the lateral canal and upward to the beginning of the crus commune. Posteriorly it crowds backward against the ductus reuniens, filling in the space between the utricle, saccule, and the proximal end of the cochlear duct. Both on its median and lateral surfaces there is no further opportunity for expansion except as the vestibule itself enlarges. The deUcate membrane-like wall of the cistern hugs closely against the parts of the membranous labyrinth on the one side and the tympanic wall of the cartilaginous vestibule on the other, being separated from them only by a thin layer of the original reticulum. Along the dorsal margin of the cistern, however, there is room for expansion, and the reticulum in this region shows enlarging spaces in the process of uniting with the main cavity. On its ventral margin, near the cochlea and extending along the apical surface of the latter, there is a definite row of reticular spaces actively coalescing and constituting the beginning of the scala vestibuli. These are shown in figure 21, which is a section of a fetus of about the same age. The spaces of the scala vestibuli lie between the cochlear duct and the cistern. This section also shows veiy well the relation of the stapes to the cistern. The scala tymi)ani is already well started at this time, but its development is quite independent of the cistern. Within the cistern can be seen scattered clumps of faintly refractive granular threads of what seems to be a coagulated constituent of the plasma.


The subsequent growth of the cistern is shown in figures 28 to 31. Figures 28 and 29 show respectively a median and lateral view of a wax-plate reconstruction of the membranous labyrinth and its periotic spaces in a human fetus 85 mm. long (Carnegie Collection, No. 1400-30). The growth of the cistern here has kept pace with the increase in size of the lab3'rinth and maintains the same general relations as regards the stapes and the parts of the membranous labyrinth. The view of the cistern in figure 28 is an oblique one which would tend to mislead one as to its width. In reality it is relatively a little wider. It has also extended upward on the dorsal surface of the utricle and is beginning to creep along the inner side of the jjostcrior end of the lateral semicircular duct. Ventrally it communicates freely with the scala vestibuli, which now extends well down along the cochlear duct.


The oldest stage studied is shown in figures 30 and 31. These show two views of a wax-plate reconstruction of these structures in a human fetus 130 mm. long (Carnegie Collection, No. 1018). At this time the periotic cistern has spread over the vestibular part of the membranous labyrinth, covering it nearly eveiywhere excepting at the macular jiortions where the nerves terminate. In figure 31 it can be seen that the mesial surface of the saccule is not covered ; this lies close against the wall of the cartilaginous vestibule. The uppermost division of the cistern, situated between the crus commune and the ampulla of the posterior semicircular duct, does not yet open into the general cavity It has formed separately and owing to the i)osition in which it lies its coalescence with the other parts of the cistern is retarded ; otherwise, free communication exists between all divisions of the cistern.


Development of the Periotic Spaces of the Semicircular Ducts

From the descriptions given of the adult the reticulum along the ducts never develops a single continuous wide periotic space like that of the cistern and the two scala?. There always remain a few trabecular, such as are seen in the cistern and scala? in their earlier stages, and these constitute partitions which traverse the space and give it in sections the a])pearance of a series of separate spaces extending along the inner margins of the semicircular ducts. Although these spaces along the ducts are inc()mi)lete as compaicd with the cistern and scahc, they are, however, entirely analogous with them in their formation.


The space along the lateral semicircular duct is the largest. Its posterior end exists as a continuation of the cistern. This can be seen in the lateral view of the model shown in figure 30, where the cistern extends for a considerable distance along the inner border of the lateral duct. Along the other two ducts of the same specimen (130 mm. crown-rump length) the reticulum has commenced the process of space-formation, but complete channels are not yet established. A typical section through one of the semicircular ducts in a fetus of this size, and this is the oldest fetus studied, is shown in figure 23. As compared with the scalse in the same fetus, as shown in figure 20, the space-formation along the ducts is very much retarded.


Development of Scala Tympani and Scala Vestibuli

The scala vestibuli may be regarded as an extension of the cistern downward into the region of the cochlea and as such its growth starts from a focus opposite the fenestra vestibuli (ovalis) . The scala tympani in a similar way makes its first appearance opposite the fenestra cochleae. From these two foci the scalae extend gradually downward along the cochlear duct as two separate spaces which do not communicate with each other until they reach the tip of the duct, where there is finally developed a free opening between them known as the helicotrema.


In their formation they go through a series of histogenetic changes essentially in the same manner that has been followed in the case of the formation of the cistern ; this (as we shall see) consists of the enlargement of the spaces of the periotic reticulum that originally occupied this region, the enlargement being a result of the disappearance of the protoplasmic bridges of the reticulum, whereby adjacent spaces unite in the formation of composite larger spaces. This process continues until there is a single continuous space extending down along the cochlear duct representing each scala and at the margins of each of them there is developed a membranous arrangement of the reticular cells which completely walls ofif the space from the surrounding tissue. In these alterations in the reticular mesh and in the formation of the surrounding membrane there is an active change in the form of the reticular cells, which repeatedly adapt themselves to the new conditions. There is no evidence to indicate that smy other cells take part in the formation of the scalae.


The first evidence of the formation, of scalae is found in fetuses about 40 mm. long, which stage is a little later than the first appearance of the cistern. In a fetus 43 mm. crown-rump length (Carnegie Collection, No. 886), along the proximal part of the cochlear duct on its basal surface there is a distinct widening of the meshes of the periotic reticulum. This is the beginning of the scala tympani. On the opposite side of the cochlear duct, where one would look for the scala vestibule, the periotic reticulum retains its primitive appearance characterized by a narrow and rather uniform mesh. Thus the scala tympani makes its appearance slightly in advance of the scala yestibuli that is, if we regard the latter as distinct from the cistern.


In fetuses 50 mm. long both the scala tympani and the scala vestibuli can be plainly identified, although they are still very incomplete. A wax-plate reconstruction of them, representing their form and their relation to the membranous labyrinth in a human fetus 50 mm. crown-rump length (Carnegie Collection, No. 84), is shown in figures 26 and 27, being a median and a lateral view respectively.


In preparing this and tlie models shown in figures 28 to 31, it is to be remembered that only those periotic spaces are included that were outlined by a membranelike margin. In the adjacent reticulum there are spaces that are actively coalescing and gradually uniting with the main cavity. No attempt, however, was made to show such spaces in the models. From figures 26 and 27 it will be seen that the scala tympani is larger and more advanced in its development than the scala vestibuli. The hitter is in its earliest stage and consists of hardly more than a row of enlarged reticular spaces which extend downward from the cistern along the dorsal and apical surface of the cochlear duct. A section through the scala vestibuli in another fetus of about the same age (Carnegie Collection, No. 448) is roughly shown in figure 21, the spaces of the scala being situated between the cistern and the cochlear duct.


The scala tympani consists of an elongated oval space lying along the basal surface of the proximal part of the cochlear duct, about corresponding to the proximal half of the first turn of the duct. In the main part it is a single space with a distinct margin separating it from the general periotic reticulum. In the more apical portion it tapers off into multiple incompletely united smaller spaces which actively coalesce as the process advances into the new territory along the duct. It is of interest to note that the most mature and the largest part of this scala, representing the focus at which it first appeared, is opposite the fenestra cochleae (rotunda), just as the cistern forms opposite the stapes and the fenestra vestibuli. The scala tympani always begins at the same place and extends downward along the cochlear duct, at first a little in advance of the scala vestibuli, but subsequently the latter catches up with it and the two reach the tip of the duct at about the same time.


It is well known that the proximal portions of the cochlear duct mature sooner than the distal portions. One might expect that the accompanying periotic spaces would correspond in their development to the maturity of the duct and therefore the proximal parts of the scalse would differentiate first. In other words, the maturation of the cochlea proceeds as a wave from the proximal end to its tip, involving all of its constituent structures as it passes along, including mesenchymal parts as well as epithelial.


This conception might explain the direction of development of the scalae, but can hardly be applied to the cistern, the vestibular representative of the scala vestibuli. One can not say that those portions of the membranous labyrinth lying opposite the focus of development of the cistern (that is, the lateral walls of the saccule and utricle) mature in advance of the rest of the labyrinth. There is no indication that a wave of differentiation passes through the epithelial elements of the labyrinth in the same direction and synchronously with the extension of the cistern as it advances from its primary focus upon the roof of the utricle and over on its median surface. In the case of the cistern it seems much more likely that the point at which it first apjiears is determined by the position of the stapes, which is doubtless an expression of the physical relation that subsequently exists between the two. By analogy this would yield additional significance to the relation existing between the fenestra cochlea; and the point of beginning development of the scala tympani.


In dealing with the cistern and also with the scalse one should not consider them as insignificant accessories that merely fill in the waste intervals between the membranous labyrinth and the surrounding cartilage. From studying their development it becomes apparent that they have a morphological individuality in many respects as definite as that of the ossicles themselves. They make their appearance at a definite time and at definite places, they spread in a definite manner, and eventually they attain a form and structure that are adapted to a definite function. This becomes more and more evident as we examine older stages.


The form and relations of the scalie in fetuses between 12 and 13 weeks old are shown in figures 28 and 29. These figures show median and lateral views of a waxplate reconstruction of the membranous labyrinth and the surrounding periotic spaces in a human fetus 85 mm. crown-rump length (Carnegie Collection, No. 1400-30). Attention has already been directed to these figures in the description previously given of the cistern. The scala vestibuli can be seen in figure 28. Above, it opens freely into the cistern and extends downward along the apical side of the duct as a single main space, possessing a rather uniform diameter. It extends along the first two turns of the duct, gradually tapering off and showing a less mature character in its distal portions. Along the second turn of the duct the spaces are incompletely fused and the contour becomes correspondingly irregular. As a rule the peripheral margin of the scala is less mature and more irregular than the central margin. The scala, vestibuli does not connect with the scala tympani at any point as yet. The two are separated in the first place by the cochlear duct and then more centrally by a framework of connective tissue in which are the radiating bundles of the cochlear nerve with the nodes of ganglion cells that form the spinal ganglion. These latter structures are not shown in the model; they occupy, however, the V-shaped groove seen between the two scalae.


The scala tympani, as can be seen in figure 29, extends downward on the basal side of the cochlear duct along its first two turns. This corresponds to about the same linear dimension as that of the scala vestibuli. In its proximal portion it shows a greater area in cross-section than the latter, but further toward the apical region it is of about the same size and in some places it is even smaller. The peripheral margin of the scala tympani is distinctly more irregular than the central margin. The irregularity is due to spaces along this margin that are actively coalescing with the main sjiace, but in which the fusion is not yet complete. The irregularity of this margin is thus an indication of the direction of the expansion of the scala. As the diameter of the whole cochlear mass increases, it is evident that the main growth of the scala must radiate outward in a peripheral direction. This is accomplished by the continual assimilation of new reticular spaces along this margin. At the proximal end of the scala tympani can be seen an oval depression which corresponds to the fenestra cochleie (rotunda) and with which it stands in intimate relation.


In fetuses about 16 weeks old the form and relations of the scalae have nearly attained the adult conditions, and this represents the oldest stage studied in connection with the present paper. The conditions found at this time are shown in figures 30 and 31, which present nuMhan and lateral views of a wax-plate model of a human fetus 130 mm. crown-rump length (Carnegie Collection, No. 1018). On comparing the scala tympani and scala vestibuli as seen in these figures with those in figures 28 and 29, it will be seen that they are larger in cross-section and more nearly cover the cochlear duct. Furthermore, they now extend to the extreme tip of the duct and communicate with each other across its central margin, thus forming a helicotrema. A section through this point can be seen in figure 25, in which these structures are shown as seen under low magnification. It will be noted that now, even as far as the tip of the cochlea, each of the scalse consists of a continuous principal space, though both are more mature and larger in their proximal portions. Along the first turn of the cochlear duct they are walled off by a smooth membranous margin which separates them from the adjacent reticular tissue. The spaces of the latter do not seem to be taking any further part in the process of enlargement of the scala. Along the second turn of the cochlear duct, a section of which is shown in figure 20, the coalescence of reticular spaces with each other and with the scalse is still in active operation. This produces a greater irregularity of the scalar than is shown in the model. The subsidiary spaces are shown as a solid mass; the slender clefts separating them are not represented. The nearer one approaches the tip of the duct the more immature are the scalse, until the condition is reached that is shown in figure 25, where the membrane-like margin is quite incomplete and the spaces merge irregularly with the surrounding reticulum. Thus a single specimen, if studied in its different parts, shows several stages in this interesting process of the formation and growth of the scalaj.


The figures grouped on plate 3 illustrate some of the histological features of this process. An early stage in space-formation is shown in figure 23. This is a section through the canal region where the changes in the reticulum are late in making their appearance. In fact, the periotic spaces never reach the same degree of differentiation here that occurs in the case of the cistern and scalse. The initial steps, however, are the same, and this figure presents very well the appearance of the periotic reticulum as it begins to open up into larger spaces. Unmodified reticulum is characterized by a rather uniform narrow mesh. The essential change in space-formation consists in the disappearance of some of the trabecule of the mesh, with the consequent coalescence of the corresponding adjacent spaces. The trabecuhe consist of the protoplasmic processes of the constituent cells of the reticulum and their disappearance is to be explained in either of two ways: It is possible that owing to some property of the fiuid element of the tissue the protoplasmic strands are dissolved or liquefied; this would account for their complete disappearance. On the other hand, the same result could be accomplished by an alteration in the form of the cell processes. A given trabecula could separate at either end, or at some jioint along its line, and the free ends of protoplasm could then retract and reshape themselves and become a part of the remaining framework. Whether we are dealing with a licjuefaction of tissue or with active motility of the cell, protoplasm involving detachment and retraction of the trabeculae can not be definitely determined by observations of fixed tissue; but the appearance of sections where the process is in active operation seems to the writer to indicate the latter.


In the above paragraph and elsewhere in this paper reference is made to trabeculae ser^dng as "partitions" between "spaces" and the disappearance of trabeculse resulting in the "coalescence of adjacent spaces." In making this use of the term "space" it should be explained that it is done in a descriptive sense, in application to the appearance of the tissue as seen in sections in which form human embryological material is mainly available. In thin sections of a reticular tissue one sees trabecule as partitions separating adjacent spaces. The same tissue in a mass would show that the spaces everywhere communicate freely with each other, like the spaces in a sponge, and that the trabeculse are thread-like strands which at the best are very incomplete partitions. Instead of a meshwork containing many small spaces, one could perhaps equally well describe reticular tissue as a single large space traversed by many trabeculse. If the latter practice were adopted, one would describe the development of the tissue-spaces with which we are concerned as a process of gradual decrease in the number of traversing trabeculae, with the result that the mesh thereby becomes coarser. For descriptive purposes, however, it is convenient to refer to the intervals between the strands of the mesh as spaces, at the same time not granting them the significance that is attached to such membrane-lined tissue-spaces as are represented by the vestibular cistern and the two scalse, though the latter are in reality derived from them.


In figure 23 the free detached ends of the trabeculse will be noted everywhere, as is characteristic of this stage of development. It is a necessary step in the coalescence of adjacent spaces. The detached trabeculse seem to be gradually retracting and adapting themselves to the formation of larger spaces. Their constituent protoplasm reshapes itself as a smooth border or as a part of other trabeculae. Larger spaces necessitate longer trabeculae, and as trabeculae become longer they also tend to become heavier. These phenomena are all in evidence in the spreading and enlargement of the scalse.


Figure 20 shows a characteristic view of the scalse as seen under low magnification. It will be noted that the scala vestibuli is relatively mature at this point; the scala tympani, however, is in the act of spreading peripherally, so as to underlie, as it eventually will do, the future basilar membrane. The scala tympani finally reaches the peripheral margin of the cochlear duct, and it does this by the coalescence of the enlarging reticular spaces, which become incorporated with the main cavity of the scala. This can be observed better in figure 22, which shows a detail of the same section as seen under higher magnification. By comparing this figure with figure 20 the exact location can be readily made out. A portion of the main cavity of the scala is indicated and to the right of this are a few enlarged reticular spaces that are uniting with each other and will in the end become part of the main space. In addition to the enlarged reticular spaces there is a certain amount of residual undifferentiated reticulum. It is this tissue that will play the part of an adventitial coat to the completed scala. The trabecular that separate the enlarged spaces seem to be under tension and about ready to snap apart. In fact, in most sections one can see the fragmentary ends of trabeculse where this interruption of continuity has apparently occurred.


The differentiation of the margin of the scalae constitutes the final feature in their maturation. During the period in which the enlargement of an individual scala is being brought about by the coalescence of enlarging reticular spaces, the margins of the main cavity can be seen to consist of smooth, delicate strands of nucleated protoplasm that resembles in all essentials that of the trabeculae between the large reticular spaces. These linear margins are interrupted here and there by openings into adjacent spaces, but they tend to form a continuous line that definitely marks off the space from the adjacent reticulum. An early stage in the formation of such a margin is shown in figure 25, where the margin is indicated at a few places, but for the most part the space abuts against the surrounding ragged reticulum. The margin of the space is more complete in the scala tympani shown in figure 22, but it is still thin and delicate and can be easily opened up to allow the taking in of new spaces. If we examine the borders of more mature spaces we find them inclosed by a firmer membrane, which finally reaches a state that will probably not admit of any further opening up for the coalesence of additional spaces. Any further growth must thereafter be limited to simple distention of the wall of the space with the consequent adjustment of its constituent cells. Such a condition is represented in figure 24. This shows a more mature section of the wall of the scala vestibuli, being a detail of the same section shown in figure 20. The only difference between such a membrane, as we must now call it, and the corresponding structure in younger stages is its density; it is wider and its protoplasm perhaps more opaque, or in other words, more protoplasm is accumulated there.


If figures 24, 22, 25, and 23 are compared and followed in that order, it will be seen that the lining membrane of the scala can be traced backward, step by step, to the ordinary trabeculie of the periotic reticulum. There is no histological evidence that any new cells enter into its formation. It seems to be simply a product of the proliferation and adaptive reshaping of the cells already there. In its final form the margin of the space resembles an endothelial membrane. One could describe, as immediately lining the space, a thin membrane with flattened nuclei, which is supported underneath by a thin coat of nucleated protoplasm thai, has the form of fibrous connective tissue. The former, judging only from its final appearance, one might designate as endothelium and thus make a distinction between it and the underlying tissue. In its histogenesis, however, it differs in no way from the rest of the wall and the difference that exists later seems to be merely the result of its adaptation to the existing physical conditions. Its early behaviour is entirely different from that of vascular- endothcliuiu. Thus if its final appearance is stressed and the term endothelium is used for its designation, it must be done with a considerable amount of reservation. It is preeminently a place where the term mesothelium could be used with great advantage.

Communication of Periotic Spaces with Arachnoid Spaces

The relation of the scala tympani and scala vestibuli to the subarachnoid spaces surrounding the hind-brain is of considerable interest, both on account of the possibiUty of their functional relationship and on account of the similarity that exists in their development. For a satisfactory investigation of the establishment and the character of the communications that are formed between these two allied systems of tissue-spaces, one should resort to other methods than those used in the present study, and, furthermore, one should examine older fetuses than those described here. In fact, a problem lies here that would be well worth careful study.


Certain observations, however, were made in the course of the above investigation that bear relation to these matters, and they will be briefly outlined here. In the first place, the histological picture of the periotic reticulum is essentially the same as that of the early stages of the pia-arachnoidal tissue investing the central nervous system. The enlargement of the meshes of the latter and the formation of the subarachnoid spaces and the arachnoid cistern, as has been recently described by Weed (1917), correspond exactly with the appearance seen in the histogenesis of the periotic spaces in the ear. The periotic spaces are not, however, extensions of the arachnoid spaces that have invaded the cavity of the cartilaginous labyrinth. If this were so we should find them first appearing among the rootlets of the vestibular and cochlear nerves, along which the subarachnoid space extends for some little distance. Instead, they begin at points where there can be no connection with the arachnoid tissue and their direction of growth is quite independent of it. The periotic spaces maj' be analogous to the arachnoid spaces, but they are not identical with them, nor are they an extension of them.


According to the descriptions of the adult anatomy of the ear, a communication becomes established between the scala tympani and the subarachnoid space near the fenestra cochleae, the so-called aquaeductus cochleae. Vague and conflicting statements are also made concerning a communication through the internal auditory meatus connecting the arachnoid spaces with the scalae. Such communication must be estabhshed quite late. In the oldest fetus examined, 130 mm. crown-rump length, they did not yet exist. As to the latter communication, it can be seen that the arachnoid spaces extend peripherally through the internal auditory meatus along the trunk of the acoustic nerve-complex, and slender pockets and clefts from them extend along the larger bundles of the cochlear nerve; they terminate, however, before reaching the margins of the scalae, and there is no evidence at this stage that there is ever to be a conununication between them and the scalae. As to the aquaeductus cochleae, in the 130 mm. fetus it can be plainly seen that it is already forming as a derivative of the arachnoid spaces, although the communication with the scala tympani is not yet established. The arachnoid spaces invest the glossopharyngeal nerve and extend down along its trunk and pass directly by the region of the fenestra cochleae (rotunda). A thin-walled tubular pouch projects from these spaces, leaving the nerve trunk and extending obliquely toward the scala tympani in a direction that would meet it just distal to the fenestral impression on its basal surface. This fundament of the aqua?ductus cochleae is present in fetuses 85 mm. crown-rump length, but is longer in the 130 mm. fetus, where it nearly reaches the scala tympani. The communication must be established soon after this.



Carnegie Institution No.20 Otic Capsule: Introduction | Terminology | Historical | Material and Methods | Development of cartilaginous capsule of ear | Condensation of periotic mesenchyme | Differentiation of precartilage | Differentiation of cartilage | Growth and alteration of form of cartilaginous canals | Development of the periotic reticular connective tissue | Development of the perichondrium | Development of the periotic tissue-spaces | Development of the periotic cistern of the vestibule | Development of the periotic spaces of the semicircular ducts | Development of the scala tympani and scala vestibuli | Communication with subarachnoid spaces | Summary | Bibliography | Explanation of plates | List of Carnegie Monographs


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