THE SEGMENTATION OF THE OVUM. 



95 



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 proto- 

 plasm 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 roimded or polygonal bodies, in the 

 interior of each of which one of the above nuclei with its protoplasm 

 is present. This process, known as the secondary segmentation of the 

 yolk, is really part of the true segmentation, and the bodies to which 

 it gives rise are true cells. 



Other examples of this type may be cited. In Apliis ' Metschnikoff 

 shewed that the first segmentation nucleus divides into two, each of which 

 takes up a position in the clearer periphei'al protoplasmic layer of the egg 

 (fig. 52, 1 and 2). Following upon further division the nuclei enveloped in 

 a continuous layer of protoplasm arrange themselves in a regular manner, 

 and form a syncytium, which becomes segmented into definite cells 

 (fig. 52, 3 and 4). The existence of a special clear superficial layer of pro- 

 toplasm has been questioned by Brandt. 



\ / 



FIG. 52. SEGMENTATION OF APHIS ROSAE. (Copied from Metschnikoff.) 

 lu all the stages there is seen to be a central yolk mass surrounded by a layer of 



protoplasm. 



In this protoplasm two nuclei have appeared in 1, four nuclei in 2. In 3 the nuclei 



have arranged themselves regularly, and in 4 the protoplasm has become divided into 



a number of columnar cells corresponding to the nuclei. 



iv. pole of the blastoderm which has no share in forming the embryo. 



In Tetranychus telaritis, one of the mites, Claparede found on the surface 

 of the ovum a nucleus surrounded by granular protoplasm (fig. 51) ; whicli 

 is no doubt the first segmentation nucleus. By a series of divisions, all 



1 Metschnikoff, "Embry. Stud. Insecten," Zcit.fiir u-iss. ZooL, Bd. xvi. IRfiO. My 

 own observations on this form accord in the mnin with those of Metschnikoff. 



