3IO 



WHEELER. 



[Vol. X. 



now in the form of distinct loops made up of microsomes, is ag- 

 gregated in two discrete masses in the equatorial plate (Fig. lo). 

 The nucleoli which have appeared and developed during the 

 growth of the pronuclei, are cast out of the spindle into the 

 cytoplasm, where they dissolve away ; the chromatin loops split- 

 ting, meanwhile, and migrating towards the poles. I estimate 

 the number of loops, which finally arrive at each pole, at 24. 



During these movements on the part of the first cleavage 

 spindle, the Q.gg, when viewed from the side, has the shape 

 of a trefoil. The first cleavage plane passes obliquely from 

 the indentation in which the polar bodies lie to one side 



Fig. 9. Fig. io. 



of the median lower protuberance, thus cutting off about one- 

 third of the ^%^. The blastomeres become rounded off, and 

 the 2-cell stage may be described as having a large and a small 

 blastomere. Beard's description ^ of this and the other cleav- 

 age stages of Myzostoma is incorrect. 



The above observations on Myzostoma are of a nature to 

 restrict the generalizations which the papers of Fol,^ Guignard,^ 

 and Conklin^ have called forth. In Myzostoma there is every 

 reason to believe that the female pronucleus alone is provided 



1 Mitth. a. d. zool. Staz., Neapel, Bd. V, 1884. 2 loc. dt. 



3 Nouvelles fitudes sur la Fecondation. Ann. des Sciences Nat. Bot., t. XIV, 

 1891. 



* The Fertilization of the Ovum. 

 Holl, 1894. 



Lectures of the Marine Biol. Lab. of Woods 



