144 CTENOPHORA. 



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 formative 

 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 mean time remain relatively 

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

 or less irregularly, while in Eucharis they divide into sixteen. 

 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 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 



