744 



CH/ETOGNATHA. 



cells are placed nearer the median line, and two externally. The 

 two inner cells form the eventual testes, and the outer the 



IG. 408. THREE STAGES IN THE DEVELOPMENT OF SAGITTA. (A and C after 



Biitschli, and B after Kowalevsky.) 

 The three embryos are represented in the same positions. 



A. Represents the gastrula stage. 



B. Represents a succeeding stage, in which the primitive archenteron is com- 

 mencing to be divided into three. 



C. Represents a later stage, in which the mouth involution (;) has become con- 

 tinuous with the alimentary tract, and the blastopore is closed. 



m. mouth ; al. alimentary canal ; ae. archenteron ; bl.p. blastopore ; pv. peri- 

 visceral cavity ; sp. splanchnic mesoblast ; so. somatic mesoblast ; ge. generative 

 organs. 



ovaries, one half of each primitive cell thus forming an ovary, and 

 the other a testis. 



FIG. 409. Two VIEWS OF A LATE EMBRYO OF SAGITTA. A, from the dorsal 



surface. B, from the side. (After Biitschli.) 



m. mouth ; al, alimentary canal ; v.g. ventral ganglion (thickening of epiblast) ; 

 ep. epiblast ; c.pv. cephalic section of body cavity ; so. somatopleure ; sp. splanchno- 

 pleure ; ge. generative organs. 



