110 



DEVELOPMENT OF AMPHIOXUS. 



blastula-wall. This side now becomes flattened, as shown 

 in Fig. 59 A. Next, the flattened side of the blastula 

 becomes gradually tucked up or invaginated into the 



blastoccel (Fig. 59 B) until, 

 finally, the segmentation 

 cavity is completely obliter- 

 ated, and the invaginated 

 layer of cells becomes tightly 

 fitted up against the outer 

 layer (Fig. 59 Q. 



c 9% The embryo, now known 

 as the gastrnla, is a double- 

 layered sac, the cavity of 

 which was produced by in- 

 vagination, and is known as 

 the primitive gastral cavity, 

 or archenteron. This cavity 

 is widely open to the ex- 

 terior by the orifice of invagi- 

 nation, or blastopore, which 

 in German is designated by 

 the expressive term Urmnnd. 

 The two layers of cells which 

 constitute the wall of the 

 gastrula are the primitive 

 germ-layers ; the outer layer 

 is the primitive ectoderm, 

 and the inner layer, sur- 

 rounding the gastral cavity, 

 is the primitive endoderm ; the two layers are continuous 

 with one another round the margin of the blastopore. 



The blastopore is at first a very wide oval opening, 

 but it soon becomes narrowed down to a small aperture 



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