200 



EMBRY0L0GICAL TYPES 



undivided. The margin of the blastoderm merges with the 

 yolk round it, forming the periblast ; and beneath the blastoderm 

 is a cavity, the blastoccel, which separates it from the underlying 

 yolk. This stage, when the blastoderm is but a single layer 

 (though of many cells), represents the blastula of Amphioxus 

 and the frog. 



Gastrulation. — A layer of cells becomes split off from the 

 under side of the blastoderm, between it and the underlying 

 yolk. This layer soon extends over the under surface of the 

 blastoderm and is known as the " lower layer," or secondary 

 endoderm. It is continuous with the upper layer all round 



Fig. 88. — Gallus : transverse sections through the primitive streak of the 

 blastoderm of a hen's egg. (From Jenkinson.) 



A after 10 hours', B, after 15 hours' incubation. //, lateral portion of 

 the primitive streak corresponding to the lateral lip of the blastopore (cf. 

 Fig. 76) ; mes, mesoderm ; pd, endoderm ; yp, primitive groove. 



the margin, and, like it, merges into the periblast. The 

 blastoderm extends gradually over the yolk, and in so doing 

 it forms a margin of overgrowth. In this region, all round 

 the edge of the blastoderm (which is called the germ- wall), 

 the protoplasm is thicker than in the centre. When therefore 

 a blastoderm is looked at by transparency, two zones are 

 distinguishable. Centrally there is a relatively clear area 

 pellucida ; and round the edge is a denser area opaca. The egg 

 is usually at this stage when it is laid, some twenty-four 

 hours after fertilisation, 



