234 BIOLOGY: GENERAL AND MEDICAL 



In the ontogenetic development of the higher animals 

 the conditions are different, for the nourishment of the 

 embryonal tissues is provided for by the egg yolk or by 

 the placenta, so that the digestive organs need not be 

 early perfected. The size of the embryo and the arrange- 

 ment of its parts are also such that the circulatory 

 organs need not be active before a considerable general 

 complexity of structure is attained. There is, however, 

 among vertebrates a general dominance of the nervous 

 system, so that instead of the organic systems developing 

 one after the other, as in phylogenetic progress, ontoge- 

 netic development is so modified that the dominant sys- 

 tem first makes its appearance, and in vertebrate em- 

 bryos the central nervous system which takes precedence 

 of all others in importance is one of the first to appear. 



However, though the importance and hence the order 

 of development is changed, the general plan of develop- 

 ment for each organ or system of organs is quite compar- 

 able to that seen in phylogenesis. 



"When one traces the course of development of any 

 vertebrate, he finds, speaking in general terms, that 

 those fundamental characteristics more or less common 

 to all vertebrates first appear, being followed by second- 

 ary characteristics distinguishing one class from another." 

 In vertebrate embryos, however, before the develop- 

 ment reaches a certain point, distinct resemblances to 

 invertebrate forms are met, and the younger the embryo 

 is, the more it has in common with embryos in general, 

 until at the very beginning we come to the single germi- 

 nal cell which is the starting point of every embryo. 

 These facts have found expression in the statement that 

 "the ontogeny recapitulates the phytogeny." This as a 

 theory is certainly justified; as a fact it must be further 

 explained. Von Baer, in 1828, gave us the following 

 generalizations: 



1. That which is common to a large group of animals develops in 

 the embryo earlier than that which is special. 



