576 



DEVELOPMENT OF THE LANCELET 



obliterated, so that the ectoderm and endoderm are in con- 

 tact (A, B). The general resemblance of the gastrula to a 

 simplified Hydra, 1 devoid of tentacles, will at once be ap- 

 parent, and the stage in the development of the frog's egg re- 

 presented in Fig. 64, F, though much modified by the quantity 

 of food-yolk, will be seen to correspond to the gastrula-stage. 

 As in the frog, the blastopore soon closes, the mouth and 



FIG. 152. Three stages in the formation of the gastrula of Amphioxus. In A 

 the nuclei of the endoderm-cells are omitted. (From Korschelt and Heider's 

 Embryology, after Hatschek.) 



anus being subsequently formed from the stomodeeum and 

 the proctodseum respectively (p. 204). 



The gastrula becomes elongated, flattened on one side, 

 and convex on the other. The flattened side corresponds 



1 It must, however, be remembered (pp. 308 and 321) that the 

 ectoderm and endoderm of Hydroids are differentiated before the 

 mouth is formed, so that the mouth does not correspond to the blasto- 

 pore of the gastrula. 



