II 



ONTOGENY OF CNIDAEIA 



131 



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

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



FIG. 100. Three early stages of development of a Ctenophore (Callianira), after Metschnikoff, 

 somewhat diagrammatic, cc, Ectoderm ; en, endoderm ; me, mesoderm ; d, gastric cavity ; st, oeso- 

 phagus (stomodseum). 



out more and more towards the vegetative pole, 

 which we can already observe, besides the two 

 primitive germ layers, a mesodermal rudiment (It). 

 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 (<7), 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 Ctenophore. 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 Ctenophora 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 



me/ 



FIG. 101. Two further stages 

 of development of Callianira, 

 after Metschnikoff. en, Endoderm ; 

 me, mesoderm ; mei, mesenchyme ; t, 

 tentacle ; sk, sensory body ; d, gas- 

 tric cavity; st, oesophagus (stomo- 

 dseum) ; g, jelly. 



