42 EVOLUTION AND GENETICS 



Eggs of the first type give rise by a succession of cleavages to a 

 hollow sphere called the blastula, made up of cells very nearly 

 alike in size (Fig. 19). The presence of a moderate amount of 

 yolk gives rise to a slight modification of this form as shown m the 

 blastula of the frog (Fig. 20), while the relatively enormous amount 

 of yolk in the egg of a bird crowds all of the cytoplasm to one pole, 

 with the result that cleavages cannot pass entirely through the 

 sphere. In these eggs the protoplasm alone is cut up into a cap of 

 cells underneath which a small space is equivalent to the cavity of 

 the spherical blastula or blastocoele, so conspicuous in the other 

 forms (Fig. 21). 



Gastrulation. Following cleavage the simplest blastulas (Fig. 

 19) cave in on one side until the concave layer of cells is in 

 contact with the convex side, thus establishing a sac with a two- 

 layered wall by whose formation the blastocoele has been ob- 

 literated. The enclosed cavity, or archenteron, opens to the ex- 

 terior of the embryo by the blastopore. If we compare the 

 series of figures of frogs' eggs (Fig. 20) we note at once that the 

 blastocoele is relatively so small that this process cannot be accom- 

 plished. Instead, a crescentic lip evaginates from the thinner part 

 of the blastula, grows down and around the structure until its ends 

 meet and form a rounded opening, and the same result is obtained. 

 The blastocoele is obliterated, while the cavity present is enclosed 

 by a two-layered wall and opens to the exterior of the embryo. 

 That this archenteron is almost completely filled by the remaining 

 yolk mass which bulges from the inner cells is merely incidental 

 to the accumulation of yolk. In the bird's egg the cap of cells turns 

 under at one point, and l)y proliferation from this point finally 

 becomes d ,,ible (Fig. 22). The yolk is then capped by two layers 

 of cells in place of one, although it is so bulky that these layers do 

 not enclose it until much later. For homologies here we must note 

 that the blastopore has another definite characteristic : it is bounded 

 by the united outer and inner layers, or ectoderm and endoderm, 

 and this is also characteristic of the point at which the cells on the 

 hen's egg turn under to form the second layer. 



The Neurenteric Canal. In order to compare the early devel- 

 opment of the mammal it is necessary to pass on to other struc- 

 tures. The mesoderm in all vertebrates arises between the two 

 original layers, and from the three all structures of the body de- 

 velop. As an early step a region along the mid-dorsal line of the 



