302 bulletin: museum of comparative zoology. 



which arise from the chromosomes in the telophase of second matura- 

 tion, prepare to fuse together to form the female pronucleus. When 

 these vesicles begin to unite, linin threads appear, and small granules of 

 chromatin are found on them (Plate 12, Fig. 85). A distinct increase 

 in the volume of chromatin follows, and several plasmosomes appear ; 

 all of the chromatin, as distinguished from the plasmosomes, gathers on 

 the linin threads, and the result is the deeply staining, typical sponge- 

 work characteristic of the quiescent nucleus (Plate 8, Fig. 51). 



The next step in the metamorphosis of the chromatin is the breaking 

 up of the extensive spongework into isolated masses, which in turn are 

 resolved into spherical bodies, either grouped or single (Plate 12, Fig. 88). 

 Many of the spheres of chromatin are further changed into ring-shaped 

 bodies, which are connected with one another by linin threads (Plate 12, 

 Fig. 87; Plate 13, Fig. 89) ; the chromatin bodies which are not thus 

 metamorphosed become hollow spheres (Plate 12, Fig. 86 ; Plate 13, 

 Figs. 90, 93). The solid and the vacuolated chromatin bodies form the 

 chromosomes of the first cleavage spindle, while the remainder of the 

 chromatin (including the plasmosomes) disappears, either immediately 

 before or after the nuclear wall breaks down (Plate 12, Figs. 81, 8G, 87 ; 

 Plate 13, Figs. 89, 90, 93). In one nucleus (Fig. 89) there are two large 

 masses of chromatin which have not yet broken up into spheres. 



(b) Origin and Fate of the Cleavage Astets. 



The centrosome at the deep pole of the second maturation spindle 

 disappears when the chromosomes fuse to form the female pronucleus. 

 From this time until the chromatin has broken up into spherical bodies 

 (Plate 12, Fig. 88) there are no distinguishable centrosomes in the egg. 

 The earliest observed condition of the first cleavage centrosome (Fig. 88) 

 shows two distinct and widely separated asters, unconnected with each 

 other; each is associated with one of the two pronuclei. In this case a 

 large portion of the upper nucleus was removed with the section preced- 

 ing the one figured. The aster nearer the surface of the egg is the 

 deeper in the section and has already begun to push in the wall of the 

 larger (deeper in the section) pronucleus. The other aster lies at a high 

 level in the section and has its rays associated with the small portion of 

 the upper pronucleus that in cutting fell to this section. Another fairly 

 early condition in the formation of the cleavage spindle is shown in 

 Figure 87. The outline of the pronucleus uppermost in the section 

 coincides with that of the lower one, so that it is difficult to distinguish 

 one from the other ; but in the region of the asters, there is evidence 



