130 



COMPARATIVE ANATOMY 



CHAP. 



neighbouring endodermal walls of every 2 gastric pouches apply themselves to each 

 other and form the partition walls or septa (If, se), which are continued also, with 

 free axial edges, into the central gastric cavity, and there form the so-called gastric 

 ridges. The first 4 tentacles (E, t), arise around the oral disc as outgrowths of the 

 ectoderm and endoderm. The endodenn forms a solid axis in the tentacles, which 

 arise over the 4 gastric pouches, and which increase in number later. 



In the Coral polyps the formation of the tentacles, gastric pouches, and partition 

 walls, if not exactly like the above, is still similar, and the endodermal axes of the 

 tentacles are hollow from the first. 



The most important facts about the development of the Scyphula into the Acra- 

 specle Medusa have already been given on pp. 77, 106, 107, and 116. 



The direct development of the Acraspede Medusa out of the egg is not yet suffi- 



FIG. 99. Development of the Scyphula of Aurelia Aurita out of the Planula, after GOtte. 

 A, Planula. J5, The same after it has attached itself. C, Invagination of the oesophagus. D, Break- 

 ing through of the enteric aperture. E, Scyphula. F, Transverse section through the stage C. 

 G, The same through stage D at the level of the line a-b. H, Transverse section through the Scyphula 

 (E) at the level a-b. I, the same at the level c-d. Black, ectoderm ; streaked, endoderm. s, (Eso- 

 phagus ; se, septa ; mt, gastric pouches ; t, tentacle ; m, gastric cavity ; st', st, septal funnels. 



ciently known. Compare what was said of the development of the mesoderm of the 

 Coral (p. 100). 



The development of the Ctenophora is in a certain way opposed to that of all 

 other Cnidaria. The total unequal furrowing of the telolecithal egg has already been 

 described, as also the formation of a two-layered germ by epibole. We here again 

 resume our account of the development. 



After the 8 macromeres have formed a cap of micromeres which yield the whole 

 ectoderm, they themselves divide into 16 macromeres, which arrange themselves as 

 a plate on the vegetative side of the germ. Thereupon each of the 16 macromeres 

 constricts off one micromere on the under side, i.e. towards the vegetative surface 

 of the germ (Fig. 100). The 16 micromeres so formed are a part of the mesodermal 

 rudiment, the rudiment at any rate of the tentacle mesoderm. They may be con- 

 sidered, perhaps, as part of the endoderm, as an early product of separation from it. 

 The mesodermal axis of the Ctenophoran tentacle might then be compared with the 

 endodermal tentacle canals or tentacle axes of the other Cnidaria. 





