No. 4.] 



BULL A SOLITARIA. 



FIG. 6. The female pro- 



inner pole of the second 

 maturation spindle. A 

 few astral rays are pres- 

 ent. Zwischenkorper 

 of second polar body 

 present. 



The structure of the two pronuclei when they have come 

 together (Fig. 6) is the same. The male pronucleus is usu- 

 ally regular in outline and slightly smaller. The irregularities 



in the outline of the female pronucleus often 

 persist until the central corpuscles of the 

 first cleavage appear. The chromatin stains 

 very slightly and is connected by delicate 

 linin threads. The changes through which 

 the chromatin passes before the equatorial 

 plate is formed may be described under 

 three stages. First, the chromatin rapidly 

 nucleus is irregular in increases in staining power, forming a dense 



outline, surrounded by .... 



the cortical zone of the reticulum, often containing stellate masses 

 of solid chromatin. Second, the chromatin 

 begins to assume a definite form. The most 

 conspicuous shape is the stage where the 

 masses of chromatin have begun to break up 

 into rings but are still attached to one another. The chromo- 



O 



somes have not yet become hollow. They stain uniformly. 

 Third, the chromosomes have broken apart from each other, 

 and have become hollow, round bodies. At first there is a 

 delicate meshwork connecting them (Fig. 7), but this is soon 

 lost and each pronucleus is filled with ring- 

 shaped chromosomes. While the chromatin 

 is undergoing the last two changes, the 

 central corpuscles (the so-called cleavage 

 centrosomes) of the first cleavage spindle 

 make their first appearance. I have found 

 them in a much earlier stage than the one 



r , . . , -,, FIG. 7. Origin of cleav- 



figured, but in each case there was no con- age centrosomes . One 



nection between them ; but these corpuscles 



with their rays have a definite relation with 



the pronuclei, that is to say, each pronucleus 



has an aster and central corpuscle with 



a faint cortical zone. My observations upon the cleavage 



centrosomes would tend toward the position, first, that they 



arise de novo ; and, second, that one arises in connection with 



each pronucleus. 



in connection with each 

 pronucleus. The cen- 

 tral corpuscles are 

 surrounded by a faint 

 cortical zone. 



