Vol. 3 1 Torreij. — Biological Studies on Corymorpha. 



283 



membrane between hydranth and stem from the annular thick- 

 ening of the endoderm in that region. 



(a) The fenestrated memdrane begins to develop in advance 

 of the peripheral canals. It has been noticed already in Figs. 

 11 and 13, where it is composed of a ridge of columnar cells 

 which have become longer than their neighbors. Fig. 26a shows 

 a slightly later condition, which is detailed in Fig. 29. Fig. 

 29rt and b represent longitudinal optical sections through a polyp 

 with four distal and seven proximal tentacles. Fig. 29c is a 

 similar section through another polyp of the same age. The 

 projecting ridge has fused from all sides across the enteron in 

 each case, forming a partition between hydranth and stem cavi- 

 ties with a tritling intrusion of mesogloea at its outer edge. In 

 Fig, 27 the partition has thinned out into a membrane with 

 f enestrae, one of which is shown 

 in the section. The mesogloeal 

 intrusion has increased into a 

 narrow shelf. Fig. 30 repre- 

 sents a cross section, through 

 the membrane, of a polyp of 

 a1)out the same age, in which 

 tive to eight canals are develop- 

 ing and fourteen proximal ten- 

 tacles are present. The outer- 

 most white zone is ectoderm. 

 ,,„ T . , , T .,, tisruous transverse sections of a young 



The adjacent broad zone with , • xi i ^t ^ I >- ^ 



^ polyp, passing through the lenestratecl 



concentric striations is the hor- membrane, 

 izontal shelf of mesogloea on 



which lies a thin layer of fibrils which are by no means so con- 

 spicuous as the figure indicates. The central area is occupied 

 by a thin membrane, pierced by nine irregular f enestrae and com- 

 posed of numerous small non-vacuolated cells with their longest 

 diameters approximately radial. The fenestrae correspond with 

 eight canals that are taking shape jiLst below in the stem. 



An attempt to explain the formation of the fenestrated mem- 

 brane naturally recalls the formation of the membrane across 

 the open end of a wounded Tuhularia stem. Though the latter 



Fig. 30. A composite of two con- 



