THE ONTOGENESIS OF VERTEBRATES 69 



ments for the later developments, hardly recognizable on com- 

 paring it with the more expanded and simple form found 

 among the lower types. Aside from such modifications as 

 those mentioned, which are explained through mechanical 

 exigencies, there appear to be differences in the origin of the 

 first mesoderm cells themselves, differences which tend to 

 shake our faith in the absolute homology of the germ layers. 

 Since, however, in spite of such variation in the early history, 

 the same embryonic elements eventually appear in all cases, 

 so that the anlagen of the principal organs are the same for 

 all, it is hardly possible that the early modifications, however 

 profound, have any deeper significance than that of caenoge- 

 netic adaptations to the various changed conditions of develop- 

 ment. 



The presence of yolk has a great modifying influence, both 

 on the general shape of the early embryo and upon the definite- 

 ness of its stages. Yolk is an inert substance, the presence of 

 which in large quantities within the cells interferes with their 

 normal division and with their assumption of normal positions. 

 Beyond a certain proportion, in fact, no cell division is possi- 

 ble, and the egg comes to consist of two portions, (i) the 

 protoplasmic area, in which all cell divisions take place, and 

 which ultimately becomes developed into the embryo, and (2) 

 the yolk-sac. These two areas are indicated in some eggs, as 

 in those of the frog, by a difference in color, the protoplasmic 

 area being deeply pigmented and the yolk area not. The 

 extreme of disproportion is seen in the bird's egg, where the 

 protoplasmic area is represented by the light yellow embryonal 

 disc, about 4-5 mm. in diameter, which floats on the upper 

 surface of the huge, non-cellular yolk mass. In such cases, 

 the embryo, when passing through the early stages, or until 

 after the establishment of the mesodermic somites and the 

 formation of head and tail, is spread out on the surface of the 

 spherical yolk, in proportion to which it is so small as to be 

 almost flat, but later on becomes nearly separated from it, re- 

 taining its connection by a narrow yolk-stalk attached in the 

 umbilical region. The embryo grows at the expense of the 

 yolk-sac, and as the former increases in size, the latter dimin- 



