302 



CARTILAGINOUS SKULL OF A 21 MM. HUMAN EMBRYO. 



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 phylogenetic evidence regarding the form of the skulls of our remote 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 embryo increases, that both it and its various organs are at all stages as 

 characteristically human as are the adult body and its organs. One can distinguish 

 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 characteristically 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 embryology 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 study 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. 



TABLE 1. 



THE CHONDROCRANIUM AS A WHOLE. 



The chondrocranium at this stage forms a continuous mass of cartilage and 

 precartilage. It constitutes but a small part of the brain covering, as does that part 

 of the adult cranium which is ossified 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 compared 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 human anatomy a large literature on the 

 ossification centers of both the cartilaginous and the membranous bones. This is, 



