THE SEGMENTATION OF THE OVUM. 1 15 



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. 



The ovum, like that of Eupagurus before segmentation, is 

 now a syncytium. Eventually the nuclei, having increased by 

 division and become very numerous, travel, unless previously 

 situated there, to the surface of the ovum. They then either 

 simultaneously or in succession become, together with protoplasm 

 around them, segmented off from the yolk, and give rise to a 

 peripheral blastoderm enclosing a central yolk mass. In the 

 latter however many of the nuclei usually remain, and it also 

 very often undergoes a secondary segmentation into a number 

 of yolk spheres. 



The eggs of Insects afford numerous examples of this mode 

 of segmentation, of which the egg of Porthesia 1 may be taken as 

 type. After impregnation it consists of a central mass of yolk 

 which passes without a sharp line of demarcation into a peripheral 

 layer of more transparent (protoplasmic) material. In the 

 earliest stage observed by Bobretzky there were two bodies in 

 the interior of the egg, each consisting of a nucleus enclosed in a 

 thin protoplasmic layer with stellate prolongations. This stage 

 corresponds with the division into two, but though the nucleus 

 divides, the preponderating amount of yolk prevents the egg 

 from segmenting at the same time. By a continuous division 

 of the nuclei there becomes scattered through the interior of the 

 ovum a series of bodies, each formed of nucleus and a thin layer 

 of protoplasm with reticulate processes. After a certain stage 

 some of these bodies pass to the surface, simultaneously (in 

 Porthesia) or in some cases successively. At the surface the 

 protoplasm round each nucleus contracts itself into a rounded 

 cell body, distinctly cut off from the adjacent yolk. 



The cells so formed give rise to a superficial blastoderm of a 

 single layer of cells. Many of the nucleated bodies remain in 

 the yolk, and after a certain time, which varies in different forms, 

 the yolk becomes segmented up into a number of rounded or 

 polygonal bodies, in the interior of each of which one of the 



1 Bobretzky, Zeit.f. wiss. Zoo!., Bel. xxxi. 1878. 



82 



