Eugene Howard Harper 31 



activity of the sperm nuclei, rather than any mechanical driving of them 

 from the inner region. What chemotactic influences there may be pres- 

 ent we of course have no means of knowing. 



Some Peatuees oe Mitosis in the Pigeon's Egg. 



Centrosomes and asters are structures which are frequently asserted 

 to be absent from eggs heavily laden with yolk. For example, in the 

 maturation stages of some amphibians they are said to be wanting. 

 Speaking of the egg of unio, Lillie says that "rays form more readily 

 in protoplasm free from yolk granules." The bird's egg is certainly 

 as heavily laden with deutoplasmic granules as any, and these granules 

 are of relatively large size. 



This question cannot be considered properly without taking into view 

 more than the earliest phases of development. If we take these into 

 view in connection with the later cleavage, we find that there is a pro- 

 gressive increase in the distinctness of achromatic structures as devel- 

 opment proceeds. A typical mitosis from a rather late cleavage stage is 

 shown in Eig. 39. Here the centrosome is a very large, well-defined 

 granule, and spindle and astral fibers are distinct. 



In inquiring into the reason for the feeble development of astral fibers 

 in the maturation stages of the pigeon's egg, it does not seem that the 

 interference of yolk granules in all cases accounts for the fact. Eor 

 occasionally yolk granules are not especially near to the spindle, and the 

 structures in question are not exceptionally well developed in these cases. 

 In the maturation stages in the pigeon's egg, the spindle is sometimes 

 in an area free from granules, but the achromatic structures are essen- 

 tially similar in all cases. Centrosomes and asters are inconspicuous, 

 but the alveolar structure about the poles of the spindle, when copied 

 with the camera, shows a somewhat regular radiate arrangement. There 

 is no well defined centrosome, more than perhaps a cluster of minute 

 granules difficult to make out. 



Some light is thrown on this matter of asters and centrosomes by 

 mitosis in the sperm nuclei. As has been pointed out, these nuclei when 

 they reach the periphery of the disc in their migrations, come to rest 

 and become surrounded by large areas of cytoplasm, which is identical 

 in appearance with the sphere substance associated with the nuclei 

 earlier. As has been suggested, this cytoplasm is apparently the product 

 of the activity of these nuclei, and is evidently of a highly plastic nature, 

 giving rise in division to very regular mitotic figures, and well defined 

 centrosomes. 



In the later cleavage stages of the blastoderm, the same is true. The 



