392 F. M. BALFOUK. 



thne more or less dcfiuite layers, in which the fibres run at riyht 

 angles. It forins a detitiite ovarian tunic. The pseuciu- 

 epithelium is much more distinct, and the tails of its cells, so 

 conspicuous in previous stages, can no longer be made out. 



Formation of ike permanent ova and the follicular epithelium. — 

 In my monograph on the development of Elasmobranch Fishes 

 an account was given of the earliest stages in the develop- 

 ment of the primitive ova, and I now take up their 

 development from 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 0035 mm., and the 

 medium-sized examples of about O'O-i mm. The larger nuclei 

 iiave a diameter of about 0"10 mm., but their variations in size 

 are considerable. If the above figures be compared with those 

 on page 131 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 (PI. XVII, fig. 1) are usually aggregated in masses, 

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

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

 ])lasin is clear,and their nuclei, whichare somewhat passive towards 

 t^taining 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 (p. 135, 136, PI. XI, 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 import- 

 ance. Professor Semper has described similar appearances in the 

 male primitive ova of a later period. 



As to the distribution of the primitive ova in the germinal 

 epithelium, Professor temper's statement that the larger primitive 

 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 sulUcienlly constant to lay so much 

 stress on it as he does. 



The passive condition of the primitive ova becomes suddenly 

 broken during stage Q, and is succeeded by a period of re- 

 markable changes. It has oidy 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 

 1 do not understand. 



Very shortly after stage q, in addition to primitive ova with a 

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



