In cases like those of Penæus, Eupagurus, etc., the yolk in the interior is at first nearly homogeneous, but at a later period it generally becomes divided up partially or completely into a number of distinct spheres, which may have nuclei and therefore have the value of cells. In many cases nuclei have however not been demonstrated in these yolk spheres, though probably present; yet, till they have been demonstrated, some doubt must remain on the nature of these yolk spheres. It is probable that not all the nuclei which result from the division of the first segmentation nucleus become concerned in the formation of the superficial blastoderm, but that some remain in the interior of the ovum to become the nuclei of the yolk spheres.
Fig. 50. Segmentation and formation of the blastoderm in Chelifer.
(After Metschnikoff.)
In A the ovum is divided into a number of separate segments. In B a number of small cells have appeared (bl) which form a blastoderm enveloping the large yolk spheres. In C the blastoderm has become divided into two layers.
In Myriapods (Chilognatha) a peculiar form of segmentation has been observed by Metschnikoff[59]. The ovum commences by undergoing a perfectly normal, though rather irregular total segmentation. But after the process of division has reached a certain point, scattered masses of very small cells make their appearance on the surface of the large spheres. These small cells have probably arisen in a manner analogous to that which characterizes the formation of the superficial cells of the blastoderm in the types of centrolecithal ova already described. They rapidly increase in number and eventually form a continuous blastoderm; while the original large segments remain in the centre as the yolk mass. In the interesting Arachnid Chelifer segmentation takes place in nearly the same manner as in Myriapods ([fig. 50]).
Fig. 51. Four successive stages in the segmentation of the egg of Tetranychus Telarius. (After Claparède.)
It is clear that it is not possible in centrolecithal ova to have any type of segmentation exactly comparable with that of meroblastic ova. There are however some types which fill the place of the meroblastic ova in the present group, in as much as they are characterised by the presence of a large bulk of food-yolk which either does not segment, or does not do so till a very late stage in the development. The essential character of this type of segmentation consists in the division of the germinal vesicle in the interior, or at the surface of the ovum into two, four, etc. nuclei ([fig. 51]). These nuclei are each of them surrounded by a specially concentrated layer of protoplasm ([fig. 51]) which is continuous with a general protoplasmic reticulum passing through the ovum [not shewn in [fig. 51]]. The yolk is contained in the meshes of this reticulum in the manner already described for other ova.