IV 



COELENTERATA 



93 



Just before the lower pole of the macromeres 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 



Fig. 75. — Two optical sections through the embryo of Beroe forskalii. (After Chun.) 



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



end, cavity of the gastric sac. (, 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 (Eig. 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 

 this jelly wandering cells 

 are budded from the cross 

 of " mesoderm " cells. 

 These cells become con- 

 nected with one another, 

 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. 



Fia. 76. — Larva of Beriie forskalii four days ohl, viewed 

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



The paragastric canals have forked and united at X with 

 the sub-ventral or sub-" stomachic " canals. The sub-tentac- 

 ular canals are still short. Letters as before, x, forks of 

 paragastric canal ; "p.g, paragastric canal ; s.sty sub-stomachic 

 canal ; s.ten, sub-tentacular canal. 



