CCELENTERATA. 



175 



interchange on the part of the one or the other of these authors of the two 

 poles of the embryo, in that according to Agassiz the formation of the mouth 

 takes place at the formative pole, and according to Kowalevsky at the pole 

 opposite to this. 



Without attempting to decide between the above views, we shall speak of 

 the pole at which the mouth is formed as the oral pole. 



The formation of the alimentary cavity commences shortly 

 after the complete investiture of the embryo by the epiblast 

 cells. At the oral pole an invagination of epiblast cells takes 

 place (fig. 83 B), which makes its way towards the opposite pole. 

 More especially from the figures given by Agassiz, and from the 

 explanation of his plates, it would seem that a large chamber is 

 formed in the hypoblast at the end of the invaginated tube, into 

 which this tube soon opens (fig. 83 C). The invaginated tube 

 would seem to give rise to the so-called stomach, while the 

 chamber at its aboral extremity is no doubt the infundibulum, 

 which as may be gathered from Kowalevsky 's statements, is lined 

 by a flattened epithelium. At a later period the gastrovascular 

 canals grow out from the infun- 

 dibulum as four pouches, which 

 are surrounded by, and grow at 

 the expense of, the large central 

 cells, which have in the mean- 

 time arranged themselves in 

 four masses, and appear to serve 

 as a kind of yolk. The nuclei 

 of these large cells according to 

 Kowalevsky disappear, and the 

 cells themselves break up into 

 continually smaller masses. 



The main difficulty in the above 

 description of Agassiz is the origin of 

 the infundibulum. In the absence of 

 definite statements on this head it 

 seems reasonable to conclude that it arises as a space hollowed out in the 

 central cells, and that its walls are formed of elements derived from the yolk 

 cells 1 . On this interpretation the alimentary canal of the Ctenophora would 



s.c 



s.c 



FlG. 83. FOUR STAGES IN THE DE- 

 VELOPMENT OF IDYIA ROSEOLA. (After 

 Agassiz. ) 



s.c. sense capsule; st. stomodasum. 



1 Chun (No. 174) gives a short statement of his observations, which accords with 

 the interpretation in the text. 



