THE BLASTOPORE 245 



The conditions in the reptile lead on easily to those which 

 obtain in birds. Here, again, the endoderm is formed pre- 

 cociously as a lower layer split off from the underside of the 

 superficial layer of the blastoderm. The blastopore, however, 

 never is a real aperture, because its lateral lips are fused 

 together all along their length forming the primitive streak. 

 The dorsal lip is the primitive knot beneath which a solid 

 strand of cells is tucked in to form the notochord. As the 

 primitive streak moves backwards over the blastoderm, it 

 pays in a stream of cells into the hinder end of the notochord. 

 The primitive streak gives off mesoderm to each side. In the 

 bird, therefore, the blastopore is closed from the start, and its 

 aperture is represented only by the depression of the primitive 

 pit just behind the primitive knot, and by the primitive groove 

 which runs along the centre of the primitive streak. The 

 bird's blastopore begins where that of the frog leaves off, for 

 in the latter it will be remembered that the blastopore which 

 was spherical becomes oval, and its lateral lips become apposed 

 to one another, forming what is in fact a short primitive streak. 

 In the bird, there is no invagination, and no archenteron, and 

 the walls of the alimentary canal are derived (as in reptiles) 

 from the lower layer. 



In mammals, the embryo develops from a primitive streak. 

 In some cases the blastopore is a real aperture, or in other 

 words, the primitive pit sinks down and opens into an 

 archenteron beneath the superficial layer of the floor of the 

 amniotic cavity (corresponding to the blastoderm). In others, 

 the blastopore is reduced. The primitive streak and archen- 

 teron give rise to the notochord and mesoderm, and the 

 endoderm is formed from the lower layer. 



In reptiles, birds, and mammals, therefore, the blastopore 

 either closes or arises already closed without the yolk becoming 

 enclosed. For the anterior edge of the blastoderm does not 

 grow down under the yolk to form a true ventral lip of the 

 blastopore. , J 



In Amphioxus where there is little yolk, the rim of the 

 blastopore is formed as the result of simple invagination of the 

 vegetative hemisphere. Thereafter the rim of the blastopore 



