94 GAKDIAEJi. [Vol. XV. 



with it, and a more or less broken spireme is formed. The 

 origin of the linin is difficult to account for, unless it is formed 

 from the substance which has been drawn from the cyto- 

 plasm and which surrounds the sperm head (Figs. 23-25). 

 At about this stage, at a point between the two pronuclei, but 

 much nearer to the male than to the female, a small but 

 distinct aster appears in the cytoplasm. The rays are very 

 straight and clear, are not affected by the stain, and the central 

 point shows no trace of a vesicle or centrosome. It can be 

 described simply as the central starting point of these rays, 

 and shows no structure which in any way differs from these 

 rays. Subsequent stages show conclusively that this aster 

 gives rise to the centrospheres of the cleavage spindle in 

 which the large black-staining centrosomes lie. 



It is, therefore, evident that here, as in the beginning of the 

 first maturation spindle, the substance forming the centre- 

 sphere does not preexist as such (unless too small to be seen), 

 or that its chemical structure is so different that it will not 

 react under the same stains as later. In regard to the origin 

 of this aster it appears to be purely cytoplasmic. There is 

 no evidence that it is in any way connected with the sperm 

 nucleus. With the study of this point in view, hundreds of 

 sections have been most carefully examined, but in no one 

 could any distinct particle resembling a centrosome or vesicle 

 be detected. A cytoplasmic origin of the centrosomes and 

 centrosphere of the male pronucleus and consequently of the 

 first cleavage spindle is too important a variation from the 

 usually ascribed origin of these bodies to rest on anything but 

 the best of evidence, and in this case the evidence is more of 

 a negative than of a' positive nature. It agrees, however, with 

 the origin of the male centrosome as described by Wheeler (7) 

 in Mysostoma and in AllolobopJiora by Miss Foot (12). 



As this aster increases in size the chromatin in both pronuclei 

 increases also, and except for the absence of nucleoli and their 

 smaller size, either might be mistaken for the original germinal 

 vesicle. The nuclear wall is, however, absent or but little 

 developed. Soon the aster centers divide (Fig. 15), the rays 

 from each extending out to the surfaces of the approaching 



