THEORY OP THE EMBRYONIC PHASE OF ONTOGENY. 325 



Sedgwick's Theory of the Embryonic Phase of 

 Ontogeny as an aid to Phylogenetic Theory. 



By 



E. TV. MacBi'idc, B.A., 



Fellow of St. Jolm's College, Cambridge ; Demonstrator in Animal Morpho- 

 logy to the University of Cambridge. 



In a recent Eumbei'i of this Journal there appeared a paper 

 by Mr. Adam Sedgwick ou the significance of the embryonic 

 phase in development, which embodies a principle which, if 

 true^ seems to me well fitted to throw light on some obscure 

 problems in morphology. 



It is not the object of the present essay to discuss the cor- 

 rectness of Mr. Sedgwick's views, but rather, assuming them 

 to be true, to point out some of their consequences. 



These views may be briefly stated as follows. Making a 

 broad survey of the facts of ontogeny, we find that there are 

 two main types or phases of development — the larval and the 

 embryonic. In the former case the immature organism pur- 

 sues a free life, engaging in the struggle for existence ; in the 

 latter case the developing animal is shut off" from the influence 

 of external conditions, either inside an egg-membrane or in 

 the uterus of the mother; but in both cases it is relieved from 

 the necessity of having to seek its own living, since nourish- 

 ment is provided for it either in the shape of food-yolk or fluid 

 nourishment exuded from the uterine walls. 



In many cases the whole course of the ontogeny of an animal 



* " On the Law of Development commonly known as von Baer's Law ; and 

 on the Significance of Ancestral Rudiments in Embryonic Development," 

 Quart. Journ. Micr. Sci.,' April, 1894. 



VOL. 37, PAET 3. NEW SER. Y 



