284 Cell OS Basis oj Organic Activity 



3. Eggs with a large amount of yolk which is concentrated at the vegetal 

 pole : highly telolecithal eggs. Cleavage is confined to the protoplasmic area 

 (germinal disc) alone: mcrohlasHc cleavage. Example: bird. 



Although the niainiiiaHan egg is of the isolecithal type, its early 

 embryology is highly modified. Until the formation of the blastula, 

 cleavage is holoblastic and equal ; after this, the divisions are confined 

 to a germinal disc. This modification which represents a heritage from 

 remote reptilian ancestors, plays an important role in the development 

 of embryonic membranes. 



Primitive Streak. — The eggs of some reptiles and birds, with 

 their cleavage confined to an embryonic disc, show marked modifica- 

 tions of gastrulation. This modification is due to the tremendous 

 amount of yolk and the necessity of making it available immediately 

 to the rapidly growing embryo. Blood vessels must get over its surface 

 at once. In the bird, the formation of the three germ layers is very 

 rapid and does not involve the invagination and inrolling as seen in 

 Amphioxus. From a practical viewpoint, this is a very reasonable 

 development inasmuch as the yolk makes such invagination quite dif- 

 cult if not impossible. Along the central axis of the embryo, the cells 

 of the germinal disc thicken and migrate and proliferate into the in- 

 terior. This thickening is clearly visible to the naked eye and is known 

 as the primitive streak (Fig. 86). Roughly it corresponds to the dorsal 

 roof of the archenteron which gives rise to the mesoderm and notochord 

 in Amphioxus. 



In the bird, the entoderm delaminates (cuts off) from the rest of 

 the germinal disc before the primitive streak forms and is in contact 

 with the yolk. Then a pushing in of cells occurs between the entoderm 

 and residual outer layer, the ectoderm. These cells which push in give 

 rise to the mesoderm. This whole thickening is the primitive streak. 



Early Human Embryology. — The need for implanting the em- 

 bryo at once and a long evolutionary history are the two most important 

 factors which influence the course of the early cleavage stages of the 

 human ovum. The human ovum is an isolecithal egg, but is peculiar 

 in its being surrounded by the heavy zona pellucida. This acts as a 

 restricting membrane which persists for a short time. The very early 

 cleavage stages are holoblastic and equal until the formation of the 

 morula. At that time, the zona pellucida starts disintegrating, and the 

 embryo begins to increase in size. 



