KARYOKINESIS. 25 



the egg from the lower side, and during the prophase of the first cleavage the germ 

 nuclei usually, though not invariably, occupy the same relative positions, the egg- 

 nucleus being above and the sperm nucleus below, figs. 42-55. The positions of 

 the spheres relative to the germ nuclei is not perfectly constant, though the sperm 

 sphere usually precedes the sperm nucleus and the egg sphere lies on the central 

 side of the egg nucleus. The spheres remain distinct during the approach of the 

 germ nuclei, one being quite as evident as the other, and neither showing any trace 

 of degeneration. A number of yolk spherules are carried before the sperm into the 

 protoplasmic area surrounding the egg nucleus and sphere, and thus it happens that 

 several yolk spherules are usually found between the two germ nuclei and spheres, 

 _ and more or less isolated from the remainder of the yolk. The germ nuclei first 

 come into contact, as shown in figs. 44 and 45, and afterwards the spheres meet, 

 inclosing still some of the yolk between them ; the spheres then completely fuse, 

 figs. 45, 46, 47, 49, 50, 51, FS. 



Before fusion the spheres consist of masses of faintly staining granules, and a 

 more or less distinct boundary line separates them from the remaining cytoplasm ; 

 from this boundary a few fibres or rows of microsomes radiate. This boundary line 

 is sharper in some cases than in others, but is always faintly marked. Immediately 

 before and after the fusion of the spheres it can be seen that the coarse granules in 

 the spheres are nodal points in a very delicate reticulum, figs. 45-47 and 49-51. 

 As soon as the spheres have fused, their substance surrounds the nuclei and spreads 

 in a faintly staining mass into the cytoplasm above the nuclei and immediately 

 below the polar bodies. A similar area of darkly stained protoplasm has been 

 observed by Coe ('99) in Cerebratulus (see his figs. 23-28), and is said by him to 

 be derived from the germinal vesicle. In Crepidula there can be no doubt that 

 this area is derived from the egg and sperm spheres, though these in turn may be 

 derived from material escaped from the germinal vesicle. All this time very faint 

 radiations proceed from the periphery of the fused spheres, figs. 46, 49, 50. In 

 Arenicola, according to Child ('98), the germ nuclei, when they meet, are sur- 

 rounded by an area of reticular cytoplasm from which radiations run into the sur- 

 rounding substance of the egg. Child regards these radiations as possibly the result 

 of the absorption of liquid by the germ nuclei, while the reticulum, he thinks, may 

 indicate an accumulation of liquid around the nuclei. 



In Crepidula the spheres are present during the period when the germ nuclei 

 are growing most actively ; they lie in close contact with these nuclei and appear to 

 be associated with their rapid growth. I am inclined to regard them as the expres- 

 sion of certain chemical and physical processes, taking place between the nuclei and 

 the cytoplasm, rather than as structures of high morphological significance. 



5. Origin of Cleavage Centrosomes. — In several cases I have observed two 

 large granules among the microsomes at the periphery of the spheres, from which 

 stronger radiations proceed into the cytoplasm, but not into the spheres, figs. 47, 50, 

 51. These granules are but little larger than others in the peripheral layer of the 

 spheres, and the radiations proceeding from them are but a trifle stronger and more 



4 JOURN. A. N. S. PHILA., VOL. XII. 



