STRUCTUllE AND DEVELOPMENT OF VERTEBRATE OVARY. 401 



the act of being formed, and many of tliese ])resent had reached 

 a considerable size, measuring as much as 0"07 mm. Nests of 

 the typical forms were present as before, but the neuclei in them 

 were more granular than at the earlier period, and on the average 

 slightly smaller. A series measured had the following 

 diameters : — 



001 mm. 



0.312 mm. 



0014 mm, 



0-016 mm. 



One of these nests is represented on PI. XVIII, fig 20. 

 Many nests with the outlines of the individual ova distinct were 

 also present. 



On the whole it appeared to me, that the second mode of forma- 

 tion of permanent ova, viz. that in which the nest does not 

 come into the cycle of development, preponderated to a greater 

 extent than in the earlier embryonic period. 



PosT-EMBEYONic DEVELOPMENT OF THE OVA. — My investiga- 

 tions upon the post embryonic growth and development of the ova, 

 have for the most part been conducted upon preserved ova, and it 

 has been impossible for me, on this account, to work out, as com- 

 pletely as I should have wished, certain points, more especially 

 those connected with the development of the yolk. 



Although my ovaries have been carefully preserved in a large 

 number of reagents, including osmic acid, picric acid, chromic 

 acid, spirit, bichromate of potash, and Miiller's Euid, none of 

 these have proved universally successful, and bichromate of potash 

 and Miiller's fluid are useless. Great difficulties have been expe- 

 rienced in distinguishing the artificial products of these reagents. 

 My investigations have led me to the result, that in the gradual 

 growth of the ova with the age of the individual the changes are 

 not quite identical with those during the rapid growth which 

 takes place at periods of sexual activity, after the adult condition 

 has been reached — a result to which His has also arrived, with 

 reference to the ova of Osseous Fish. I propose dealing sepa- 

 rately with the several constituents of the egg-follicle. 



Egg membranes. — A vitelline membrane has been described 

 by Leydig^ in Raja, and an albuminous layer of the nature 

 of a chorion^ by Gegenbaur^ in Acanthias — the membranes 

 described in these two ways being no doubt equivalent. 



^ ' Rochen u. Haie.' 



^ By chorion I mean following E. van Beneden's nomenclature, a mem- 

 brane formed by the follicular epithelium, and, by vitelline membrane, one 

 formed by tlie vitellus or body of the ovum. 



3 " Bau and Eutwicklung d. Wirbeltbiereier," &c., ' Mull. Archiv.,' 1S61. 



