" ONTOGENY OF CNIDARIA 131 



After the 16 mesoderm micromeres [me) have been formed, the macromere 

 plate becomes depressed, while, at the same time, the ectoderm cap spreads 



Fio. 100.— Three early stages of development of a Oteuopliore (Oallianira), after MetsolmlkofiF, 

 somewhat diagrammatic, ec, Ectoderm ; en, endoderm ; liie, mesoderm ; cf, gastric cavity ; st, o3so'- 

 phagus (stomodEeum). 



out more and more towards the vegetative pole, 

 which we can already observe, besides the two 

 primitive germ layers, a mesodermal rudiment {B). 

 The latter, by the invagination of the 16 macro- 

 meres which represent the rudiment of the endo- 

 dermal gastro- canal systems, come to lie inside, 

 towards the gastric cavity. Later on it reaches 

 a position quite at the animal pole, beneath the 

 ectoderm [C), its elements at the same time increas- 

 ing by division. The ectoderm at the vegetative 

 pole becomes depressed inwards round the blasto- 

 pore, and thus forms a stomodseum (st), the rudi- 

 ment of the oesophagus (erroneously stomach) of 

 the adult OteTwpliore. The mesoderm at the aboral 

 pole, viewed from this pole, assumes the shape of 

 a cross. Two opposite limbs of the cross stretch 

 out into the rudiments of the two tentacles, which 

 appear as outgrowths of the ectoderm. The ecto- 

 derm thickens at the aboral pole to form the 

 sensory body (Fig. 101, sk). Swimming plates 

 appear as fused cilia in 8 meridians, arranged in 

 pairs at the surface of the ectoderm. At first 

 only a few swimming plates are formed in each 

 row, but their number gradually increases. The 

 hollow spaces of the gastro-canals appear as fissures 

 penetrating from the gastric cavity to the circum- 

 ference. Between the endoderm and the stomo- 

 dseum on the one side, and the ectoderm on the 

 other, a clear mass of jelly is secreted. The various 

 elements which in the Gtenophora occupy the 

 jelly are, according to some investigators, derived 

 from cells migrating inwards from the ectoderm ; 

 according to others they are yielded by the 

 rudimentary mesoblast, which has been described 

 above. 



A ccelogastrula thus arises, in 



H .t 



Pig. 101.— Two further stages 

 of development of Callianlra, 

 after Metsohnikoff. ere, Endoderm; 

 me, mesoderm ; mej, mesenchyme ; t, 

 tentacle ; sk, sensory body ; d, gas- 

 tric cavity ; st, oesophagus (stomo- 

 dfeum) ; g, jelly. 



