3o8 



WHEELER. 



[Vol. X. 



may be likened to the more extensive middle-plate in plants. 

 This thickening persists for some time on the inner surface of 

 the second polar body (Fig. 6). 



Both the nucleus of the second polar body and the female 

 pronucleus often consist at first of several distinct vesicles, but 

 these soon fuse to form typical spherical or oval nuclei. While 

 the enlarging female pronucleus is moving back from the apical 

 pole towards the center of the o.^^, a mass of archoplasm con- 

 taining two centrosomes may be distinguished near its lower 

 face (Fig. 5). / have never been able to find any traces of such 



Fig. 5. Fig. 6. 



archoplasm or any centrosome in connection with the male pro- 

 nucleus. 



Before this time both of the pronuclei have reached exactly 

 the same stage of development, and continue to keep accurate 

 pace with each other. At first 12 very minute Diplococcus-like 

 chromosomes may be made out in each pronucleus. Each 

 chromosome increases to several times its original volume 

 during the growth of the pronuclei, and then breaks down into 

 a string of small karyo-microsomes. A few of the large 

 spherical granules, however, remain as nucleoli (Fig. 8). 



In many eggs of this stage I have looked in vain for any 

 traces of archoplasm or centrosome, but whenever these struc- 

 tures could be brought out by means of the iron-alum haema- 

 toxylin, they were always close to the female pronucleus and 



