CCELENTERATA. 



145 



s.c 



FIG. 83. FOUR STAGES IN THE DEVELOP- 

 MENT OF IDYIA ROSEOLA. (After Agassiz.) 

 s.c. sense capsule; st. stomodasum. 



be gathered from Kowalevsky 's statements, is lined by a flattened 



epithelium. At a later period the gastro-vascular canals grow out 



from the infundibulum as four 



pouches, which are surrounded by, 



and grow at the expense of, the 



large central cells, which have in 



the meantime 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 consist, as in the Acraspedote Medusse and Actinozoa, 

 of two sections: (1) A true hypoblastic section consisting of the infun- 

 dibulum and the gastro-vascular canals derived from it ; and (2) an 

 epiblastic section the stomodseum forming the stomach. 



The observations of Kowalevsky on the alimentary system do not 

 wholly tally with those of Agassiz. He finds that the oral side of the 

 embryo becomes hollowed out, and that the hollow, lined by flattened 

 cells, becomes constricted off as the infundibulum, from which the radial 

 canals subsequently grow out. To the infundibulum there leads a 

 narrow canal lined by a columnar epithelium which becomes the gastric 

 cavity. 



While the alimentary canal is becoming formed a series of 

 important changes takes place in other parts of the embryo. The 

 rows of locomotive paddles first appear as four longitudinal equi- 

 distant linear thickenings of the epiblast near the aboral pole 

 (fig. 83 D). On the projecting surface of these ridges stiff cilia 

 appear which coalesce together to form the paddles. While the 

 embryo is still within the egg the rows of paddles are quite short 

 and also double. There are in Pleurobrachia about eight or nine 

 pairs of paddles in each row. Each double row eventually separates 

 into two. 



In all the forms except the Eurostomata (Beroe) two tentacles 

 grow out as thickenings of the epiblast (fig. 84 B, .). They are placed 

 at the opposite poles of the long transverse axis of the embryo. 



1 Chun (No. 1 74) gives a short statement of his observations, which accords with 

 the interpretation in the text. 



B. E. 



10 



