METAZOA COELENTERA. 121 



common object. Three investigators 1 worked together 

 on the eggs of Renilla, keeping them under continuous 

 observation, and "were thus enabled to determine with 

 all possible certainty the fact that at least five or six well- 

 marked modes of yolk cleavage with many minor varia- 

 tions, may occur as normal phenomena of development, 

 that the segmentation may be at first equal or unequal, 

 complete or partial, regular or irregular, and that a great 

 amount of variation exists in the duration of the various 

 stages of activity and quiescence." 



PL 177, figs. 1-14, illustrates the consecutive stages of 

 development in one individual, the time required being 

 115 minutes. The egg first divided into eight spheres 

 (one third of the specimens examined divided in this way, 

 but usually this stage is skipped and the egg cleaves at 

 once into sixteen spheres). When the sixteen-sphere 

 stage is reached the process of delamination begins. 

 This process does not go on simultaneously in all the 

 spheres but occurs later in some cells than in others. 

 Fig. 15 is a section of the embryo in which the inner ends 

 of the cells are separating or have just separated from 

 the outer portion. The cavities seen in the figure are 

 caused by shrinkage. Fig. 16 shows the process of 

 delamination completed. At this time the egg consists 

 of a solid mass of cells in which there is no trace of the 

 segmentation cavity. The cells of the outside and of the 

 central mass grow smaller in size as they increase in 

 number. Fig. 17 gives in outline the shape of a larva 

 twelve hours old. When the embryo is about twenty 

 hours old a change begins to take place, and a few hours 

 later the endoderm appears by a differentiation of those 

 cells of the central mass lying just under the outer layer. 

 This is well seen in figs. 18 and 19. 



Fig. 20 is a twenty-four hours old embryo. It is now 

 covered with cilia which at first do not possess the power 



1 See E. B. Wilson, Phil. Trans., London, 1883, p. 723. 



