IV 



COELENTERATA 



93 



Just before the lower pole of the macromeres is completely covered 

 by the advancing ectoderm, each macromere bud* off downwards, 

 a small cell. We thus get a circle of sixteen small cells at the 



A 



-S.fen 



FIG. 75. Two optical sections through the embryo of BeriJe forskalii. (After Chun.) 



A, through the stomodaeum. B, above the stomodaeum near the aboral pole. 

 end, cavity of the gastric sac. t, taeniola. 



vegetative pole ; these are the rudiment of the so-called mesoderm. 

 The macromeres now rotate in such a way that the " mesodermal " 

 cells are rotated upwards 

 and inwards, and eventu- 

 ally come to lie at the 

 upper pole of the macro- 

 meres. Here they undergo 

 rapid division and form a 

 cross, the two longer arms 

 of which extend in the 

 direction of the long axis 

 of the embryo, while the 

 two shorter ones are trans- 

 verse to that axis (Fig. 72). 

 The ectodermal skin 

 is now completed, and the 

 macromeres now undergo 

 rapid division, forming a 

 parenchyma - like tissu e. 

 Between ectoderm and 

 endoderm jelly appears 

 as a secretion; and into 

 this jelly wandering cells 



are budded from the CrOSS The paragastric canals have forked and united at x with 



of "mesoderm" Cells. the sub-ventral or sub-" stomachic" canals. The sub-tentac- 



These cells become con- ular ca " a ! s are f 11 short L f ters as , befo f - forks f 



paragastric canal; p.g, paragastric canal; s.st, sub-stomachic 

 nected With One another, canal ; g.ew, sub-tentacular canal. 



with the ectoderm and 



with the endoderm by their processes, so as to produce a cell-network 



which simulates connective tissue ; it is, however, certain that many, 



FIG. 76. Larvu of BeroeJ'vfxki'Hi four days old, viewed 

 from "stomach-plane." (After Chun.) 



