112 



CENTROLECITHAL SEGMENTATION. 



Thus in Eupagurns prideauxii^ (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 



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

 EUPAGURUS PRIDEAUXII. (After P. Mayer.) 



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 seg- 

 mented. The ovum before segmentation is therefore in the 

 condition of a syncytium. 



The segmentation of Asellus aquaticus 2 is very similar to that of Eupagu- 

 rus, 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. 



1 Mayer, Jenaische Zeitschrift, Vol. XI. 



2 Ed. van Beneden, Bull. d. FAcad. roy. Belgique, 

 1869, p. 54. 



seiie, Tom. xxvm. No. 7, 



