1/4 CTENOPHORA. 



of granular protoplasm. Each segment contains a nucleus. A 

 second similar division at right angles to the first gives rise to 

 four segments (fig. 82 B), and the segments so formed become 

 again divided into eight (fig. 82 C). In the division into eight, 

 which takes place in a vertical plane, the segments formed are of 

 unequal size, four of them being much smaller than the others. 

 The eight segments are arranged in the form of a slightly curved 

 disc round a vertical axis the future long axis of the body ; 

 and there is a cavity in this axis which, like the segmentation 

 cavity of Sycandra raphanus^ is open at both extremities. The 

 disc with its concavity on the side of the formative pole has the 

 shape sometimes of an ellipse (fig. 82 C) and sometimes of a 

 rectangle, in which the four small spheres occupy the poles of 

 the longer axis. A bilateral symmetry is thus even at this stage 

 clearly indicated. 



In the next phase of segmentation the granular layer sur- 

 rounding each segment again forms a protuberance at the forma- 

 tive pole, but, instead of each segment becoming divided into 

 two equal parts, the protoplasmic protuberance alone is divided 

 off from the main segment. In this way sixteen spheres become 

 formed, of which eight are large and are formed mainly of the 

 yolk material of the inner part of the ovum, and eight are small 

 and entirely composed of the granular protoplasm. The eight 

 small spheres form a ring on the formative surface of the large 

 spheres (fig. 82 D). 



The small spheres now increase very rapidly (fig. 82 E), partly 

 by division and partly by the formation of fresh cells from the 

 large spheres ; and spread over the large spheres, forming in this 

 way an epibolic gastrula. They constitute a layer of epiblast. 

 (Fig. 83 A.) The large cells in the meantime remain relatively 

 passive, though during the process they divide, in some cases 

 more or less irregularly, while in Eucharis they divide into six- 

 teen. The axial segmentation cavity would seem during the 

 process to become obliterated. 



There is an important discrepancy between the statements of Kowalevsky 

 and Agassiz as to the course of the growth of the small cells. According 

 to Agassiz the small cells grow most rapidly at the formative pole and cover 

 this before they meet at the opposite pole. The reverse statement is made 

 by Kowalevsky. It would seem that the above discrepancy is due to an 



