THE BLASTOPORE 243 



blastoderm. The posterior edge of this blastoderm grows 

 back over the yolk and tucks cells in beneath itself, like the 

 typical dorsal lip of the blastopore which it is. Overgrowth 

 also takes place at each side of the dorsal lip, and the blastopore 

 becomes crescentic. Eventually the two horns of the crescent 

 meet and the blastopore is then a closed circle. But the 

 anterior edge of the blastoderm has not moved, it has not grown 

 round underneath the yolk, and it takes no share whatever in 

 the formation of the blastopore. At the same time it is to be 

 noticed that the blastopore is a real aperture, through which 

 the yolk can be seen from the outside. The cavity which 

 communicates with the exterior through the blastopore is, 

 of course, the archenteron, and the lining of this cavity is the 

 endoderm, formed by the activity of the edge of the blastopore. 

 In the reptiles, yolk is abundant, and cleavage leads to the 

 formation of a blastoderm. At a place which marks the 

 posterior end of the future embryo, cells are proliferated under 

 the blastoderm, forming a lower layer between the blastoderm 

 and the yolk. This lower layer is really the endoderm, which 

 has been formed precociously, probably serving the function 

 of digesting the large quantity of yolk. A dorsal lip of a 

 blastopore arises (not at the extreme hind edge of the blasto- 

 derm, but well within its margin) as a rim beneath which cells 

 become tucked in and passed forwards beneath the blastoderm 

 and above the lower layer. The rim of the blastopore extends 

 to the sides, and so the lateral lips come into being. Eventu- 

 ally the lateral lips extend backwards, and lie parallel to one 

 another. The blastopore is now slit-like, and resembles a 

 primitive streak. The lateral lips of the blastopore join 

 posteriorly, and the blastopore is then closed. The cells 

 which get tucked in by the lips of the blastopore line a cavity 

 which is the archenteron, so that here as in fish, frogs, newts, 

 and Gymnophiona, the blastopore is a real aperture. The 

 archenteron extends far forwards as the result of invagination, 

 and its roof in the middle line becomes the notochord ; on 

 each side the roof becomes mesoderm. The floor of the 

 archenteron fuses with the underlying lower layer and then 

 disappears, so that the blastopore leads right down through 



