3i6 MEAD. [Vol. X. 



The evidence in support of my position may be summarized as 

 follows : 



(i) By the method employed, male centrosom.es, with their 

 powerful radiations, are perfectly distinct at every step in the 

 process, from the time of the expulsion of the first polar body up 

 to at least the four-cell stage. 



(2) The female centrosomes and their rays are also perfectly 

 distinct throughout the maturation stages, are always found 

 with the female pronucleus at first, and thenceforth gradually 

 become fainter, until, before the pronuclei have met, not a trace 

 of them can be seen ; though the male centrosomes and rays, 

 meanwhile, are brought out with ever-increasing clearness. 



(3) The female centers of radiation, so long as they can be 

 seen, never take a position in advance of the female pronucleus. 



(4) If the female centers should Join those of the male they 

 would be obliged to make their way through the closely packed 



fibers of the latter ; and though the female centers themselves 

 might not be brought out by our method, they would necessarily 

 displace the fibers of the male centers. No such disturbance in 

 the fibers can be seen. 



In a stage like that represented in Fig. 8, just before the for- 

 mation of the cell-membrane between the two blastomeres, a 

 fusiform bundle of connecting fibers ( Verbindungsfasern) may 

 be seen running from one nucleus to the other. As the mem- 

 brane forms there is developed in it a well-defined middle 

 plate (Zivischenkorper). With the appearance of the latter the 

 bundle of fibers becomes constricted in the middle (Fig. 10). 

 Similar phenomena take place during the division of the two 

 blastomeres. 



In the early stages of the middle plate its component cor- 

 puscles can be distinguished (Fig. 10). In its later stages it 

 becomes more compact and homogeneous: meanwhile the con- 

 necting fibers appear to lose their connection with the nuclei 

 (Fig. II). 



The two centrosomes at each end of the first cleavage 

 spindle, whose origin has been mentioned above (compare 

 Fig. 6), continue to move apart during the reconstitution of 

 the nuclei. Ultimately, each nucleus becomes spindle-shaped. 



