PYKOSOMA DEVELOPMENT OF THE CYATHOZOOID. 



393 



At the time when the separation of the germ-layers takes place 

 systems of cavities appear in the mesoderm (SALENSKY) these 

 being connected with imaginations on the lower (entodermal) surface 

 of the germ-disc. One of these imaginations is rather large and lies 

 near the posterior edge of the germ-disc (Fig. 185, ch). It is con- 

 nected with a system of cavities running forward in the median line 

 of the disc. The two other (paired) imaginations (Fig. 186 A, c) 





Fi<;. 1st). Two tnuisvrrsc sections through a young germ-disc of /'///-uxo/tni (after 

 SAI.ENSKY). . 1 , through the posterior, and B, through the anterior region, c, coelom ; 

 t'./., cavity of the chorda ; ec, ectoderm ; en, entoderm ; ms, mesoderm ; n, nervous 



system. 



lie laterally and somewhat in front of the first and probably com- 

 municate with the lateral system of cavities.* These are regarded 

 by SALENSKY as the rudiments of the coelomic sacs, and the axial 

 system of cavities as the equivalent of the chorda. SALENSKY was 

 unable to decide whether there are here a number of separate spaces 

 serially arranged or a continuous, but somewhat bent longitudinal 

 canal. 



B. Development of the Uyathozooid. 



The next changes to be noticed in the germ-disc are the appearance 

 of the rudiment of the nervous system of the Cyathozooid and the 

 development of the peribranchial sacs. The nervous system arises as 

 an ectodermal thickening in the anterior part of the germ-disc (Fig. 

 1 s 7, //), which later becomes depressed as a furrow, and in this way 



" [SALENSKY figures the paired coelomic imaginations at an earlier stage 

 than that shown in Fig. 186 A, and further he regards the depression seen on 

 the right of the swelling containing ch as the coelomic invaginatiou, and in 

 his tigure, of which the above is a copy, letters it as such. FA).] 



