92 CENTROLEC1THAL SEGMENTATION. 



segmentation that the spheres fuse in the centre of the egg. 

 Such ova belong to a type which is really intermediate between the 



B 



D 



FIG. 49. TRANSVEESE SECTION THROUGH FOUR STAGES IN THE SEGMENTATION OF 

 EUPAGURUS PRIDEAUXII. (After P. Mayer.) 



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 segmen- 

 tation is therefore in the condition of a syncytium. 



The segmentation of Asellus aquaticus 1 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 segmen- 

 tation 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 fiat blastoderm cells is completely segmented 

 off from the yolk below (fig. 49 D). 



In cases like those of Peneus, 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 ; 



1 Ed. van Benedeu, Bull. d. VAcad. roy. Belgique, 2 me serie, Tom. xxvin. No. 7, 1860, 

 p. 54. 



