586 EEPTILIA. 



through the different orders of Reptiles, the student will not fail to 

 observe many beautiful illustrations of progressive improvement. 



The finny tribes, incapable of social intercourse, were content 

 with the simple extrusion of their eggs into the sea, leaving them 

 to be impregnated by the casual approach of a male of the same 

 species : but even in the Amphibious Reptiles some steps are 

 gained in associating the sexes with each other ; and although the 

 eggs are still impregnated out of the body of the mother, in the 

 Frog this is accomplished in exitu, and not subsequent to their 

 expulsion. 



Frogs, during the breeding season, are found to pair, and the 

 male having selected his mate mounts upon her back, clinging to 

 her with unwearying pertinacity during the whole period of 

 oviposition, and vivifying her eggs by the aspersion of the seminal 

 secretion as they are successively expelled in long gelatinous chains. 

 During this protracted embrace the male Frog is assisted in 

 retaining his hold by the developement of a peculiar papillose 

 structure upon the first toes of the fore-feet, which disappears at 

 the end of the time appropriated to reproduction. Of course no 

 intromittent apparatus is as yet required, and we may naturally 

 expect to find the male organs still exhibiting great simplicity of 

 construction. 



(649.) The testes and their excretory ducts are, in fact, the only 

 parts as yet met with ; but the anatomy of these parts, although most 

 accurately investigated by Swammerdam upwards of a century ago, 

 is still very generally misunderstood. The testicles are situated 

 in the loins, surrounded by several tongue-like masses of fat, pre- 

 senting a peculiar granulated appearance. Each testis is invested 

 by a delicate capsule, and, on removing this very carefully, the 

 entire viscus is seen to be made up of short caeca ; the blind 

 extremities of which alone appearing at the periphery of the 

 organ caused Cuvier to describe it as being " an agglomeration of 

 little whitish grains interwoven with blood-vessels." The semen 

 elaborated by these caeca is taken up by several small excretory 

 ducts that pierce the kidney, in the immediate vicinity of which 

 the testis lies, and open into the ureter, that here forms the com- 

 mon excretory duct, whereby the urine as well as the seminal fluid 

 is discharged, both escaping into the cloaca at a little distance from 

 the orifice of the allantoid bladder, to be ultimately ejected through 

 the vent. 



(650.) Neither is the generative system of the female Frog less 



