CELL-DIVISION AND DEVELOPMENT. 



141 



standing the fact that they are unicellular, differentiation of 

 the protoplasm into myophanes, for example, occurs. How far 

 organic continuity obtains between the spherules in cases of 

 total cleavage is something upon which we have as yet but 

 little information, but in typical cases of centrolecithal cleavage 

 there seems to be little question of its existence. It may be 

 pointed out, however, that in the practically alecithal ovum of 

 Peripatus capensis the syncytial condition exists according to 

 the observations of Adam Sedgwick, 1 and it is interesting to 

 note that the ova of this form show indications of having lost 

 a considerable amount of yolk in correspondence with their 

 intra-uterine method of development, the ancestral species 

 of Peripatus having probably possessed an ovum provided 

 with a considerable amount of yolk and resembling somewhat 

 the ovum of P. Novae-Zealandiae which undergoes a cen- 

 trolecithal segmentation. 



Of course the comparison of the syncytial ovum of Jaera 

 with an Infusorian is not perfectly just, since the Infusorian 

 possesses but a single nucleus, and the differentiation seen in 

 the Jaera ovum might be ascribed to influences exerted by 

 each nucleus on the protoplasm in its immediate vicinity. 

 What the nature of this influence may be, whether or not it 

 even exists, is a question at present without an answer, but a 

 case may be mentioned which seems to me to point very 

 clearly to the probability of a cytoplasmic differentiation 

 occurring independently of any direct nuclear influence. The 

 case I refer to is presented by the developing ova of the 

 terrestrial Isopods, Porcellio and Armadillidium. In the un- 

 segmented stage these ova resemble in all structural peculiari- 

 ties those of Jaera, differing only in their greater size. One 

 finds in them the central nucleus surrounded by a mass of pro- 

 toplasm which is joined by a network with a peripheral proto- 

 plasmic layer, which up to the stage in which four nuclei are 

 formed, is uniformly distributed over the entire surface. Now 

 it must be premised that in the later stages of Jaera there is a 

 concentration of the cells towards one surface of the ovum, 



1 A. Sedgwick: The Development of the Cape Species of Peripatus. Quart, 

 fourn. Micr. Sci., XXVI, 1886. 



