iiS 



CENTROLECITHAL SEGMENTATION. 



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 proto- 

 plasmic 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 segmentation 

 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 next becomes separated off as a nucleated layer 



FIG. 54. SURFACE VIEW AND OPTICAL SECTION OF A LATE STAGE IN THE 



SEGMENTATION OF PHILODROMUS LIMBATUS (Koch). (After Hub. Ludwig.) 



bl. blastoderm ; yk. yolk spheres. 



(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. 



