8 Hybridization oj Echinoids. 



enteron with one another is striking. There seems to be Httle surface 

 tension in the wall at this time, each cell being rounded and having 

 rather a minimal contact with its neighbors. Many of the cells are 

 in phases of mitotic division. Slightly later, cells begin to move 

 away from the outer surface of the archenteron and the conditions 

 represented in plate 3, figures i and j, may be seen. The cells protrude 

 long protoplasmic processes, which come into contact with one 

 another and with the processes pushed out by adjacent cells; these 

 then fuse and give rise to the characteristic networks which may be 

 seen in echinoid gastrulse (fig. 3a). 



At about 50 hours the migration of mesenchyme has come to an 

 end and the enterocoele arises as a single pouch from the inner end 

 of the archenteron. On the left side this opens to the exterior through 

 a dorsal water-tube (fig. 2c). The single enterocoele, about 5 hours 

 later, divides into two, a right and a left, the left retaining connection 



Fig. 2a, Optical section living Cidaris gastrula; 23 hours. X 180. 

 26, Optical section living Cidaris gastrula; 30 hours. X 140. 

 2c, Optical section living Cidaris gastrula; 55 hours; enterocoele 

 and pore canal present, X 140. 



with the dorsal water-tube. By this time the wall of the archenteron 

 has become thin, the cells composing it no longer standing out dis- 

 tinctly from each other. 



About this time the spheroidal form of the embryo changes, due 

 apparently in part to the more rapid growth of the dorsal region, 

 which causes the blastopore to open on the ventral surface, and in 

 part to the modification in form of the future oral area. The anal 

 area remains rounded, while the oral area becomes first flat and then 

 concave. A cihated band appears along the posterior edge of the 

 oral area. This is the posterior ciliated band. It becomes well 

 established before there is any indication of an anterior ciliated band. 

 The anterior band is present at 66 hours (fig. 4a). A stomodseal 

 invagination appears about the seventieth hour and pushes back 

 until it comes into contact with the future esophagus, the inner end 

 of the archenteron having been directed toward the ventral side. 



