314 STRUCTURE OF THE VERTEBRATES 



greatest role in postnatal growth and differentiation. There 

 naturally is a great overlap in the influence of these regulators. 

 Each tissue probably influences others throughout life, and the 

 internally secreting (endocrine) glands exert an influence at a 

 very early stage of development. 



The differentiation which exists in the egg varies widely in 

 the classes of vertebrates. In all there appears to be an antero- 

 posterior specialization; and in some there is evidence that 

 there is a transverse and dorso-ventral differentiation. The frog 

 is an illustration of this highly regulative type of egg. In others 

 the protoplasm is more generalized in nature, with less localiza- 

 tion of developmental potentialities. 



In the latter group the first cleavage apparently adds nothing 

 to the specialization of parts already present. This is indicated 

 by the so-called identical twinning, two individuals developing 

 from a single fertilized ovum. It is well established that in 

 Amphioxus each of the first four, or eight, cells has the poten- 

 tiality of forming a complete animal. This has been proved 

 true of some mammals, identical twinning being known to 

 occur in a number of groups. The armadillos offer the best 

 illustration of this process in the mammals. One species regularly 

 develops four embryos from a single egg, and another species 

 has eight young at a time when only one egg is given off from 

 the ovary. In both cases the embryos are attached to a single 

 placenta. 



In all vertebrates, however, this generalized condition of the 

 protoplasm disappears after a few cleavages; and with the 

 formation of the primary germ layers each region becomes 

 specialized and limited in its ability to develop various tissues. 

 A biological determinism has taken place within the cells. But, 

 although there is an apparent specialization controlling the 

 fate of each group of cells, many internal and external factors 

 may influence future growth. The external influences, such as 

 mutilations, temperature and food supply, may be disregarded 

 in this discussion. The internal influences, the organizators of 

 the body, are more relevant to a discussion of anatomy. 



Only the surface has been touched in the study of these 

 organizing substances. Working upon the frog it has been found 

 (Brachet and others) that the anterior region exerts a more 



