l'\l;oS()\[ \ DKYKI.OI'MKNT OK TMK (.' V \TIlOX( )l) I I ). 



393 



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

 three systems of cavities appear in the mesoderm (Salknsky) these 

 being connected with invaginations on the lower (entodermal) surface 

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

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

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

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



Tig. 186. Two transverse sections through a young germ-disc of Pyrosoma (after 

 SaLBKSKT). .1 , through the posterior, ami IS. through the anterior region, c, eoeloui ; 

 cavity of the chorda ; ec, ectoderm : en, entoderm ; ms, mesoderm ; n, nervous 

 systen . 



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

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

 I'V Salknsky 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 

 illy 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 tin' peribranchial sacs. The nervous system arises as 

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

 187, M. which later becomes depressed as a furrow, and in this way 



" [Salensky figures the paired coelomic invaginations at an earlier stagi 

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

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

 his figure, of which the above is a copy, letters it as such.— Ed. | 



