THE SEGMENTATION OF THE OVUM. 



97 



which finally assume a radiating form diverging from the centre of 

 the egg. In the centre of the radiate figure is a protoplasmic mass, 

 probably containing a nucleus, which sends out protoplasmic filaments 

 through the columns (fig. 53 A). After a certain period of repose 

 the figure becomes divided into two rosette-like masses, which remain 

 united for some time by a protoplasmic thread : this thread is 

 finally ruptured (fig. 53 B). The whole egg does not in this process 

 divide into two segments, but merely the radiate figure, which is 

 enclosed in a finely granular material. The two rosettes next become 

 simultaneously divided, giving rise to four rosettes (fig. 53 C) : and the 

 whole process is repeated with the same rhythm as in a regular seg- 

 mentation till there are formed thirty-two rosettes in all (fig. 54 A). 

 The rosettes by this time have become simple columns, which by mutual 

 pressure arrange themselves radiately around the centre of the egg, 

 which however they do not quite reach. 



When only two rosettes are present the protoplasm with its 

 nucleus occupies a central position in each rosette, but gradually, in 

 the course of the subsequent subdivisions, it travels towards the 

 periphery, and finally occupies, when the stage with thirty-two 

 rosettes is reached, a peripheral position. The peripheral protoplasm 



FIG. 54. 



SDKFACE VIEW AND OPTICAL SECTION OP A LATE STAGE IN THE SEGMENTATION 

 OF PHILODBOMUS LIMBATUS (Koch). (After Hub. Ludwig.) 

 bl blastoderm; yk yolk spheres. 



next becomes separated off as a nucleated layer (fig. 54 B). It forms 

 the proper blastoderm, and in it the nuclei rapidly multiply and finally 

 around each an hexagonal or polygonal area of protoplasm is marked 

 off ; and a blastoderm, formed of a single layer of flattened cells, is thus 

 constituted. The columns within the blastoderm now form (fig. 54 B) 

 more or less distinct masses, which are stated by Ludwig to be with- 

 out protoplasm. 



From observations of my own I am inclined to differ from Liulwig as to 

 the nature of the parts within the blastoderm. My observations have 

 been made on Agelena labyrintliica and commence at the close of the seg- 

 mentation. At this time I find a superficial layer of flattened cells, and 

 within these a number of large polyhedral yolk cells. In. many, and I 

 believe all, of the yolk cells there is a nucleus surrounded by protoplasm. 

 It is generally placed at one side and not in the centre of a yolk cell, and 



B. E. 7 



