Eugene Howard Harper 25 



however, as they reach the marginal yolk they become surrounded by 

 wide areas of protoplasm free from granules of yolk, owing, according 

 to the assumption, to the cessation of their rapid movements and their 

 delimitation to fized areas, which results in the accumulation about them 

 of the products of their metabolism, instead of its diffusion into the 

 surrounding protoplasm. 



Mitosis in the Sperm Nuclei. — The mitosis of the accessory nuclei 

 appears to be normal in the early divisions, at least in the sense that it 

 results in an equal division of the chromosomes. The details of mitosis 

 have not been compared with that of the cleavage nuclei, although such 

 a study might indeed be valuable. The determination of the number of 

 chromosomes in the spindle has a bearing upon the origin of the nuclei, 

 of course. It is not asserted, since it has not been definitely proved, that 

 mitosis is always perfectly normal, even at this stage, since abnormalities 

 do appear later which lead eventually to amitosis. N"o pluripolar 

 spindles have, however, as yet been observed. Regular equatorial plate 

 stages are found. The reduced number of chromosomes is present, which 

 is eight. In the metabolic nuclei the chromatin network is somewhat finer 

 than that of the cleavage nuclei. This difference extends to the fully 

 formed chromosomes, which are narrower and somewhat more elongated 

 than those of the cleavage nuclei. In the prophases very long, slender 

 chromosomes are formed which become shorter and thicker as they 

 approach the equatorial plate stage. A typical longitudinal splitting of 

 V-shaped chromosomes takes place (Fig. 33), and as the daughter 

 chromosomes pass to the poles, one end of each becomes thicker. Gradu- 

 ally the chromatin accumulates at this end (Pig. 34) until in the late 

 anaphase the chromosomes appear as short oval bodies approaching a 

 spherical shape (Fig. 35). The achromatic structures are well defined. 

 The centrosome is a sharply defined, deeply staining spherical granule, 

 not so large as in the late cleavage nuclei, but decidedly more conspicuous 

 than the centrosome of the maturation and early cleavage stages. The 

 spindle fibers are distinct and the spindles are very regular in form. 

 These characteristics of mitosis and their similarity to that found in 

 later cleavage stages and dissimilarity with that found in the early 

 cleavage seems clearly correlated with the nature of the substance 

 by which ' the nuclei are surrounded, which is in the one case a 

 highly plastic cytoplasm, the immediate product of the nuclear activity, 

 and in the other is the largely unmodified egg cytoplasm, which from 

 its coarse alveolar structure reacts differently to the mitotic forces and 

 gives less evidence of their operation by a change in form than does 

 the more plastic medium. 



