THE CARTILAGINOUS SKULL OF A HUMAN EMBRYO TWENTY-ONE 



MILLIMETERS IN LENGTH, 



BY WARREN H. LEWIS. 



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 literature, gradually passing into 

 oblivion, on the segments of an unsegmented skull. Its effects, however, are still 

 to be noticed in the attempts to imagine a segmentation in the unsegmented brain. 

 In the slowly accumulating literature 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 his 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 realize 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 phylogeny?" 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 with 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 phylogenetic 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 same species. The series 



301 



