THE SEGMENTATION OF THE OVUM. 99 



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 man- 

 ner mentioned in the previous paragraph, but at the close of the first 

 period of segmentation 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 segmenta- 

 tion, 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 mero- 

 blastic 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 morphological 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. 



(1) 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. 



(2) The distribution of food-yolk in the protoplasm of the 

 ovum exercises an important influence on the segmentation. 



The rapidity with which any part of an ovum segments varies 

 ceteris paribus with the relative amount of protoplasm it contains ; 

 and the size of the segments formed varies inversely to the relative 

 amount of protoplasm. When the proportion of protoplasm in 

 any part of an ovum becomes extremely small, segmentation does 

 not occur in that part. 



Ova with food-yolk may be divided into two great groups accord- 

 ing to the eventual arrangement of the food-yolk in the protoplasm. 

 In one of these, the food-yolk when present is concentrated at 

 the vegetative pole of the ovum. In the other group it is concen- 

 trated at the centre of the ovum. Ova belonging to the former 

 group are known as telolecithal ova, those to the latter as centro- 

 lecithal. 



In each group more than one type may be distinguished. In 

 the first group these types are (1) unequal segmentation, (2) partial 

 segmentation. The features of these three types have been already 

 so fully explained that I need not repeat them here. 



In the second group there are three distinct types, (1) equal 



1 Ed. van Beneden, Bull. Acad. Belgique, Tome xxvni, 1869. 



72 



