View source for Book - Contributions to Embryology Carnegie Institution No.39

=The Cartilaginous Skull Of A Human Embryo Twenty-One Millimeters In Length=

by Warren H. Lewis.




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==Introduction==



The human skull has for generations attracted the attention of anatomists. 
From the imaginative comparative anatomists, imbued with the doctrine of the 
segmentation theory, we have inherited a bulky Uterature, gradually passing into 
oblivion, on the segments of an unsegmented skull. Its eflfects, however, are still 
to be noticed in the attempts to imagine a segmentation in the unsegmented brain. 
In the slowly accumulating hterature on the cartilaginous skull of man and other 
vertebrates another phase is manifested. It was anticipated that in the so-called 
primordial or cartilaginous skull there would be found many indications of the 
phylogenetic relationships, and by a comparison of the cartilaginous skulls of the 
various vertebrates Gaupp and lus school expected to show this even more clearly. 
The theory expressed in the terse phrase, "ontogeny repeats phylogeny," formed the 
basis for such expectations. We have, however, gradually come to reaUze that 
there is more untruth than truth in it. For example, how is it possible to fit the 
conditions found in embryo No. 460 (Carnegie Collection), which are shown in 
figures 7 and 8, to the theory that "ontogeny repeats ph\^logeny?" Who would 
claim that our primitive ancestors had more brains than skull, except perhaps the 
few who believe in the downfall of man? And yet in this embryo the brain is enor- 
mous in size as compared with the cartilaginous skull or "primordial cranium." 
Even with the maximum development of the cartilaginous skull the conditions are 
essentially the same. 


Comparative anatomy shows that in lower mammals and vertebrates the skull 
is relatively large as compared vdth. the brain, while human ontogeny shows exactly 
the reverse. The whole assumption that "ontogeny repeats phylogeny" was based 
upon the erroneous notions concerning evolution that were prevalent before the 
present-day conceptions of the germ-plasm were introduced. If the various steps 
in evolution have come about primarily through the modification of the germ- 
plasm, then we should expect changes to appear in the egg and in the subsequent 
stages of ontogeny, and the entire development would thus be modified as much 
as the adult. There undoubtedly are fleeting indications of our primitive ancestors 
in the development of the embryo, but they are not very numerous and are usually 
extremely difficult of interpretation. 


It is probable that in the phjdogenetic history some sort of a membranous 
skull preceded the cartilaginous skull, and the latter preceded the osseous; but it 
is apparent from recent studies on vertebrate cartilaginous skulls that they no more 
form a phylogenetic series than do the adult skulls of the san\e species. The series of cartilaginous human skulls with which we are now familiar, modeled from 
embryos varying in length from 13 to 80 mm., has failed to add much or any addi- 
tional phylogonotic evidence regarding the form of the skulls of our remf)te ancestors. 
In fact, these cartilaginous skulls are as characteristically human as the adult skull 
is human. It is becoming more and more clear, as our knowledge of the anatomy of 
the human embr>'o increases, that both it and its various organs are at all stages as 
characteristicalh' human as are the adult body and its organs. One can distinguisli 
with ease between the cartilaginous skull of man and that of the ape, the pig, the 
cat, the rabbit, or the mole, as each is as characterLsticallj^ formed as are the adult 
skulls of the same species. Homologies and similarities are to be found in the 
cartilaginous skulls just as in the adult skulls, but it is doubtful if much additional 
evidence of phylogenetic relationships will be revealed by a comparison of carti- 
laginous skulls. 


The great need in the embrj^ology of the human skull, as of other vertebrates, 
is a more complete and detailed series of the various stages, showing the gradual 
development from the primitive membranous stages to the adult. The present 
communication is concerned primarily with a particular stage that helps to fill in 
one of the many gaps still existing in the literature. The studj' of each stage is 
necessarily laborious, since it is practically impossible to picture such complicated 
forms without resort to models. The various structures in the head and neck, 
including the cartilaginous skull of this embryo, 21 mm. in length, were modeled 
with the plaster of paris technique. 

Table 1 includes all of the human chondrocrania that have been reconstructed and described within recent years. 


==The Chondrocranium as a Whole==

The chondrocranium at this stage forms a continuous mass of cartilage and 
precartilage. It constitutes but a small jiart of the brain covering, as does that part 
of the adult cranium which is o.ssified in cartilage. One can best understand the 
cartilaginous skull by comparing it with that portion of the human adult skull 
which is ossified in cartilage rather than with the cartilaginous skulls of the lower 
vertebrates. To do this with accuracy and precision an extensive series of stages 
intervening between the stage to be comi^ired and the adult should be at hand. 
Unfortunately, but few stages are known, although in man more has been done than 
with any other vertebrate. We have in hunuui anatomy a large literature on the 
ossification centers of both the cartilaginous and the membranous bones. This is of course, a great help towards understanding the cartilaginous skull. Such a 
literature for any of the lower vertebrates is scarcely e.xistent, and therefore very 
little is now to be gained through a comparison of the cartilaginous skulls of different species. 

It is well to bear in mind the great difference in size between the cartilaginous skull of the embryo and that part of the adult skull ossified in cartilage. Table 2 
gives a few comparative measurements. Unfortunately, the embryonic measurements are all from the one cartilaginous skull under consideration; it would be most 
desirable to have several such specimens, since the size of the skull varies considerably in embryos of corresponding ages. To realize the extent of this variability 
one has but to compare the length of the basal plate, from the foramen magnum 
to the hypophysial canal, in embryos of approximately the same age. Similar 
differences exist in the adult, and the adult measurements here used are the averages for six skulls. 

Table 2. 


These figures serve to indicate in a general way the greater immaturity of the anterior end of the cartilaginous skull at this stage. The adult basioccipital is about 13 times as long as the cartilaginous basioccipital; the adult nasal septum about 30 times as long as the embryonic nasal septum. In agreement with this we find that the basioccipital consists of more advanced cartilage than the nasal septum. In one of the following paragraphs I have stated reasons for believing that the alar process of the sphenoid at a later stage becomes incorporated into the body of the sphenoid; this is partly substantiated by comparing the ratios for 
the distances between the alar processes of the embryo and between the Ungulse of the adult with the width between the outer edges of the optic foraminae or the 
distance between the foraminae rotundte (the latter formed in precartilage in the embryo) in both embryo and adult. These ratios are the same in each case, namely, 1:14, thus indicating that if the rate of growth in each of these regions of the sphenoid is the same the alar process must be looked upon as ultimately forming the lateral part of the body of the sphenoid. If, however, we compare the width of 
the basisphenoid in the embryo (not including the alar processes) with that of the adult (including the lingulse) it will he seen that the ratio is 1 :25, which corresponds 
exactly to the ratio for the width of the basioccipital between the otic capsules in the embryo and in the adult. One would naturally expect the two to grow at about 
the same rate and, if this is true, then the alar process should not be looked upon as ultimately forming part of the basisphenoid. Factors such as the possible pressure of the otic capsules on this region of the basioccipital may, however, account for the discrepancy, since the ratio for the distance between the hypoglossal foramina? (1 :15) is about the same as that for the basisphenoid, including in the embrj'o 
the alar processes and in the adult the Ungulae.

==DORSAL ASPECT==

The most familiar aspect of the chondrocranium is the upper surface of the 
base, shown in figures 1 and 2. These figures should be compared with a some- 
what similar view of the upper surface of the base of the adult skull shown in figure 
4, where the bone, ossified in cartilage, is colored blue. One is surprised, somewhat 
startled in fact, bj' the general similarity between the cartilaginous skull of the 
embryo and that part of the adult skull which is ossified in cartilage. The general 
resemblance is even more marked in later stages when the chondrocranium is more 
completely de^'eloped, as shown in the figures by Macklin and Hertwig of skulls 
from embryos 40 and 80 mm. in length. Again, if the dorsal aspect of the base 
of the entire skull, as shown in figure 3, is compared with that of the adult, new 
resemblances will appear. In figure 3 is shown not only the cartilage, but also 
the precartilage, blastema, and part of the dorsal membrane which enters into the 
formation of the brain capsule. In comparing the adult skull with figures 1 and 
3, it must be borne in mind that these figures are drawn with the basioccipital 
horizontal and not inclined, as in the usual view of the adult skull or in the view of 
the cartilaginous skull shown in figure 2. At the caudal end of the cartilaginous 
skull is the basioccipital with the two lateral parts, or exoccipitals, one on each side. 
The exoccipitals are continued into the occipital squamse or nuchal plates which are 
not yet united in the middorsal line, so that the foramen magnum is incomplete. 
The basioccipital is continuous in front into the basisphenoid, and from it the two 
wings project on each side. The mesethmoid is continuous from the basisphenoid, 
without Une of demarcation, to the anterior end of the chondrocranium, and con- 
nected with it are the nasal capsules. Lateral to the anterior half of the basi- 
occijntal and fused with it are the large otic capsules corresponding to the i:)etrous 
bones of the adult, each being separated from the exoccipital by the large jugular 
foramen and the mastoid plate. 

The basioccipital consists of a flattened plate extending from the foramen 
magnum to the basisphenoid. The caudal edge bordering the foramen magnum 
is more deeply notched than in the adult. This notch is continued into a deep 
groove on the cranial side of the cartilage. At the apex of the groove is the oi)en- 
ing where the notochord, after passing through the condensed mesenchyme which 
fills the groove, enters the body of the occipital. 


Both in the embrj^o and in the adult each exoccipital arises from the side of 
the posterior half of the basioccipital and is perforated at its origin by the hypo- 
glossal canal. In tliis embryo the right hypoglossal canal is divided on the cranial 
side into two parts by a cartilaginous bar; on the pharyngeal side there is but a 
single orifice, a condition not uncommon in the adult. The a.\is of the hypo- 
glossal canal is nearly dorso-ventral in the chondrocranium, while in the adult 
it passes almost laterally. 

The lateral borders of the foramen magnum are formed by the lateral parts 
of the occipital or exoccipitals and the squamae. There is a general similarity in 
direction, in position, and in form of the exoccipital in the embryo and adult. 
Included in the lateral occipital and constituting much of it, as seen in this view, 
is the occipital neural arch, which is partially separated near its dorsal end from 
the exoccipital and squama by a fissure (the occipital fissure) which is filled with 
condensed mesenchjine. 

That part of the exoccipital (the alar lamina) which continues upward from the 
jugular process is concave on its inner surface for the large transverse sinus. The 
sinus continues upward and outward, as in the adult, across the exoccipital and 
mastoid portion of the temporal, but turns forward over the dorsal margin of the 
otic capsule instead of backward over the squama of the occipital, as in the adult. 
At the bottom of the transverse sulcus the exoccipital joins the mastoid portion of 
the temporal. Along the upper part of this junction j'oung cartilage is present, but 
towards the jugular foramen the two are joined by precartilage and at the jugular 
foramen by dense mesenchjone or blastema, which in older embrj'os changes into 
cartilage. There is no line of demarcation between the exoccipital and the squama. 
The latter continues upward from the exoccipital as a broad, curved plate of cartilage. 

Above the region of the exoccipital the occipital squama is continuous with 
the mastoid portion of the temporal. There is no Une of demarcation between the 
squama and mastoid, and therefore much confusion exists in the literature regard- 
ing this region. This has come about through the use of the term parietal plate 
for the plate of cartilage lying above the outer edge of the otic capsule and partly 
continuous with it. We naturalh^ associate the word parietal with the membrane 
bone of the same name, and the assumption has been that the parietal plate dis- 
appeared at a later stage and was replaced by the membrane bone. Its fate, how- 
ever, has never been carefullj' traced, and it is probable that this so-called parietal 
plate, which extends backward into the nuchal plate, is part of the mastoid, for in 
the adult the mastoid is continued upward, around the posterior and even the dorsal 
edge of the petrous bone. 

The foramen magnum is incomplete dorsally except for a thin membrane of 
connective tissue, the dorsal membrane. In later stages (as shown bj^ Le\T[, 
Hertwig, and Mackhn) the foramen is completed by a bridge of cartilage formed 
by the junction of the occipital squamae, which gradually' extends around the 
brain. This cartilaginous bridge or band, usually called the tectum posterius 
or tectum syyioticum, in older stages extends around the brain from the lateral 
occipital and mastoid plate of one side to the lateral occipital and mastoid plate of the other, and corresponds to tlio nuchal j^hite of the adult occijMtal squama, 
which, as is well known, ossifies in cartilage. It seems quite unnecessary to intro- 
duce into human embryology a term appUed to a somewhat similar bridge of 
cartilage, arching over the brain from one auditory capsule to the other, in the 
cartilaginous skulls of ami)liibia and reptiles. The nuchal plate in the human 
embryo is not homologous to the tectum synoticum of the reptiles, since in man it 
is jirimarily continuous with the lateral parts of the occijiital, and in reptiles with 
the auditory capsules only. It would be better to avoid entirely the term tectum 
posterius in human embryology, and to use the term nuchal plate, or squatna, as 
Levi has done, which at once gives to it its true significance in the liigher vertebrates, 
in man at least. 

The relations about the foramen magnum can be appreciated best by com- 
paring figures 2 and 4. Figure 2 is drawn in the same position as the adult skull 
and the squama is intact. In its growth around the central nervous S3\stem it 
precedes that of the occipital neural arch and (as has been pointed out by MackUn) 
fuses with its fellow of the opposite side to form the primitive foramen before the 
occipital neural arches meet or nearly meet. 

The large jugular foramen hes between the exoccipital and the otic capsule, 
as in the adult, extending laterally to the junction of the exoccii^ital with the 
mastoid cartilage, and medially to the junction of the exoccipital with the otic 
capsule. In the embryo and in the adult the same structures pass through the 
foramen, but both the nerves and the vein are proportionally much larger in the 
embryo. These structures have, however, approximately the same relations to 
each other; the transverse sinus occupies the posterior and lateral part, the vagus 
and the accessory nerves are just anterior and medial to it, and the glossopharyngeal 
is still more anterior and medial and lies in the notch on the posterior edge of the 
otic capsule. 

The temporal cartilage, as in the adult, has an otic capsule or petrous part 
and a mastoid plate or mastoid part. The otic capsule forms one of the most 
conspicuous features of the cartilaginous skull, especiallj' at this stage, for at this 
time the relatively small size of the occipital squama behind and of the orbital 
wing of the sphenoid and the ethmoid cartilage in front gives e\en greater j^romi- 
nence to the otic capsules than at a later stage when the squama and anterior end 
of the chondrocranium are fully developed in cartilage. 

The otic capsule consists of a medial cochlear part fused with the lateral side of 
the anterior half of the basioccipital, and a lateral canalicular portion that forms part 
of the lateral wall of the chondrocranium. The otic cai)sule is broader in j^ropor- 
tion to its length than the ))etr()us bone. The internal acoustic meatus, the oiHMiing 
of the aqueductus vestibuli, and the fo.ssa subarcuata are all relatively enormous at 
this stage. In the infant the fossa subarcuata also forms a relatively large depres.sion. 

The direction of the axis of the otic capsule or petrous portion of the temporal 
is very similar to that of the adult. The upper surface of the otic capsule, as 
alreadj' noted, contains the internal acoustic meatus, the opening of the aque- 
ductus vestibuli, and the large fossa subarcuata under the superior semicircular canal. In the adult these openings are on the posterior surface of the petrous 
bone. If the adult skull is rotated so that the cranial surface of the basioccipital 
is horizontal, as in figure 1, of the embryonic chondrocranium, it will be seen that 
the posterior surface of the petrous bone is directed upward and corresponds in 
position to the upper surface of the otic capsule as seen in figure 1. In the embrj^o 
one leaf of the tentorium cerebelU is attached to the anterior margin of the otic 
capsule, and in the adult the tentorium is likewise attached to the corresponding 
margin of the petrous bone, so that the upper surface of the otic capsule faces the 
posterior cranial fossa, as does the corresponding surface in the adult. All of the 
mastoid plate (except its most anterior tip) and the squama of the occipital are pos- 
terior to the tentorium and face the posterior cranial fossa, as do the mastoid 
and nuchal plates of the adult. 

Surrounding the caudal and dorsal edges of the canaUcular part, and con- 
tinuous with it, is the mastoid plate or cartilage. This cartilage forms part of the 
outer wall of the chondrocranium and is interrupted by a large mastoid foramen. 
We have already' noted the fusion of the mastoid plate with the exoccipital and occip- 
ital squama. It is also grooved for the transverse sinus, and the latter covers 
practically all of the inner surface of the cartilage. That portion of the mastoid 
cartDage above the outer edge of the otic capsule corresponds, therefore, more 
nearly to the upper part of the mastoid bone lying caudal to the upper part of the 
outer end of the petrous bone. This becomes the more apparent when we consider 
that the upper surface of the otic capsule, as seen in this aspect of the chondro- 
cranium, presents both the internal acoustic meatus and the opening of the aque- 
ductus vestibuli, and thus corresponds to the posterior surface of the petrous bone 
and faces the posterior cranial fossa. 

The basisphenoid, which continues forward from the basioccipital without line 
of demarcation, occupies about the same relative position as in the adult. The 
dorsum sellse, although well marked, is but imperfectlj' developed and consists 
mostly of precartilage and blastema, not indicated in figures 1 and 2 but shown 
in figure 3. To this blastema is attached the medial part of the tentorium. Be- 
tween the dorsum sellse and the tip of the otic capsule the abducens nerve passes 
forward towards the orbit. In front of the dorsum sellse is the large shallow sella 
turcica. The hypophysial canal is seen in the center of the sella and contains 
remnants of the h\i)ophysial stalk. Connected with the basisphenoid are the 
temporal and orbital wings. The greater size of the orbital wings in the cartil- 
aginous skull is exactly what we would expect when it is taken into consideration 
that the part of the adult temporal wing which is ossified in cartilage is smaller 
than the adult orbital wing ossified in cartilage. 

The temporal wings project laterally from the sides of the basisphenoid, just in 
front of dorsum sellse and at a lower level than the floor of the sella turcica. These 
wings form a small part of the very incomplete floor of the- lateral joarts of what 
will later be the middle cranial fossa. Each lateral part of the middle cranial 
fossa, although indicated here by the depressed area between the orbital wings 
and the otic capsules, is entirely filled by the lower end of the embryonic tentorium cerebelli. The boundaries of the middle cranial fossa are more clear in figure 3, 
where the lateral blastemal walls are shown. The temporal wing extends into 
tliis lateral blastema and thus completes the medial portion of the floor of the 
middle fossa. Between the temporal wing and the otic capsule is the large middle 
lacerated foramen, through wliich pass the internal carotid arter_y and the mandib- 
ular nerve. The facial nerve, with the geniculate ganglion and the greater super- 
ficial petrosal nerve, also passes through the foramen. The large otic ganglion lies 
across the foramen (figs. 12 and 13). The precartilaginous tissue attached to the 
anterior aspect of the temporal wing completes the foramen ovale and separates it 
from the supraorbital fissure. Between the temporal wing and the orbital wing, 
with its precartilage, is the large supraorbital fissure, and through it the oculomotor, 
trochlear, abducens, and ophthalmic nerves pass into the orbital cavity (fig. 15). 

The embrj'onic tentorium cerebelh consists of two lateral prismatic masses of 
mesenchjTne connected across the midline by a band of condensed mesenchjone 
that extends upward from the whole breadth of the dorsum sellae. The bases 
of the prisms are against the lateral walls of the membranous skull and the apical 
edges at the membranous band that extends upward from the dorsum sellae. The 
lower end of each prism fills one of the lateral depressed areas of the potential 
middle cranial fossa, between the otic capsule in back, the orbital wing in front, 
and the lateral wall of the membranous skull. The posterior wall of each pris- 
matic mass of the tentorium is composed of a thin layer of condensed mesenchjone, 
and its lower edge is attached along the anterior edge of the otic capsule and 
extends upward. The anterior wall of each prismatic mass, hkewise composed of 
a thin layer of condensed mesenchyme, is attached to the posterior border of the 
orbital wing and its precartilage and extends upward. These two membranes 
come together along the medial apical edge where they are continued into the 
medial connecting band. Laterally, the two membranes fuse with the lateral 
membranous wall of the skull. The interior of each prism is filled with loose 
mesenchyme in which are imbedded the semilunar gangUon, large blood-vessels, 
and nerves. The trochlear and oculomotor nerves pass from above downward 
through the entire length of the prism. The tentorium obUterates entireh^ the 
potential middle cranial fossa. Later, when this fossa develops, the anterior wall 
of the tentorium must be pushed down and back into the floor and against the 
posterior wall of the fossa. 

Each ala temporaUs consists of two distinct parts, a medial alar process joined 
to the basisphenoid by young cartilage, and a lateral part wliich is, more strictly 
speaking, the cartilaginous temporal wing. It is attached to the under surface of 
the alar jprocess by condensed mesenchyme and Ues at a lower level (figs. 1, 2, 3, 
5, 6, 10, 11, and 14). Each alar process is usually regarded as forming the hngula, 
but since the internal carotid artery, which enters the cranial cavity between it 
and the apical end of the otic capsule, passes over its cranial surface, it seems 
prol^ablc that it gives rise as well to that part of the basisphenoid wliich forms the 
carotid sulcus. The relation of the greater superficial i)etr()sal nerve to the alar 
process also indicates that the latter is incorporated into the body of the sphenoid since, in the passage of the nerve from the geniculate ganglion to the spheno- 
palatine ganglion, part of its course is just beneath the alar process medial to the 
medial end of the temporal wing proper. It is here that the junction of the body 
with the temporal wing and the pterygoid process occurs and where later develops 
the pterygoid canal (fig. 13). Only the lateral part of the ala temporalis of the 
embryo, then, corresponds to that part of the temporal wing of the adult which 
ossifies in cartilage. In the adult the carotid sulcus and the lingula are at a higher 
level than the temporal wing, a condition which e.xists in the embryo if we consider 
the alar process as forming a part of the body of the sphenoid. 

Since most of the temporal wing and both plates of the pterj-goid process are 
ossified in membrane, the cartilaginous wing, even when fully developed, repre- 
sents but a small part of the temporal or greater wing and jjterygoid process of the 
adult. The maxillary nerve lies in front and the mandibular nerve behind this 
cartilage. In later stages the cartilage grows around the maxillary nerve and 
separates it from the supraorbital fissure, thus forming the foramen rotundum 
(figs. 3 and 10). Cartilage does not grow around the mandibular nerve, and the 
foramen ovale is said to be formed by membrane bone. At this stage, therefore, 
the large cleft between the temporal wing and the orbital wing represents more 
than the supraorbital fissure. 

The alar process and the temporal wing proper each has its own center of 
chondrification, as pointed out by Bardeen and Fawcett. The alar process chondri- 
fication unites with the basisphenoid before the temporal center unites with the 
alar process. 

The cartilaginous orbital wing, Uke the temi;)oral wing, is very incomjjletely 
developed, as will be seen by comparing it with the older stages of Levi, ]\Iacklin, 
and Hertwig. It consists of two parts, a proximal or basal part, and a lateral, 
sickle-shaped part, which springs upward, outward, and forward around the optic 
nerve and has much the same general position as that part of the orbital wing 
immediately about the optic nerve of the adult. The basal part of the orbital 
wing is connected to the basisphenoid by young cartilage and to the lateral part 
by cartilage not quite so far advanced as that in either the basal part or the lateral 
part. It seems probable that both parts of the orbital wing maj- arise from inde- 
pendent centers of chondrification. From the general position of this basal part 
and its relation to the optic nerve and to the general mesench^-me of this region, 
and from the fact that it gives origin to all of the muscles of the orbit except the 
superior obUque, I think it must ultimately become incorporated into the body of 
the sphenoid. The sickle-shaped lateral part of the orbital wing is intimately 
related to a larger precartilaginous part indicated by the green-colored structure 
in figure 3. This precartilaginous part shades off into the frontal blastema and 
there is in realitj^ no sharp line between the two. The cartilaginous and pre- 
cartilaginous parts together have somewhat the same form as the orbital wing of 
the 40 mm. embrj'o described by ^lacklin. 


Ill the cartilaginous skull the optic foramen is incomplete but, as shown in 
figure 3, is completed by the precartilage extending from the tip of the orbital wing 
to the anterior part of the body of the sphenoid. 

In figure 1 it can be seen how the anterior end of the basisphenoid rapidly 
narrows and continues into the mesethmoid. Both in figures 1 and 2 the dorsal 
thin edge of the mesethmoid can be seen; it corresponds in the adult to that part 
of the mesethmoid which bisects the posterior part of the cribriform plate. The 
mesethmoid extends into the large crista galli, which consists mostly of precarti- 
lage. In figure 3 the position of the future cribriform plate is indicated by the large 
olfactorj' foramina, one on either side of the short dorsal edge of the mesethmoid. 
On either side of the mesethmoid, and connected with the anterior edge in front of 
the crista galli, are the lateral wings of the nasal capsules. As seen in figures 1 
and 2, the posterior edge of each capsule is presented. 

The anterior cranial fossa is not evident in the cartilaginous skull. In the 
combined cartilaginous and membranous skull shown in figure 3 its hmits are 
clearly indicated. The floor of the fossa is formed by the precartilage of the orbital 
wing, frontal blastema, and dorsal membrane, and in the center are the mesethmoid 
and crista galli and the olfactory and ethmoid foramina. Figure 3 shows not 
only the floor but also a considerable extent of the anterior wall formed by the 
dorsal membrane. 

MEDIAN SAGITTAL ASPECT. 

In order to compare the view from the median sagittal plane shown in figure 
5 with a similar view of the adult skull, the latter should be rotated so that the 
cranial surface of the basioccipital is horizontal. In the median sagittal section 
of the adult skull before me the pharyngeal edge of the basioccipital measures 26 
mm., the basisphenoid 24 mm., and the upper edge of the vomer (from the basis- 
phenoid to the anterior nasal spine of the maxilla) 54 mm. The latter distance 
corresponds to the ventral edge of the mesethmoid. In the cartilaginous skull the 
corresponding distances are 2 mm., 1.6 mm., and 1.8 mm. The ratios, then, are 
respectively 1:13, 1:15 and 1:30. The basisphenoid is thus proportionally shorter 
in the embryo at this stage than the basioccipital, and the mesethmoid much more 
so. This is what one would expect to find in the less differentiated regions at the 
anterior end of the .skull. 

The angles which the pharyngeal surface of the basioccipital make with the 
same surface of the basisphenoid, and the latter with the mesethmoid, arc prac- 
tically identical w^ith those found in the adult. Since the edge of the basisphenoid 
is concave, a straight line connecting the two ends was used. It is interesting 
to note in this connection that the angle made by the cranial surface of the basi- 
occipital, projected sagittally on to a line parallel with the upper edge of the zygo- 
matic arch, is almost exactly the same in tliis embryo as in the adult. Such 
measurements serve to indicate that the cartilaginous skull is laid down from the 
very beginning on much the same plan as that part of the adult skull ossified in 
cartilage. 


The basioccipital shows in its median sagittal section two parts — a thin caudal 
part hang beneath the notochord connecting the posterior part of the two lateral 
thickened masses of the body from which the exoccipitals arise, and a long wedge- 
shaped part in front of and above the notochord; this Ls thickest where it joins the 
basisphenoid. The diagonal course of the notochord through the basioccipital 
and its reentrance into the basal plate at or near the junction of the basioccipital 
with the basiphenoid is also clearly shown. The three roots of the occipital neural 
arch arise from the posterior half of the basioccipital and unite lateral to the 
hypoglossal foramina into a tapering, rod-Uke cartilage. The serial relationship 
of the occipital neural arch with the cervical neural arches is very apparent in 
figure 5. The dorsal tip is in line with the dorsal tips of the cervical neural arches. 
The occipital squama is very prominent in this view; it appears to continue 
upward from both the exoccipital and the otic capsule, and even to project in 
front of the otic capsule. There is no hne of demarcation between it and the 
mastoid plate which encircles the posterior and upper edges of the canalicular 
part of the otic capsule and thus no definite limits can be given to either cartil- 
aginous area. The anterior part of this plate is usually called the parietal plate, 
a term which we have already discussed and discarded, since it is not apparent 
that it includes anji,hing more than what might be embodied under the term 
mastoid plate. In considering this region it should be borne in mind that figure 5 
is drawn with the basioccipital horizontal. The large mastoid foramen (capsulo- 
parietal fissure) Ues at the bottom of the transverse sulcus and interrupts the 
continuity of the mastoid plate. Above the region of the otic capsule the masto- 
squamal plate is grooved for the endoljonphatic sac. Whether this groove Ues 
on the mastoid or squamal part is uncertain. The fate of the various regions of 
the masto-squamal plate will remain obscure until we have a more complete series 
of stages. 

The surface of the otic capsule presented in this A-iew corresponds to the 
posterior surface of the petrous bone and faces the posterior cranial fossa. Its 
diagonal position also corresponds to the adult when the latter is seen from the 
same angle. The cochlear part is mostly above the level of the basioccipital, pro- 
jecting but shghtly below the level of the latter. The canaUcular part shows the 
position of part of the superior semicircular canal in front and of the posterior 
canal in back. The endolymphatic sulcus serves to mark the position of the 
common duct. 

The medial section of the basisphenoid extends only to the hypophj'sial canal 
and a diagonal cut in front of this cuts off the orbital wing and extends to the edge 
of the cartilage. It is impossible to determine the anterior and posterior hmits 
of the basisphenoid. The posterior Umit is probably at or near the notochord 
and the anterior limit somewhere near the line (*). The body is biconcave in the 
region of the sella turcica. Anteriorly, it narrows from side to side, as shown in 
figure 1, and increases in its sagittal thickness, as shown in figure 5. The tem- 
poral wing projects below the body and the orbital wdng above. The mesethmoid is shown intact after removal of the nasal capsule. The large crista galli and 
anterior end consist largely of young cartilage and precartilage. 

INFERIOR ASPECT. 

In comparing the inferior surface of the chondrocranium with that of the adult, 
only the occipital and otic regions lend themselves readily to our purpose, unless we 
imagine as eliminated the membranous bones, the maxillse, the palatines, the vomer, 
the jitorygoid jiroeess, most of the great wing of the sphenoid, and the tjinpanic part 
of the temporal. Figure (i is drawn with the pharyngeal surface of the basioccipital 
parallel to the plane of the pai)er. This is not the usual view of the adult skull, and 
the latter must be rotated into the corresponding jilane for comi)arison. 

The basioccipital and the basisphenoid present no new features of especial 
interest not already considered, except the openings of the canal for the notochord. 
The anterior opening where the notochord reenters the basal plate is probably 
near or just back of the junction of the basioccipital and basisphenoid. The 
exoccipital shows the large hypoglossal foramina; these are not covered by the 
large condj'les, as is the case in the adult. The condyles have scarcely begun to 
develop, and the occipital is united to the atlas by dense mesenchjTne without 
articular surface. The tip of the occipital neural arch is quite distinct and sepa- 
rated from the squama. The prominent transverse or jugular process Ues just 
caudal to the jugular foramen and, as in the adult, extends shghtly lateral to the 
outer border of the foramen. The process is more prominent than in the adult 
and has more the character of the vertebral transverse processes. 

The immature condition of the inferior surface of the otic capsule and mastoid 
region is accentuated by the fact that such structures as the carotid artery and 
the facial nerve are not inclosed by cartilage, but Ue partly in sulci beneath the 
capsule. The internal carotid artery passes forward and medially in the mesen- 
chyme beneath the cochlear part, with only the beginning of a groove formed by 
precartilage, not indicated in this figure. It enters the cranial cavity between the 
apex of the cochlear part and the alar process. The facial nerve, which enters the 
otic capsule through the internal acoustic meatus, soon emerges again from 
the facial foramen located in the anterior region of the capsular sulcus between 
the cochlear and canalicular parts. The geniculate ganglion hes just outside this 
foramen (figs. 12 and 13). The greater part of the facial nerve, which in the adult 
is inclosed in bone, is extracapsular and passes backward and outward in a groove 
on the under surface of the canalicular part. The groove lies ])etween the fossa 
incudis and the fenestra vestibuli or ovalis, and medial to the root of the styloid. 
The cartilaginous auditory ossicles Ue beneath the canalicular jiart (fig. 14). 

The serial relationshij:) of the jugular process, the mastoid jirocess, the upper 
end of the styloid, and the incus is clearly indicated in figure G, in which the 
jugular, mastoid, root of the styloid, and fossa incudis are .shown. It is impossible 
to determine at present whether this has any jjhylogenetic significance or not. 
In some of the embryos of al)out this stage there is a small sejjarate cartilage in the blastema of the mastoid process to which are attached the digastric and stapedius 
muscles. The serial relationship shown in the embryo is quite different from that 
of the adult, where the mastoid process which gives attachment to the digastric 
is some little distance lateral to the jugular process, and the styloid process is im- 
mediately in front of the latter. Just in front of the jugular foramen is the opening 
for the aquaeductus cochlese. 

The sphenoid shows the basisphenoid with the opening of the hypophysial 
canal near its center and the two wings on each side. Part of the orbital wing is 
obscured by the lateral part of the nasal capsule, as it would be in the adult from 
this point of view. 

The line of fusion of the nasal capsule with the mesethmoid is shown, and the 
anterior nares directly face the observer as they do in the adult. The junction 
of the nasal capsules with the mesethmoid is effected along the middle tliird of the 
anterior edge of the mesethmoid by precartilage. The cut edge of this junction 
is shown in figure 5. 

LATERAL ASPECT. 

The cartilaginous skull covers but a small area of the lateral surface of the 
brain, namely, part of the medulla, part of the cerebellum, and a small area in the 
region of the optic nerve (figs. 7 and 8). The cartilaginous skull, even in its 
completed form, is a very inadequate protector for the brain, never covering more 
than a small fraction of its surface. The condensed mesenchyme or blastema 
covers a much larger surface, but even the cartilage and blastema together form 
at this stage a very incomplete brain capsule (fig. 9). The capsule is completed 
by the thin dorsal membrane. The inclosing of the central nervous system by the 
gradual spreading of the blastema and cartilage, wliich invade and replace the dorsal 
membrane, is similar to the well-known development of the thoracic and abdominal 
walls and the disappearance of the ventral membrane. 

The blastema covers almost all of the lateral surface of the cartilaginous skull. 
A small part of the occipital cartilage, including the transverse process, part of the 
squama and occipital neural arch, part of the orbital wing of the sphenoid, and 
part of the lateral surface of the nasal capsule, are uncovered (figs. 9 and 15). 
Into the blastema covering the squamal cartilage, rather than into the cartilage 
itself, are inserted the various occipital muscles (figs. 14 and 15). The blastema 
covering the squama and the lateral surface of the otic capsule probably fases 
later with the perichondrium, but at this stage it seems to be continuous with the 
rest of the blastemal wall which later gives rise to membrane bones. It is in the 
sphenoidal and frontal regions that the blastema greatly predominates over the 
cartilage. All of the lateral wall of the middle cranial fossa consists of blastema 
and the greater part of the floor (as well as all of the lateral wall of the anterior fossa) 
is Ukewise formed by blastema. The orbital walls are mostly of blastema; cartilage 
of the orbital wing of the sphenoid takes part in the formation of the apical region 
of the orbit about the optic foramen, and a portion of the medial wall of the orbit 
is formed by part of the lateral wall of the nasal capsule. Connected with the cartilage is considerable precartilage extending along the inner wall towards the 
apex (fig. 15). Part of the outer wall of the orbit, consisting of the zygomatic 
blastema and the zygomatic process of the frontal blastema, and also the zygo- 
matic arch, are shown in figure 9. In lateral views of the adult skull the upper 
border of the zj'gomatic arch is horizontal, and it is therefore easier to compare 
figure 9 and the other lateral views with the adult if the figure.s are rotated so as 
to bring the zygomatic arch into a horizontal position. Attention has already 
been called to the fact that the angle made by projecting the hne of the upper 
edge of the zygomatic arch on to the basioccipital is almost exactly the same in 
the embryo and in the adult. 

By rotating figure 7 to correspond to the usual adult position it will be seen 
that the occipital squama projects backward from the exoccipital and mastoid, 
and that, since it covers only part of the medulla and cerebellum, it is concerned 
with the wall of the posterior cranial fossa. It is from this lateral view that one 
gets the impression that the squama extends into the jugular process, or more 
strictly, into that part of the exoccipital which continues into the lateral part of 
the jugular process, rather than into the occipital neural arch wliich lies in a deeper 
plane. The occipital neural arch has attained only a small proportion of the growth 
necessary to inclose the medulla. The tip of the squama, on the other hand, has 
grown farther around. The condition here is not as far advanced as that found by 
Levi in a supposedly younger embryo. 

The lateral aspect of the jugular process is quite prominent, and to it are 
attached two muscles, the rectus capitis lateralis (serially related to the inter- 
transversarii) and the occipitomastoid muscle (fig. 14). The former needs 
no special comment. The latter muscle is not found in the adult and is infre- 
quent in embryos. Its presence would seem to indicate either that the mastoid 
process was separate or movable, or that the temporal and occipital cartilages 
were at one time movable on each other. We do find that the occipital and 
mastoid cartilages are separated by young cartilage or precartilage in the region 
below the mastoid foramen. Above this, however, no fine of separation can be 
found, but tliis does not necessarily mean that the two cartilages were articulated 
by a movable joint. The significance of the mastoid process is described elsewhere. 

The canalicular part of the otic capsule forms a conspicuous part of the lateral 
wall of the chondrocranium. A slight bulging on the lower part of its lateral 
surface indicates the position of the lateral semicircular canal. The styloid process 
and incus he beneath the lateral edge of the otic capsule almost flush with the 
lateral surface. 

Between the canalicular part and the occipital cartilage is the mastoid cartilage. 
It is fairly clearly defined in the region between the otic capsule and the exoccip- 
ital below the mastoid foramen. In the region above the otic capsule there is 
no line of demarcation between the squama and mastoid. The mastoid process 
projects from the lower edge of the mastoid cartilage. In this embrj'^o it consists 
of blastema; in other embryos, as will be stated farther on, a small cartilaginous 
nodule is sometimes eml)edded in the blastema. To the mastoid process are attached several muscles, namely, the occipitomastoid, the atlantomastoid, the 
digastric, the stapedius, and the longissimus capitis (figs. 14 and 15). 

The anterior part of the cartilaginous skull, the sphenoid and the ethmoid, 
bears very little resemblance to anything ordinarily seen in the lateral view of the 
adult skull, partly because most of the bone ossified in cartilage becomes covered 
or shut out from the lateral view by membrane bone. If we can imagine all of 
this membrane bone stripped away, the resemblance between the two would be 
clearly seen. 

The lateral surface of th3 body of the sphenoid, with the shallow sella turcica 
odging the hypophysis, is to be seen in front of the apex of the cochlear part of the 
otic capsule. The prominent dorsum sella projects towards the great mid-brain 
fissure, and from it (but not shown) the thin medial membranous part of the ten- 
torium projects into this fissure. The temporal wing hes for the most part below 
the level of the basisphenoid and is attached to the under surface of the alar 
process, while the orbital wing Ues, for the most part, above the level of the basis- 
phenoid and is curved around the optic nerve. The two parts of the orbital 
wing are clearly indicated in figures 7 and 14. In the description of the middle 
cranial fossa and the embryonic tentorium cerebelli it was noted that the lateral 
prismatic mass was so placed that the apical edge extended upward from the 
dorsum sellse in connection with the thin medial membranous part of the tentorium 
lying in the great mid-brain fissure. The lower end of the embryonic tentorium, 
as previously noted, occupies the space between the otic capsule and the orbital 
wing of the sphenoid, the semilunar gangUon filling up most of the gap (fig. 8). 
It hes outside of the cartilaginous skull but within the membranous skull. The 
geniculate gangUon and adjacent sections of the facial nerve are not incased in 
cartilage, and they also he in this middle cranial fossa or gap (figs. 12 and 13). 

In the lateral aspect of the chondrocranium most of the mesethmoid is shut 
out from view by the nasal capsule. Between the orbital wing and the nasal capsule 
the anterior end of the basisphenoid and a bit of the mesethmoid appear. If only 
the cartilaginous skull were considered, this region of the basisphenoid might be 
regarded as interorbital and the term inter orhital septum could be apphed. But 
this so-called interorbital septum exists only in the cartilaginous skull; in the 
more complete skull, with all the blastema and precartilage, the medial wall of 
the orbit is complete enough to shut it out from the orbit. The term inter orhital 
septum probably has no significance whatever in human embrj^ology. The 
prominent crista galU projects between the cerebral hemispheres. The lateral 
surface of the nasal capsule forms a large proportion of the inner wall of the orbit 
and probably ossifies into the lamina papyracea of the ethmoid. 

THE OCCIPITAL CARTILAGE. 

The occipital cartilage consists of an elongated, flattened body (or basioc- 
cipital), the lateral parts (or exoccipitals), and the two nuchal plates (or squamae). 

The body forms the greater part of the so-called basal plate, a term much used 
in descriptions of the cartilaginous skull. It continues without line of demarcation into the basisphenoid, and one can only approximate the future hne of junction 
between the two bones which ossify in the cartilage by the relation to the apex of 
the otic capsule and the entrance of the notochord. The basioccipital Ues in a 
plane almost exactly horizontal to the long axis of the embryo. This elongated 
quadrilateral jilate is thinner at the center, slightly concave on the cranial surface, 
and slightly convex on the pharj-ngeal surface. It becomes thicker where it joins 
the sphenoid. The rounded caudal end is bent downward and notched in the 
midline at the foramen magnum. The caudal part of the basioccipital gives much 
more of a clue to its vertebral origin than the flattened cranial part. The caudal 
part is thickened and on the cranial surface is incompletely divided into two lateral 
masses by a deep median groove. These correspond, perhaps, to the bilateral 
masses which fuse to form the bodies of the cervical vertebrae. Levi finds in his 
13 mm. embryo two bilateral cartilaginous centers medial to the hypoglossal roots. 
They probably represent, as Macklin suggests, the bilateral centers for the occipital 
vertebrse. Their position in the 13 mm. embryo corresponds to the position of the 
lateral masses in this 21 mm. embryo. These masses are united across the midline 
beneath the notochord, and anteriorly are continued into the broad flat plate of 
the basioccipital. The center of each mass contains the most highly differentiated 
cartilage of the entire chondrocranium. 

In the 14 mm. embryo described by Levi the otic capsule and the basioccipital 
are apparently separated by a wide gap. Levi probably did not include all the 
precartilaginous tissue and blastema of the otic capsule, since in embryo No. 109 
(Carnegie Collection), 11.5 mm. in length, the blastema of the otic capsule is in con- 
tact with the basioccipital. It is only in younger stages, 9 mm. length, that I find 
the blastema of the otic capsule separated from the basioccipital. The union of the 
blastema, then, precedes that of the cartilage in this region. In embryo No. 460 
the anterior half of the lateral border of the basioccipital is fused with the cochlear 
part of the otic capsule along a crescentic line. The line of fusion can still be recog- 
nized by the differences in the degree of differentiation of the two cartilages. The 
cochlear part consists of young cartilage and the nuclei are closer together than in 
the basioccipital. The border of the basioccipital is, however, not so far advanced 
as the central part. The two cartilages were so completely fused in the 17 mm. 
embryo described by Levi that he was unable to recognize any histological border 
between them. Embryo No. 128 (Carnegie Collection), 20 mm. in length, although 
supposedly smaller and younger than the 21 mm. embryo (No. 4G0), shows a more 
advanced conchtion in the cartilaginous differentiation of this region, and it is 
impossible to find a histological border between the basioccipital and the cochlear 
part except at either end of the line of fusion. In another 20 mm. embryo (No. 22, 
Carnegie Collection) there is only the very sUghtest indication of the line of fusion . 
There is probably some variation in the rate of differentiation of the cartilages of 
this region in embryos of the same age and size. 


OCCIPITAL VERTEBRA. 

Each exoccipital corresponds for the most part to a vertebral neural arch or 
hemiarch, which is so distinct in this embryo that we may properly apply the term 
occipital hemiarch. The roots, laminae, and transverse processes can be readily 
recognized. The roots are broad and thick and correspond to the roots or pedicles 
of the vertebral neural arches. On the right side, as already noted, there are three 
roots dividing the cranial end of the hypoglossal canal into two parts, a condition 
not uncommon in the adult. The caudal root on each side is the largest and con- 
sists of more advanced cartilage, and it is questionable whether the anterior roots 
should be looked upon as forming part of the occipital vertebra. It has often been 
suggested that they are part of an anterior, still more rudimentary vertebra. The 
laminae extend outward and upward from the roots, forming part of the lateral 
border of the foramen magnum. They are much thickened, rounded, tapering 
cartilages, quite distinct, and in the dorsal part entirely separate in this particular 
embryo from the squamal cartilage. The cartilage of the occipital arch is more 
differentiated than the squamal. During the further course of development, as 
Macklin has pointed out, the occipital neural hemiarches grow around the central 
nervous system and approach each other in the midUne. It is not clear whether 
they actually meet and fuse, or whether a small part of the squamal cartilage may 
not intervene. The squamal cartilages, which likewise grow around the central 
nervous system, meet and fuse before the occipital hemiarches. The ossicle of Kerck- 
ring, which develops in this region, may represent a separate ossification center 
of the nuchal plate intervening between the tips of the occipital neural hemiarches, 
or this center may possibly be looked upon as the ossification of the fused occipital 
spinal epiph3'ses. Schultz described a skull with, two bilateral ossicles in this region, 
and suggests that they may correspond to the epiphyses of the spinous processes. 

In this embryo the separation of the dorsal part of the lamina of the occipital 
vertebra from the squama of the occipital, and the more intimate relation of the 
latter with the transverse process, would seem to indicate that the squama is an 
extension upward from the transverse process rather than an outgrowth from the 
occipital neural lamina. The relation of the squama to the transverse process 
and to the occipital neural lamina is clearly shown in figures 1, 7, and 16. The 
occipital transverse process forms part of the caudal and lateral margins of the 
jugular foramen and continues up into the squama and alar lamina without 
line of demarcation. The alar lamina, as will be seen in figure 16, constitutes the 
ventral and more medial part of the squama adjoining the lamina of the occipital 
and the medial part of the transverse process. There is also a more gradual transi- 
tion as regards the degree of differentiation from the cartilage of the transverse 
process into the squama than from that of the lamina. 

In embryo No. 22 (Carnegie Collection), 20 mm. in length, the squama of the 
occipital is fused wath the lamina throughout the entire length of the latter on 
both sides of the embryo, although throughout most of this length the younger 
cartilage of the squama can easily be distinguished from the thickened, older 
cartilage of the lamina. Towards the apex of the arch, however, the two cartilages are of the same degree of differentiation and no distinction between them can be 
seen. 

In embrj'o No. 128 (Carnegie Collection), 20 mm. in length, the resemblance 
which the occipital neural arch and its transverse process bear to those in the 
cervical region is very striking. The cartilages of the occipital and cervical neural 
arches show much more advanced differentiation than the thin transverse processes. 
The muscle intertransversarius, between the transverse processes of the second 
and the first cervical vertebra?, is in line with and serially related to the rectus capitis 
laterahs, which passes between the transverse process of the first cervical vertebra 
and that of the occipital. In this embryo (No. 128) it can be seen also how the 
squama of the occipital is a continuation upward from the transverse process 
rather than from the lamina. The laminae of the occipital vertebra are also fused 
along the entire length to the squamae, but are readily distinguishable from the 
latter by their much more advanced cartilaginous differentiation. The tips of 
these laminse are connected with the tips of the laminae of the atlas by the inter- 
spinous hgament, as in embrj^o No. 460. 

In embryo No. 240 (Carnegie Collection), 20 mm. in length, the cartilage of 
the occipital neural arch on each side is Ukewise fused with the squama throughout 
its entire length. The lamina also shows more advanced differentiation than 
the squama. In embryo No. 431 (Carnegie Collection), 19 mm. in length, the 
tips of the occipital neural arches are separated from the squama on either side 
by the condensed mesenchyme forming the perichondrium. The e.xtent of the 
fusion of the laminae and the squamae is, however, more extensive than in embryo 
No. 460. These observations indicate that the occipital neural arch is more marked 
in this stage than in the adult, and that probably an extreme degree of accentua- 
tion in the embryo would precede any marked manifestation of an occipital vertebra 
in the adult. The manifestation of an occipital vertebra in adult skulls has been 
described by Gladstone, Kollman, and others. Such variations remind one of the 
conditions found in embryos at this stage, and we shall probably find that embryos 
vary as much, if not more, than adults and that such embryonic variations always 
precede the adult variations. 

The relation and attachment of the dorsal membrane to the tips of the occipital 
hemiarches are similar to its relation and attachment to the tips of the neural 
arches of the cervical vertebrae, in that in both cases the dorsal membrane is con- 
tinuous with the perichondrium on the medial side of the tip of the neural arch. 
Its attachment to the upper border of the squama is somewhat different, since here 
it is merely continuous with the thin edge. The tip of the occijjital hemiarch is 
connected with the tips of the vertebral hemiarches by a distinct band of condensed 
membrane, the interspinous ligament described by Bardeen (fig. 9). 

The transverse or jugular j^rocess springs from the occipital hemiarch at the 
junction of the roots and lamina and projects laterally back of the jugular foramen. 
We have already noted its serial rclationsliip with the vertebral transverse processes. 
The lateral extremity of the jugular process has a knob-like enlargement, and into 
this are inserted the rectus capitis lateralis muscle and the occipito-mastoid muscle 
(fig. 24). 



The occipital squama or nuchal plate springs from the upper border of the 
lateral part of the jugular process. Near its origin it is narrow in the sagittal plane 
and wide m the frontal plane. It is continuous with the basal part of the lamina 
by a curved plate, the alar lamina (figs. 1 and 16), and rapidly widens into a broad, 
thin plate, which continues upward to a thin edge that runs into the dorsal mem- 
brane. Both the jugular process and the squama extend into the mastoid cartilage. 
At the jugular foramen these two cartilages are separated by blastema and precar- 
tilage; above this they are joined by young cartilage, which gradually changes into 
cartilage indistinguishable from that of the squama or mastoid. The lateral surface 
of the squama is covered by condensed mesench>Tne into which the edges of the 
cartilage merge. It is into this mesench3'me that the occipital muscles— namely, 
the trapezius, splenius capitis, semispinahs, rectus capitis posticus major and minor,' 
and the obUquus capitis superior, appear to be inserted (figs. 14 and 15). 

==THE SPHENOID CARTILAGE==

The sphenoid cartilage consists of a body with two lateral wings or processes 
on each side, the temporal and orbital wangs (figs. 1, 2, 3, and 6). The cartilage of 
the body is continuous with the body of the occipital, and in front narrows and 
thickens as it passes into the nasal septum. The cartilaginous center from which 
the body develops is described as being caudal to the hypophysial canal. The 
chondrification spreads caudally to meet the basioccipital, upward into the dorsum 
sellae, and forward around the hypophysial canal where the cartilage fuses around 
the canal and extends forward to form the anterior part of the sphenoid and the 
nasal septum. Fawcett, after an examination of a 21 and a 19 mm. embryo, con- 
cluded that the dorsum sellae was an independent formation, since he found in each 
embryo a separate cartilaginous bar above the region of the body and separated 
from it by mesenchj^ne. Twenty embryos of the Carnegie Collection, ranging in 
length from 15.5 mm. to 24 mm., were examined, and such a cartilaginous bar was 
found in but one (No. 229), 19 mm. long. Twelve of the embryos were between 
18 and 23 mm. Great ^■ariation in the form of the dorsum sellse was observed, but 
the rather rare presence of a separate center of chondrification would indicate that 
as a general rule the dorsum sellse develops from the basisphenoid, as described by 
Levi. In front of the dorsum sellse the body is biconcave and perforated near the 
center by the hj^jophj^sial canal. There is practically no indication in front of the 
sella turcica of the tuberculum sellse. The anterior part of the bodj^ is very incom- 
pletely developed, and not until the anterior tip of the orbital wing has grown 
medialward to meet its fellow of the opposite side and fuses with, the body is the 
anterior border of the chiasmatic groove apparent (fig. 3). 

The temporal wing consists of two parts : the alar process, which springs from 
the body just in front of the level of the dorsum sellse and projects laterally and 
sKghtly caudally toward the apex of the cochlear part of the otic capsule, and the 
lateral part, attached to the under surface of the alar process and projecting later- 
ally. Each part is supposed to have an independent center of chondrification. In 
this embryo the alar process is joined to the basisphenoid by j'oung cartilage and to the lateral part by blastema. We have already given sufficient consideration to 
the fate of the alar process and its probable incorporation into the body of the 
sphenoid, and also the lateral part which corresponds more strictly to that part of 
the temporal wing that ossifies in cartilage. This part of the cartilage is still incom- 
j)letely developed; as in later stages the cartilage extends around the maxillary nerve. 
The orbital wing is even more immature than the temporal, as will be seen by 
comparing it with the older stages of Macklin and Hertwig. In figure 3 is shown the 
precartilaginous part, which is more extensive than the cartilaginous part. The 
basal part of the orbital wing lies at the apex of the orbit and gives attachment to 
all the muscles of the orbit except the superior oblique (figs. 10 and 15). 

THE TEMPORAL CARTILAGE. 

This cartilage includes the otic capsule and the mastoid cartilage, which are 
intimately fused together. 

OTIC CAPSULES. 

The otic capsules form the cartilaginous basis for the petrous bones of the 
adult. They are very prominent, being striking features of the chondrocranium, 
and extend on each side in a caudo-dorso-lateral direction to the lateral surfaces 
of the skull. Each capsule consists of two broadly united and continuous parts 
(a medial cochlear part and a lateral canalicular part) inclosing the cochlear and 
semicircular canals respectively. The two parts are more or less set off from 
each other by a broad shallow groove, the capsular sulcus. 

The cochlear part is somewhat egg-shaped and fuses with the basioccipital. 
The cranial surface is rounded and presents the large, rounded, internal acoustic 
meatus. The ventral surface is also rounded and projects below the level of the 
basioccipital, forming with it a distinct groove. The cochlear part consists of 
somewhat younger cartilage than the canalicular part. Its relation to the cochlear 
duct is shown in figure 16. 

The capsular sulcus extends entirely around the otic capsule. On its anterior 
surface is a largo foramen for the exit of the facial nerve. This foramen is separated 
by a narrow bar of cartilage from the internal auditory meatus. The facial nerve 
has but a very short course within the otic capsule; the geniculate gangUon and the 
nerve distal to it are outside of the capsule, but close against it (figs. 12 and 13). 
The capsular sulcus is broad and shallow on the inferior surface and near its center 
hes the fossa vestibularis or ovalis, in which is imbedded the base of the stapes. 
Immediately about the fossa is an area of young cartilage; the floor of the fossa 
is also covered with young cartilage. The posterior part of the sulcus, which 
borders the jugular foramen, contains the opening for the aqueductus cochloaris. 
At this stage only a small plexus of veins pass through it, but later it contains the 
perihTnphatic duct. 

The canalicular part of the otic capsule constitutes the lateral half and forms 
part of the outer wall of the chondrocranium. It is more or less oval in form 
and flattened in a mcdio-latoral direction. It consists for the most part of the 
thickened cartilaginous covering for the semicircular ducts. The locations of the latter are more or less apparent by the configuration of the cartilage. The 
position of the superior semicircular duct is quite clearly indicated; it lies within 
the prominent, rounded semicircular mass constituting the antero-dorsal portion 
of the canalicular half of the capsule (figs. 1, 2, 3, 5 and 15). The deep fossa sub- 
arcuata Ues at the center of its curve. The lateral semicircular canal produces a 
distinct bulge on the lower part of the lateral surface of the capsule, while the 
position of the posterior duct is indicated by the configuration of the posterior 
portion of the canaUcular part. The cranial surface shows a well marked sulcus 
for the endoljTnphatic duct which leads into the opening for the aqueductus 
vestibuU. The upper edge of the cartilage, where it joins the mastoid, is deeply 
grooved for the transverse sinus. The lower edge of this posterior portion projects 
ventrally in the form of a ridge, the crista parotica, to the caudal end of which is 
attached the styloid process. In front of the crista there is a slight depression, the 
fossa incudis, in which is lodged the short crus of the incus. The facial nerve lies 
in a groove just medial to the crista and styloid process (figs. 6, 12, and 13). 

MASTOID CARTILAGE. 

The mastoid cartilage is continuous with the caudal and upper border of the 
canalicular part. We have already described its fusion with the exoccipital and 
squama of the occipital. Its inner surface is hoUowed out for the great transverse 
sinus, and in the upper part of this sulcus is the large mastoid foramen. A dis- 
tinct mass of blastema or precartilage projects ventrally from the inferior edge of 
the mastoid just back of the root of the styloid process. Since to this mass are 
attached the digastric, splenius capitis, longissimus capitis, and sternomastoid 
muscles, it probably represents the mastoid process. In addition to the above- 
mentioned muscles the stapedius also appears to arise from the inner side of the 
mass. At this stage the stapedius is relatively large and entirely extracapsular. 
Two interesting rudimentary muscles, not found in the adult, are likewise attached 
to this mastoid process. One, short and thick, arises from the occipital transverse 
process; the other, longer and more slender, extends from the transverse process of 
the atlas (figs. 14 and 15). I have named these muscles the occipito-masloid and the 
atlanto-mastoid. The occipito-mastoid is serially related to the intertransversarii. 

This mastoid blastema, or mastoid-process blastema, presents interesting 
variations. In embryo No. 382 (Carnegie Collection), 20 mm. in length, there is 
a small nodule of cartilage in this region — the mastoid-process cartilage. This is 
attached to the occipital transverse process by a short ligament. Both the digas- 
tric and stapedius muscles are attached to it, but no trace of the occipito-mastoid 
or atlanto-mastoid muscles were found. In embryo No. 431 (Carnegie Collec- 
tion), 19 mm. in length, a similar nodule of young cartilage was found attached 
in the same manner to the transverse process of the occipital by a Ugament. The 
stapedius muscle is attached to its inner surface and the digastric to its lower 
surface. In both embryos the mastoid-process cartilage is closely associated with 
the mastoid blastema and the Ugament or band of condensed mesenchj-me, connect- 
ing it wdth the jugular process, occupies the same position as the occipito-mastoid 
muscle found in embryo No. 460. The attachment of the stapedius and digastric suggests that the mastoid process may be regarded as a remnant of the proximal 
end of a branchial cartilage, while its serial relation with the transverse process of 
the occipital and cervical vertebrae, and its connection with the former by muscle, 
as in embryo No. 460, or by ligament, as in embryos Nos. 431 and 382, suggest that 
it may be a rudimentary^ portion of the transverse process of an occipital or temporal 
vertebra. A still more speculative idea would be to consider the branchial bars as 
serially related to the vertebral transverse process, and the occijiito-mastoid muscle 
could be looked upon as serially related to the intertransversarii muscles on the one 
hand and to the stapedius on the other. 

THE ETHMOID CARTILAGE. 

The ethmoid cartilage consists of the mesethmoid and the lateral nasal cap- 
sules joined to the former by precartilage. The ethmoid is very poorly developed 
at this stage and all the figures include more or less precartilage. The gradual 
transition from cartilage to young cartilage and precartilage makes it impossible 
to draw sharp Unes between the cartilage and precartilage. The mesethmoid 
consists of fairly well-differentiated cartilage near its junction with the sphenoid. 
In this region the pharyngeal edge is much thicker than the dorsal edge. It gradu- 
ally tapers towards the apex, the entire septum attaining an even thickness. The 
crista galli is relatively large when both cartilage and precartilage are included, 
as shown in the various figures. 

The nasal capsules consist mostly of young cartilage and precartilage. They 
are relatively simple and show only slight indications of the turbinate processes. 
Part of the lateral surface enters into the formation of the medial wall of the orbit 
and part is covered by maxillary and nasal blastema, the precartilaginous tips 
extending beyond (fig. 15). The inner wall of the orbit is completed by frontal 
blastema and precartilage of the orbital wing of the sphenoid. This blastema 
and precartilage, as already noted, form a continuous sheet (fig. 3). The orbital 
surface of the sheet is more or less closely united to the upper edge of the nasal 
capsule (fig. 15) . On the Une of junction between the cartilage and frontal blastema 
are found, as in the adult, the anterior and posterior ethmoidal foramina, through 
which pass the nasociliary nerve and the ethmoidal branches. 



==Bibliography==



Bakdeen, C. R., 1910. MorphogcncsLs of the skeletal system. Keibel and Mall, Manual of Human Embryology. 

Fawcett, 1910. Notes on the development of the human sphenoid. Jour. Anat. :iii<l Phys., vol. ) 1, p. 207. 

Fisher, E., 1903. Zur EntwicklunK.sne.seliii-hte des Affen.sehiidels. Zeit.f.Morph.u.Anthropol.,Bd.5. 

Gladstone, 1915. Manife.station of occipital vertebra; and fusion of the atlas with the occipital. Jour. Anat. and Phj\s., vol. 49, p. 190. 

Jacoby, M., 1895. Ein Beitrag zur Kcnntnis des menschlichen Primordialcraniums. Arch. f. mikr. Anat., Bd. 44. 

KoLLMAN, J., 1905. Varianten am Os occipitale, besonders in der Umbegung des Foramen occipitale magnum. Anat. Anz., Bd. 27. Verb. d. anat. Gesellsch., p. 231-2.36. 

Levi, G., 1900. Beitrag Zum Studium der Entwieklung des knorpeUgen Primordialcraniums des Menschen. Arch. f. mikr. Anat. u. Entwick, Bd. .55. 

Lewis, VV. II., 1915. The use of guide planes and plaster of paris for reconstructions from serial sections, etc. Anat. Rec, vol. 9. 

Macklin, C. C, 1914. The skull of a human fetus of 40 mm. Am. Jour. Anat., vol. 16. 

Mead, C. S., 1909. The chondrocranium of an embryo pig, Sus scrofa. Am. Jour. Anat., vol. 9. 

ScHULTZ, A. IL, 1917. Ein Paariger Knocken am Unterrand der Squama occipitalis. Anat. Rec., vol. 12. 

Terry, R. J., 1917. The primordial cranium of the cat. Jour. Morph., vol. 29. 

Van Noorden, W., 1887. Beitrag zur An.>»tomie der knorpeligen Schiidelbasis bci menschlichen Embryonen. .Arch. f. Anat. u. Physiol, Anat. Abth. 

VoiT, Max, 1909. Das Primordialcranium des Kaninchens unter Berucksichtigimg der Deckknocken. Ein Beitrag zur MorphologiedesSiiugeticrschadels. Anat. Hefte, Bd. 38, Heft. 116.

==Explanation of Figures==

:[[Book_-_Contributions_to_Embryology_Carnegie_Institution_No.39|'''Lewis Links''']]: [[:File:Lewis1920_Plate_1.jpg|Plate 1]] | [[:File:Lewis1920_Plate_2.jpg|Plate 2]] | [[:File:Lewis1920_Plate_3.jpg|Plate 3]] | [[:File:Lewis1920_Plate_4.jpg|Plate 4]] | [[:File:Lewis1920_Plate_5.jpg|Plate 5]] | [[Book_-_Contributions_to_Embryology_Carnegie_Institution_No.39|Contribution No.39]] | [[Book_-_Contributions_to_Embryology#Volume_IX|Volume IX]] | [[Book_-_Contributions_to_Embryology|Contributions to Embryology]]

<gallery>
File:Lewis1920_Plate_1.jpg|Plate 1
File:Lewis1920_Plate_2.jpg|Plate 2
File:Lewis1920_Plate_3.jpg|Plate 3
File:Lewis1920_Plate_4.jpg|Plate 4
File:Lewis1920_Plate_5.jpg|Plate 5
</gallery>

Dorsal aspect of base of the cartilaginous skull with the basioccijjital in the horizontal plane. The cervical vertebra; are included. At the anterior end some precartilage is included with the crista galli and the nasal capsule. X 10 diameters. 

Right half of dorsal aspect of the base of the cartilaginous skull. The adult position, basioccipital inclined at an angle with the horizontal plane. X 10 diameters. 

Dorsal aspect of cartilaginous and membranous skuU. Same view as shown in figure 1. X 10 diameters. 

Dorsal aspect of base of the adult skull. Bone ossified in cartilage stained blue. Modified from Spalteholz Atlas. 

Median sagittal aspect of the cartilaginous skuU. Part of crista galli and anterior end of raesethmoid are precartilage. XIO iliameters. 

Ventral aspect of base of the cartilaginous skull. XIO diameters. 

Lateral aspect of cartilaginous skull and cervical vertebrsE, with the brain and cervical cord and hypophysis in position. Merkel's cartilage, the styloid, hyoid, and laryngeal cartilages are also shown. 
XIO diameters. 

Lateral aspect of cartilaginous skuU and cervical vertebra; with the brain, cervical cord, and nerves. XIO diameters. 

Lateral view of cartilaginous skull and cervical vertebrie with the overlying membranous skull and the dorsal membrane. X 10 diameters. 

Dorsal aspect of sphenoid cartilage, showing attachment of the orbital muscles to the basal part of the orbital wing. X20 diameters. 

Dorsal aspect of sphenoid cartilage. X20 diameters. 

Lateral view of the right otic region. Part of the malleus and incus cut away showing course of facial nerve and position of otic ganglion. X20 diameters. 

Lateral view of right otic region showing relations of facial nerve. X20 diameters. 

Lateral view of base of cartilaginous skull with deeper muscles of occipital region and of the mouth and pharynx. X20 diameters. 

Lateral view of part of cartilaginous and membranous skull. The inner wall of the orbit and part of occipito-temporal region exposed by cutting away part of the membranous skull. X20 diameters. 

Dorsal view of temporal and occipital cartilages, showing the relation of the inner ear to the otic capsule. X20 diameters. 


All of the figures except No. 4 are from models or combinations of models and graphic reconstructions of one embryo No. 460, Carnegie Collection. 

Cartilage is colored blue, except figure 4, precartilage green, nerves yellow, muscles red; the blastema remains uncolored. 

The illustrations are mainly the work of Mr. C. W. Shepard and Mr. J. F. Didusch. 



324 



EXPLANATION OF FIGURES. 



ABBREVIATIONS. 



alar, lam., 
alar, pro., 
amp. lat. tl., 
amp. p. d., 
amp. s. (1., 
ansa, hy., 
ant. arch. I. C, 
ant. nar., 
ant. rt., 
aq. vcs., 
atl. mas., 
atl. occ. art., 
basiocc, 
bas. pt., 
biventcr, 
can. pt., 
chr. pi., 
ch. ty., 
cLl. g., 
coc. d., 
coc. pt., 
com. d., 
oris, gal., 
digas., 
dor. mem., 
dor. sel., 
ed. bl., 
end. d., 
end. sul., 
epiph., 
eth. for., 
fac. for., 
[ac. sul., 
fen. ves., 
for. rot., 
fos. inc., 
fos. sub., 
fron. bl., 
gang. I. C, 
gen. g., 
gen. glo., 
gen. hy., 
gr. sup. pet. n. 
hy. for., 
hyo. glo., 
hyp. c., 

hyp. 

inf. alv., 
inf. col., 
inf. orb., 
in. sp. lig., 
int. ac. m., 
Jac. cart., 
jug. for., 
jug. pro., 
jug. v., 
lat. rec, 
lig. fla.. 
Ion. cjip., 
It. ma«., 
mand., 
mas. for., 
mas. pro.. 



alar laminsi 

alar process. 

ampulla lateral semicircular duct. 

ampulla |>osterior semicircular duct. 

ampulla superior semicircular duct. 

ansa hypoglossi. 

anterior arch atlas. 

anterior nares. 

anterior root occipital neural arch. 

aqueduct vcstibuli. 

atlanto-mastoid muscle. 

atlanto-occipital articulation. 

basioccipital. 

basal part orbital wing sphenoid. 

biventer ccrvicis muscle. 

canalicular part otic capsule. 

choroid plexus. 

chordatympani. 

ciliary ganglion. 

cochlear duct. 

cochlear part otic capsule. 

common duct. 

crista galli. 

digastric muscle. 

dorsal membrane. 

dorsum sellae. 

edge cranial blastema. 

endolymphatic duct. 

endolymphatic sulcus. 

epiphysis. 

ethmoid foramen. 

facial foramen. 

sulcus for facial nerve. 

fenestra vestibuli. 

foramen rotundum. 

fossa incudis. 

fossa subarcuata. 

frontal bl;i.stema. 

ganglion first cervical nerve. 

geniculate ganglion. 

genioglossus muscle. 

geniohyoid muscle. 

greater superficial petrosal nerve. 

hypoglossal foramen. 

hyoglossus muscle. 

h>'pophyscal canal. 

hypophysis. 

inferior alveolar nerve. 

inferior colliculus. 

infraorbital nerve. 

interspinous ligament. 

internal acoustic meatus. 

Jacobson's cartilage. 

jugular foramen. 

jugular process. 

jugular vein. 

lateral rectus muscle. 

ligamenta subflava. 

longLssimus capitis muscle. 

lateral mass basioccipital. 

mandible. 

mastoid foramen. 

mastoid process. 



max.. 


maxillary blastema. 


med. lac. for.. 


metlian lacerated foramen. 


med. rt.. 


median root occipital neural arch. 


Meek., 


Meckel's cartilage. 


meseth.. 


mesethmoid. 


nas. bl., 


iia-sal blastema. 


nas. cap., 


nasal capsule. 


neu. arch. 


neural arch. 


obi. cap. inf.. 


obliquus capitis inferior muscle. 


obi. cap. sup.. 


obliquus capitis superior muscle. 


occ. arch, 


occipital neural arch. 


occ. fis., 


occipital fissure. 


occ. mas.. 


occipitomastoid muscle. 


occ. con., 


occi|)ilal condyle. 


olf. for.. 


olfactory foramen. 


opt. for., 


optic foramen. 


opt. n., 


optic nerve. 


orb. wing, 


orbital wing. 


ot. cap., 


otic capsule. 


otic gang., 


otic ganghon. 


pal. n.. 


palatine nerve. 


para, gl.. 


parathyroid gland. 


post, rt.. 


posteiior root occipital neural arch. 


rec. cap. ant.. 


rectus capitis anterior muscle. 


rec. cap. lat., 


rectus capitis lateralis muscle. 


rec. cap. post.. 


rectus capitis posterior muscle. 


sel. tur.. 


sella turcica. 


semispi. cap., 


semLspinalis capitis muscle. 


sem. g.. 


semiluiKir ganglion. 


squama, 


squaiiKi occipital cartilage. 


sph. pal. g., 


si>hcni)palatiiic ganglion. 


sph. pal. n.. 


.sphcndpakitiiic nerve. 


ster. thy.. 


sternothyroid muscle. 


sty. glo.. 


styloglossus muscle. 


sty. hy., 


stylohyoid muscle. 


sty. ph., 


stylopharyngcus muscle. 


submax. cap.. 


subina\illary gland capsule. 


sup. can.. 


superior semicircular canal. 


sup. ling., 


superior lingual muscle. 


sup. orb. fis., 


suiierior orbital fissure. 


sup. rec., 


superior rectus muscle. 


temp, wing, 


temporal wing. 


thy. hy.. 


thjTohyoid muscle. 


thym. gl., 


thymus gland. 


thyr., 


thyroid cartilage. 


thyr. gl.. 


thyroid gland. 


tr. pro., 


transverse process atlas. 


tr. sul., 


transverse sulcus. 


zyg-. 


zygoma blastema. 


zyg. arc, 


zygomatic arch. 


zyg. pro., 


zygomatic process of frontal b'astema. 


III. 


III. cranial nerve. 


IV. 


IV. " 


V. 


V. 


VI. 


VI. 


Vll. 


VII. " 


IX. 


IX. " 


x'S;} 


X.I .. .. 
XL/ 


XII. 


XII. " 


I. C. N. 


I. cer\'ical " 


II. C. N. 


11. 



line between si)henoid and mesethmoid 








{{Glossary}}

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[[Category:Historic Embryology]] [[Category:Human]]