20 THE PHYSIOLOGY OF REPRODUCTION 



country, breeds only in August, i.e. in the South African spring. 

 Bles, however, is disposed to think that Xenopus, like Discoglossus 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 1 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. 



Annaindale^ states that in the Malay Peninsula Bhacophorus 

 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.^ 



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

 Vertebrates there is a periodicity in the occurrence of their reflex 

 responses.* It has been shown that if the region of the shoulder- 

 girdle bearing the four limbs, together with the connected skin and 

 muscles, and the three upper segments of the spinal cord, are cut out 

 from the male -frog during the breeding season (but not at other 

 times), the irritation of the skin will cause a reflex, clasping 

 movement, similar to that characteristic of the normal male at 

 this period.. In spring and early summer, after reviving from their 



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

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



2 Annandale, FasciauM malayenses, Zool., Part I., 1904. 



^ See p. 5, Chapter I. 



* 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. ait. ; Goltz, Zeit. f. deutsch. Tried. Wiss., 

 1865-66 ; Tarchanoff, Pfliiger's Arch., vol. xl., 1887 ; Albertoni, Arch. Ital. de 

 Biol, vol. ix., 1887.) 



