22 THE PHYSIOLOGY OF REPRODUCTION 



spring. Bles, however, is disposed to think that Xenopus, like 

 Discoylossus in the wild state, may breed several times during 

 the spring and summer, since the frogs in confinement in some 

 years spawned three times. 



Semper l has shown that if axolotls are kept crowded together 

 in small aquaria, without plants or seed, individuals which are 

 sexually mature will not deposit ova even though the water be 

 changed and abundant food supplied. But if they be suddenly 

 transferred to aquaria stocked with plants, and with stones and 

 sand on the bottom and running water, they can be induced 

 to spawn within a few days, and may do so as often as three or 

 four times a year. Bles states that he is able to confirm Semper 's 

 observations upon axolotls, and that he obtained similar results 

 by treating individuals of Triton waltlii and of Discoglossus in 

 the same way. 



Annandale 2 states that in the Malay Peninsula Rhacophorus 

 leucomystax and Rana limnocharis appear to breed only after a 

 heavy fall of rain, and he concludes that the stimulus set up 

 by falling water is necessary before the sexual impulse can be 

 induced. 



Thus there appears to be abundant evidence that breeding 

 in mature Amphibians does not occur cyclically merely, but it 

 takes place only in response to certain definite external stimuli. 

 Bles remarks that if this view is correct, and assuming it to 

 apply to other groups besides the Amphibia, it helps to explain 

 why many animals fail to breed in captivity ; and also how it is 

 that others (e.g. insects), in a state of nature, appear in large 

 numbers in one year and are much less numerous in another. 3 



It is interesting to note that among frogs and other cold- 

 blooded Vertebrates there is a periodicity in the occurrence of 

 their reflex responses. 4 It has been shown that if the region 



1 Semper, " Ueber eine Methode Axolotl-Eier jederzeit zu erzeugen," Zool. 

 Am., vol. i., 1878. See also Animal Life. 



* Annandale, Fasciculi Malaytntcs, Zool., Part I. 1904. 



* See page 5, Chapter I. 



4 The sexual posture of frogs in the act of copulation is maintained as a 

 spinal reflex. The tortoise is similar. The reflex is inhibited by excitation 

 of the optical lobes. (Spallanzani, loc. cit. ; Goltz, Zcut. f. deutsch. mcd. Wist., 

 1865-66 ; Tarchanoff, Pflugcr's Arch., vol. xl., 1887 ; Albertoni, Arch. Hal. de 

 Bid., vol. ix., 1887). 



