1897.] NEW YORK ACADEMY OF SCIENCES. 393 



the direction of the line connecting the two asters, as if pulled 

 out by the fibers attached to the membrane. In other sections 

 it can be seen that there are fibers connecting the two asters, 

 i. e., spindle fibers. In all cases one aster is slightly smaller 

 than the other. 



Near the peripherj^ of the egg are seen in Fig. 8 the last 

 traces of the disintegrating rays, which are so conspicuous in 

 Fig. 7. As the new asters increase in size these ra3'S break 

 down, and the cytoplasm once more arranges itself in the form 

 of ra^^s about the two centrosomes. 



The pronuclei do not fuse, but the membranes disappear, ap- 

 parently going to form at least a portion of the spindle-fibers. 

 The chromosomes arrange themslves about the equator of the 

 now large and distinct spindle, and the first cleavage begins. 

 Dissimilarit}' in size of the two asters are seen throughout the 

 cleavage. 



The phenomena recounted have a direct bearing upon several 

 questions of considerable importance. They show, first, that the 

 centrosome is not permanent, at least not in its ordinary form. 

 It is at present impossible to prove that the centrosome which 

 appears in the " male" aster did not enter with the spermatzotin 

 and exist for a time in some non-tingible form. It is difficult, 

 however, to deny the possibility of its formation, together with 

 tliat of the aster, from the egg-C3'toplasm as the result of some 

 stimulus furnished by the sperm. Again, the centrosomes 

 which result from its division ma}', in spite of their seeming 

 disappearance, still exist in some form and reappear as the cen- 

 trosomes of the cleavage spindle. The impossibility' of dis- 

 tinguishing any centrosome in the cleavage-asters in their earlier 

 stages, and the minuteness of the centrosomes, when the}' do ap- 

 pear, would seem, however, to indicate a new formation. If the 

 cleavage centrosomes are the same as tliose which accompany the 

 sperm-head and then disappear, the difference in size of the two 

 " male" asters observed in some cases may be connected with 

 the difference in size of the cleavage-asters. It seems, at au}^ rate, 

 that the difference in size of the cleavage-asters anticipates and in- 

 dicates the difference in size of the first two cells of cleavage, since 

 the cleavage is unequal. If now the continuity of the centro- 

 somes were proven, and if the difference in size of the "male" 

 asters is of general occurrence, which is by no means certain, 

 then we might, perhaps, be justified in assuming that the unequal 

 first cleavage is determined as unequal when the " male" aster 

 divides. This is a point of great interest and requires further 

 investigation. I am inclined, however, to regard the cleavage- 

 centrosomes as new formations and as not related to the " male" 

 centrosomes. 



