yk. central yolk mass.
1 and 2. Surface view and section of the stage with four segments. In 2 it is seen that the furrows visible on the surface do not penetrate to the centre of the ovum.
3 and 4. Surface view and section of ovum near the end of segmentation. The central yolk mass is very clearly seen in 4.
The peculiarity of the centrolecithal ova with regular or unequal segmentation is that (owing to the presence of the yolk in the interior) the furrows which appear on the surface are not continued to the centre of the egg. The spheres which are thus distinct on the surface are really united internally. [Fig. 48], copied from Hæckel, shews this in a diagrammatic way.
Many ova, which in the later stages of segmentation exhibit the characteristics of true centrolecithal ova, in the early stages actually pass through nearly the same phases as holoblastic ova. Thus in Eupagurus prideauxii[57] ([fig. 49]), and probably in the majority of Decapods, the egg is divided successively into two, four and eight distinct segments, and it is not till after the fourth phase of the segmentation that the spheres fuse in the centre of the egg. Such ova belong to a type which is really intermediate between the ordinary type of segmentation and that with a central yolk mass. Eupagurus presents one striking peculiarity, viz. that the nucleus divides into two, four and eight nuclei, each surrounded by a delicate layer of protoplasm prolonged into a reticulum, before the ovum itself commences to become segmented. The ovum before segmentation is therefore in the condition of a syncytium.
Fig. 49. Transverse section through four stages in the segmentation of Eupagurus Prideauxii. (After P. Mayer.)
The segmentation of Asellus aquaticus[58] is very similar to that of Eupagurus, etc. but the ovum at the very first divides into as many segments (viz. eight) as there are nuclei.
In Gammarus locusta the resemblance to ordinary unequal segmentation is very striking, and it is not till a considerable number of segments have been formed that a central yolk mass appears.
In all the above types, as segmentation proceeds, the protoplasm becomes more and more concentrated at the surface, and finally a superficial layer of flat blastoderm cells is completely segmented off from the yolk below ([fig. 49] D).