120 SUMMARY. 



species of Chondracanthus, a genus of parasitic Crustaceans. The ovum 

 divides in the usual way but somewhat irregularly into 2, 4, 8 segments 

 which meet in a central yolk mass ; but after the third division instead of 

 each segment dividing into two equal parts it divides at once into four, and 

 the division into four having started, reappears at every successive division. 

 Thus the number of the segments at successive periods is 2, 4, 8, 32, 128, etc. 

 In another peculiar case, an instance of which 1 is afforded by Asellus aqua- 

 ticus, after each of the earlier segmentations all the segments fuse and 

 become indistinguishable, but at the succeeding segmentation double the 

 number of segments appears. 



Although, as has been already stated, it does not seem possible to have a 

 true meroblastic segmentation in centrolecithal ova, it does nevertheless 

 appear probable that the apparent cases of a meroblastic segmentation in 

 the Arthropoda are derivatives of this type of segmentation. The manner 

 in which the one type might pass into the other may perhaps be explained 

 by the segmentation in Asellus aquaticus^. In this ovum large segments 

 are at first formed around a central yolk mass, in the peculiar manner men- 

 tioned in the previous paragraph, but at the close of the first period of seg- 

 mentation minute cells, which eventually form a superficial blastoderm, are 

 produced from the yolk cells. They do not however appear at once round 

 the whole periphery of the egg, but at first only on the ventral surface and 

 later on the dorsal surface. If the amount of food-yolk in the egg were 

 to increase so as to render the formation of the yolk cells impossible, and at 

 the same time the formation of the blastodermic cells were to take place at 

 the commencement, instead of towards the close of the segmentation, a mass 

 of protoplasm with a nucleus might first appear at the surface on the future 

 ventral side of the egg, then divide in the usual way for meroblastic ova, and 

 give rise to a layer of cells gradually extending round to the dorsal surface. 

 A meroblastic segmentation might perhaps be even more easily derived from 

 the type found in Insects. It is probable that the cases of Scorpio, Mysis, 

 Oniscus, the parasitic Isopoda, and some parasitic Copepoda belong to this 

 category ; and it may be noticed that in these cases the blastopore would be 

 situated on the dorsal and not on the ventral side of the ovum. The mor- 

 phological importance of this latter fact will appear in the sequel. 



The results arrived at in the present section may be shortly 

 restated in the following way. 



(i) A comparatively small number of ova contain very 

 little or no food-yolk embedded in their protoplasm; and have 

 what food-yolk may be present distributed uniformly. In such 

 ova the segmentation is regular. They may be described as 

 alecithal ova. 



1 Ed. van Beneden, Bull. Acad. Belgique, Vol. xxvm. 1869. 



