No. 2.] EMBRYOLOGY OF THE LIZARD. ^^g 



anterior part of the hind-brain. Thus, the lumen at this point 

 acquires the same appearance of a cross as in the late stages of 

 the lizard. The process of formation of this part in the lizard 

 explains the angular outgrowths in these other embryos, and the 

 lizard may be considered to represent in this regard the more 

 primitive method of development. 



PART II. 



Certain phyllogenetic considerations give the development of 

 the lizard a peculiar interest. The results of general mor- 

 phology indicate that the elasmobranchs present a relatively 

 primitive type of vertebrates,^ distinct from the other lower 

 forms in the fact that the latter have been more modified by 

 degeneration and peculiar specialization. The development 

 of the elasmobranchs appears also to be very primitive. The 

 embryological development of the Teleostei seems to be partly 

 abbreviated, and otherwise peculiarly modified and changed 

 from a primitive condition. The same is true in part for the 

 Cyclostomata and Amphibia.^ The embryology of the lizard, 

 which is probably next to the lowest form of amniota, re- 

 sembles more closely the embryology of the elasmobranchs 

 than that of any other forms of the icthyopsida. Moreover, 

 the fact that the lizard has retained one organ — the pineal 

 eye — in a condition much less degenerate than in all other 

 living vertebrates shows that the lizard may be in some respects 

 a very primitive form, in regard to this part of the nervous sys- 

 tem, — even more primitive than the elasmobranchs. That other 

 parts of the central nervous system would also be in a primitive 

 condition seems a permissible deduction; but at the same time 

 the retention of the pineal eye in the lizard shows how com- 

 plicated is the subject of ** higher and lower forms," and the 

 determination of what is primitive, and also with what care one 

 should receive deductions in this regard which are drawn from 

 the supposed position in the phyllogenetic relationship. The 

 history of the hypotheses in this connection, during the last 

 decade, shows that we should not expect to find all the differ- 



^ Gegenbaur. — Das Kopfskelett der Selachier. 



*Cf. Gegenbaur^ s x&wievi oi Goette's " Entw.-gesch. d. Unke" \n Morphologisches 

 Jahrbuch. Vol. I. 



