148 EMBRYOLOGY 



entodermal sacs, each one of which corresponds to a quadrant 

 of the Ctenophore body. The distinct separation of these 

 four entodermal sacs is materially promoted by the simul- 

 taneous appearance of the mesogloea (Fig. 69 A, g). This 

 transparent secreted mass accumulates between the stomach, 

 the entoderm, and the surface ectoderm (Fig. 68 g), and 

 forms septa-like processes, which extend especially between 

 the entodermal sacs. The rapid increase of the mesogloea, 

 into which cells soon migrate, occasions the considerable 

 increase in the size of the embryo during this stage. By 

 the formation of the mesogloea in the further course of de- 

 velopment, the fundament of the gastrovascular system is 

 forced farther and farther away from the outer surface of 

 the body. An intimate contact is retained only at the 

 points corresponding to the eight ribs and the fundaments 

 of the tentacles (Fig. 69 B) ; by a large accumulation of 

 entodermal cells the places are here indicated at which, by 

 the formation of additional diverticula, the eight rib-vessels 

 (meridional canals) and tentacular vessels are developed. 

 The origin of the two gastral vessels is to be attributed to a 

 similar formation of diverticula (Fig. 69 B, ing). 



The mode of formation of the four entodermal sacs by the deeper 

 penetration of the inner opening of the oesophagus, which has been 

 described by Chun (No. 3) and represented in his Fig. 18, Taf. vii., re- 

 calls the quite similar mode of formation of the two primary gastral 

 pouches of the Scyphistoma according to Goette. (Comp. p. 107.) 



During these changes the characteristic lateral compres- 

 sion of the stomach has already been effected (Fig. 69 B^ in). 

 On the contrary, the central part of the vascular system, 

 which is metamorphosed into the infundibulum, exhibits a 

 compression, more or less distinct in all Ctenophora, in the 

 direction of the other (sagittal) secondary axis, so that these 

 conditions could be utilized by Chun (No. 3) in designating 

 the cross-axes. The more the vascular system is developed, 

 the more do the entodermal cells acquire the histological 

 characters of the permanent walls of the vessels. 



We have traced the fundament of the mesoderm until, in 

 the progressing invagination of the gastral cavity, it arrives 



n 



