20 SEGMENTATION AND FORMATION OP THE LAYEES. 



their development. I regret, however, that the materials for 

 this comparison are to a great extent wanting ; for, although we 

 know more of the development of the West Indian species than 

 of the New Zealand one, thanks to the researches of Kennel, 

 still, as I have pointed out (Chapter I, p. 2), the latter are 

 too incomplete to permit of any profitable comparison. 



After this account of the general structure of the ovum of 

 the Cape species, I will describe the special features of the 

 unsegmeuted uterine ovum at its different stages. 



The unsegmented ova, which I have found, seem to belong 

 to two distinct stages, each of which presents special features. 

 There are (1) the stages before the conjugation of the male and 

 female pronuclei; (2) the stages after that event. 



1. The ova of this stage all belong to P. Balfouri; they 

 are distinguished externally by the small size of the dark area 

 in the living ovum {vide Chapter I, PI. I, fig. 2), and by the 

 apparent absence, in surface views, of the polar bodies. All of 

 them, at least all those of which I succeeded in preparing 

 good sections, presented indications, more or less distinct, 

 of a male pronucleus, and in all polar bodies were being 

 formed. 



The ovum contained an irregular central cavity which, how- 

 ever, was not so well marked as in later ova. The reticulum 

 ■was slightly denser round the nucleus than elsewhere. This 

 slight increase in density is the cause of the small opaque spot 

 in the fresh ovum. The nucleus was placed in the middle of 

 the long axis of the ovum near the surface, and presented a 

 different structure in every ovum of this stage which I examined. 

 In all, except one which I have figured (PI. Ill, fig. 1), it 

 appeared to be undergoing changes in connection with the 

 formation of the two polar bodies. I have four ova of this 

 stage, and they alii presented structures which I take to be the 

 male pronucleus. 



The polar bodies are two in number ; when fully formed they 

 have a diameter of about 'OW mm. Each of them contains 

 a small number of deeply- staining bodies which are placed 

 close together in the centre and represent the nucleus (PI. 



