BIOGENETIC LAW OF EMBRYONIC RECAPITULATION 349 



migrated from the various soma cells into the germ cells. This theory further 

 postulated the inheritance of acquired characters. 



If these three assumptions are granted, then it is easy to understand Haeckel's 

 contention that embryological development consists in the repetition of pre- 

 vious stages in phylogeny. For example, if we assume that the blastaea changed 

 into the gastraea by the addition of the features pertaining to the primitive 

 gut with its enteric lining, then the gastraea possessed the hereditary factors 

 of the blastaea plus the new enteric factors. These enteric features could 

 easily be added to the deric (outer-skin) factors of the blastaea, according 

 to Darwin's theory of pangenesis. Furthermore, according to assumption (2) 

 above, in the embryonic development of the gastraea, the hereditary factors 

 of the blastaea would reveal themselves during development first and would 

 produce the blastaea form, to be followed by the appearance of the specific 

 enteric features of the gastraea. And so it proceeded in the phylogeny and 

 embryology of later forms. In this way the preceding stage in phylogeny be- 

 came at once the cause of its appearance in the development of the next 

 phylogenetic stage. 



These assumptions, relative to heredity and its mechanism of transference, 

 were shown to be untenable by the birth of the Nageli-Roux-Weismann con- 

 cept of the germ plasm (see Chaps. 3 and 5) and by the rebirth or rediscovery 

 of Mendelism during the latter part of the nineteenth century. Studies in em- 

 bryology since the days of Weismann have demonstrated in many animal 

 species the essential correctness of Weismann's assumption that the germ 

 plasm produces the soma during development, as well as the future germ 

 plasm, and thus have overthrown the pangenesis theory of Darwin. The as- 

 siduous study of Mendelian principles during the first twenty-five years of the 

 twentieth century have demonstrated that a fixed relation does not exist be- 

 tween the original character and the appearance of a new character as implied 

 in the Haeckelian law (Morgan, '34, p. 148). Furthermore, that "in many 

 cases, perhaps in most, a new end character simply replaces the original one. 

 The embryo does not pass through the last stage of the original character 

 and then develop the new one — although this may happen at times — but the 

 new character takes the place of the original one" (Morgan, '34, p. 148). 



How then does one explain the resemblances of structure to be found 

 among the embryos at various stages of development in a large group of 

 animals such as the Chordata? Let us endeavor to seek an explanation. 



In development, nature always proceeds from the general to the specific, 

 both in embryological development and in the development of phylogeny or 

 a variety of forms. The hereditary factors which determine these generalized 

 states or structural conditions apparently are retained, and specialized fac- 

 tors come into play after the generalized pattern is established. Generalized 

 or basic conditions, therefore, appear before the specialized ones. An example 

 of this generalized type of development is shown in the formation of the 



