1892.] of the oosperm nucleus in certain ova. 13 



sections through a large field of ova with perfect confidence that 

 most of the stages will be represented. 



Taking advantage of a large supply of very well preserved 

 Distichoporas that was placed in my hands by Professor Haddon, 

 I have, during the last two years, prepared a very considerable 

 number of series of sections which enable me to review the con- 

 clusions I came to in my paper on Allopora. 



The ovum of Distichopora like that of Allopora is pro- 

 vided with a large amount of yolk and lies in a cup-shaped 

 trophodisc. In young immature ova, ova that is to say, that can 

 be seen to be immature by the fact that they do not contain 

 their full complement of yolk spheres, the germinal vesicle is 

 spherical, provided with a well-marked limiting membrane, a 

 large germinal spot and a fine network of fibrils with thickened 

 nodes. 



In some ova with a full complement of yolk spheres, the 

 germinal vesicle is irregular in shape and the membrana limitans 

 broken in places, so that the intra- nuclear protoplasm is perfectly 

 continuous with the extra-nuclear protoplasm. 



These irregular amoeboid germinal vesicles are I believe 

 caught in the act of travelling from the centre of the ovum to 

 the periphery. 



The stage which is most frequently found however is one in 

 which the germinal vesicle lies close to or actually on the peri- 

 phery of the ovum, and many interesting varieties in the form of 

 its outline may be observed. 



In the first place, a form in which the inner half is hemi- 

 spherical in shape with the membrana limitans well defined, while 

 the outer half is irregular and the membrana limitans not well- 

 defined, is very common. 



Secondly, there is a very common form in which the membrana 

 of the inner half has disappeared while that of the outer half is 

 indistinct, and the germinal spot absent. 



Lastly, numerous forms that may be considered to be inter- 

 mediate between these two types. 



In all of these forms well-stained specimens show a considerable 

 number of very minute irregular chromosomes lying in the central 

 parts of the vesicle. 



It seems probable that the discharge of the polar bodies and 

 fertilisation take place while the germinal vesicle is at or near the 

 periphery of the ovum, but I feel that it is impossible to assert 

 with any degree of assurance that any of the structures that I 

 have seen and believed to be polar bodies and male pronuclei are 

 really what I have believed them to be. 



I have been unable to find in any of my preparations of these 

 and other stages any trace of regular karyokinetic figures or 



