OVULATION IN CARTILAGINOUS FISHES. 597 



defined periphery of the yolk-mass is in close contact with the 

 layer of nucleated cells lining the ovisac, ib. s : and at a later 

 stage of developement, when the pigment-cells have been applied 

 to the surface of the germinal portion of the yolk, a distinct 

 external membrane is present. In the month of February the 

 maculae of the germ-cell multiply and form aggregates, with 

 envelopes, representing cells with a granular nucleus, which 

 nucleus finally disappears, and the so-rendered clear cells escape 

 by solution of the coat of the germ-cell into the surrounding 

 yolk-substance, this rupture or disappearance of the germ-cell 

 preceding, as in some Osseous Fishes, the reception of the matter 

 of the spermatozoon. The ripe ovarian ovum of the Frog (Rana 

 temporarid) is from T ^ to T 1 ^ inch diameter. The outer membrane 

 (ectosac) of the ovum, after quitting the ovarium, is surrounded 

 and defended, before exclusion, by the gelatinous secretion of the 

 oviduct : it does not show the structure of that of the Fish. If 

 a 'micropyle' exist prior to impregnation, as Prevost and Dumas 

 record, 1 it has escaped the express research of some later observers. 2 

 The full-sized ovum of the oviduct retains its spherical form, fig. 

 420, A. In the Newts the ectosac of the ovum is elliptical, 

 fig. 420, b, and a clear albuminous fluid intervenes between it 

 and the yolk. In Triton cristatus the yolk is bright yellow : in 

 Lissotriton punctatus it is ash-coloured : in the Land-Salamander 

 it is orange. In tailless Batrachia the colour is limited to the 

 surface of the yolk, which is grey beneath : in the Toad the 

 pigment is almost black, in the Frog it is dark brown : in both 

 it covers all the surface, save one small round spot ; in Alytes 

 obstetricans it covers half the yolk : it stains vertically from 1 

 to j^ of the diameter of the yolk : in all it defines the germinal 

 part of the yolk. 



§ 113. Ovulation in Cartilaginous Fishes and Scaled Reptiles. — 

 If the generative organs and products were exclusively to govern 

 the classification of Animals, the Chimreroids and Plasnostomes 

 would be separated from other Anallantoids, and be combined 

 with scaled Reptiles, with which they agree in the type and 

 grade of their generative organs, and in the more important 

 characters of the structure of the ovum. Its essential con- 

 stituent, the germ-cell, has a single nucleus, and it becomes 

 surrounded, while in the circumscribed ovary, with a large mass 

 of vitelline substance, consisting in great part of oily and 

 albuminous matter, inclosed in delicate vesicles. Besides these, 

 there is a small proportion of the yolk, consisting of minute 

 1 In the centre of the brown hemisphere, cccxi. p. 104. 2 ccexm. 



