OF THE VERTEBRATE OVARY. 559 



the point at which it was left off in that work. From their first 

 formation till the stage spoken of in my monograph as P, 

 their size remains fairly constant. The larger examples have 

 a diameter of about 0*035 mm., and the medium-sized examples 

 of about O'O3 mm. The larger nuclei have a diameter of about 

 O'i6 mm., but their variations in size are considerable. If the 

 above figures be compared with those on page 350 of my 

 monograph on Elasmobranch Fishes, it will be seen that the 

 size of the primitive ova during these stages is not greater than 

 it was at the period of their very first appearance. 



The ova (PL 24, fig. i) are usually aggregated in masses, 

 which might have resulted from division of a single ovum. The 

 outlines of the individual ova are always distinct. Their proto- 

 plasm is clear, and their nuclei, which are somewhat passive 

 towards staining reagents, are granular, with one to three 

 nucleoli. I have noticed, up to stage P, the occasional presence 

 of highly refractive spherules in the protoplasm of the primitive 

 ova already described in my monograph (pp. 353, 354, PI. 12, 

 fig. 15). They seem to occur up to a later period than I at first 

 imagined. Their want of constancy probably indicates that 

 they have no special importance. Professor Semper has de- 

 scribed similar appearances in the male primitive ova of a later 

 period. 



As to the distribution of the primitive ova in the germinal 

 epithelium, Professor Semper's statement that the larger primi- 

 tive ova are found in masses in the centre, and that the smaller 

 ova are more peripherally situated is on the whole true, though 

 I do not find this distribution sufficiently constant to lay so 

 much stress on it as he does. 



The passive condition of the primitive ova becomes suddenly 

 broken during stage O, and is succeeded by a period of remark- 

 able changes. It has only been by the expenditure of much 

 care and trouble that I have been able to elucidate to my own 

 satisfaction what takes place, and there are still points which I 

 do not understand. 



Very shortly after stage O, in addition to primitive ova with 

 a perfectly normal nucleus, others may be seen in which the 

 nucleus is apparently replaced by a deeply stained irregular 

 body, smaller than the ordinary nuclei (PI. 24, fig. i i, d. //.). 



