146 



EMBRYOLOGY 



veloped from a thickening of the ectoderm (Fig. 68). The 

 otoliths, which are at first small and then increase in size, 

 are formed in certain of these cells; they are finally 

 extruded upwards, to constitute the otolithic mass sus- 



tained on the four S- 

 shaped springs (cilia). 

 In many cases the first 

 otoliths are formed in 

 the epithelium in four 

 groups, corresponding 

 to the different quad- 

 rants of the body of 

 the Ctenophore (A. 

 Agassiz,Fol). The bell- 

 shaped case developed 

 over the sensory oi*gan 

 arises, like the swim- 

 ming plates, from four 

 groups of long cilia 

 fused with one another 

 (Figs. 68, 70, 72). The 

 ciliated polar plates 

 (Fig. 63 p) are formed 

 in connection with this 

 sensory body as thick- 

 ened regions of the ectoderm, which at first are rounded, but 

 subsequently much elongated. 



We have seen that two parts can be distinguished in the 

 fundament of the gastrovascular system (Fig. 67 C) : a 

 lower, which arose as an ectodermal invagination, the 

 inner surface of which is soon covered with a coat of 

 cilia, and out of which the so-called stomach subsequently 

 arises; and an upper (d), bounded by entodermal cells, which 

 represents the fundament of the infundibulum and the 

 vessels. The differentiation of this upper, entodermal 

 portion into its individual parts can be considered as essenti- 

 ally a kind of formation of diverticula. As can be seen 

 in Fig. 67 C, the entoderm cells, as the macromeres after 

 giving off the ectodermal and mesodermal elements may 



Fi6. 68.— Diagram of a Ctenophore embryo 

 at the time of the formation of the entodermal 

 sacs, all organs in transverse section, except 

 the fundaments of the ciliary plates, r, which 

 correspond to the outer surface, ot, otoliths ; 

 t, fundament of the tentacular apparatus; ms, 

 mesoderm ; en, entoderm ; ec, ectoderm ; g, 

 mesogloea ; m, stomach ; c, central intestinal 

 cavity; d, diverticula of the same (fundaments 

 of the entodermal sacs). 



