CHAP. XXXVI.] VERTEBRATA. 535 



they may be found in large numbers, and from which they escape by a small 

 opening situated near the anus. The male organs are to the unaided eye so like 

 the female, that it is only in the spawning season that they can be distinguished : 

 the spermatozoa escape into the peritoneal cavity in the same manner as the ova. 

 Secondly, in the Osseous Fishes the ovary, or roe, consists of a large membranous 

 sac, enclosing the ovigerous folds, between the layers of which the ova are 

 developed just as in those we have described, so that when the ova escape 

 they are discharged, not into the general peritoneal cavity, but into this 

 ovarian sac, and thence find their way out by a tubular prolongation of it, or 

 excretory duct, which opens just behind the anus. The testicle is strictly 

 analogous. In neither of these classes does copulation take place ; the spawn 

 is cast abroad into the water, and left to be fecundated by the sperm dis- 

 charged over it by the male, to be devoured, or to perish in an unfecun- 

 dated state, as chance may direct. One of the most remarkable points in the 

 history of osseous fishes is their immense fecundity : it is calculated that a 

 Cod discharges nine millions of ova in a single spawning season. The reason of 

 this unparalleled fertility appears to be, that there may be the greater chance 

 of some escaping and surviving the many perils to which they are exposed. 

 Thirdly, in the Cartilaginous Fishes, as the Sharks and Rays, we have a much 

 higher type of the generative function. Copulation takes place ; the male is 

 furnished with an intromittent organ, and with certain accessory parts, called 

 " claspers," for seizing and embracing the female during the act of impregna- 

 tion. In the female, the ovaries are racemose, from the increased size and 

 diminished number of the eggs, which, instead of escaping into the peritoneal 

 cavity, are seized by the patulous orifices of two long oviducts, whereby they 

 are conveyed out of the body, and by which they are furnished with that 

 peculiar horny shell, which serves at once to protect and to attach them to 

 some fixed point. 



Eeptilia. The Amphibia exhibit, in the function of generation, an interest- 

 ing link between the piscine and the true reptilian structure ; there is no intro- 

 mittent organ, and yet copulation takes place, and the ova are fecundated 

 neither after extrusion, as in the osseous fish, nor before extrusion, as in true 

 reptiles, but during the very act of discharge, in exitu. At the commence- 

 ment of the spawning season, a remarkable papillary structure is developed 

 on the thumbs of the male frogs, highly sensitive, and giving rise, when 

 stimulated, to a forcible reflex action, by which the upper extremities are ap- 

 proximated, and tightly embrace anything placed between them (vol. i. p. 335). 

 By means of this, the male frog firmly embraces the female, and continues to 

 do so through the whole time of the expulsion of ova, without any expen- 

 diture of voluntary action, and impregnates the ova as they pass from the 

 female beneath him. The vas deferens passes through the structure of the 

 kidney, and opens at once into the ureter, the two being thence continued as 

 one duct a sort of prolonged genito-urinary cloaca. In the Triton, we go a 

 step further, and find the ureter and vas deferens distinct to their termination, 

 and impregnation taking place internally, although there is no rudiment of a 

 penis. The Ophidians present a further advance, and show the first trace of 

 a penis : it consists of two erectile corpora cavernosa, which, however, are 

 quite separate, and constitute rather an organ of prehension than of intro- 

 mission. In the Saurians and Chelonians another step is gained, and the two 

 corpora cavernosa are united in the middle line ; there is still, however, no 



