IV 



COELENTERATA 



93 



Just before the lower pole of the inacromeres is completely covered 

 by the advancing ectoderm, each macromere buds off downwards, 

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



A 



sten 



sst 



FIG. 75. Two optical sections through the embryo of Beroe forskulii. (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 rnacromeres now rotate in such a way that the " rnesoderrnal " 

 cells are rotated upwards 



and inwards, and eventu- ^^^ dp 



ally come to lie at the /aK '^ ** 



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 tissue. 

 Between ectoderm and 

 endoderm jelly appears 

 as a secretion ; and into Fl( , 76- _ Larva of BerSe f(/rskaUi four days oltlj viewed 



this jelly wandering cells from "stomach-plane." (After Chun.) 



are DUQaed irom the CrOSS The paragastric canals have forked and united at x with 



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



These cells become con- ular canals are stm short Letters as befo ' e - x> forks of 



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



nected Wltn One another, canal ; s.ien, sub-ten tacular 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, 



