AMPHIBIA 195 



specialized orders. In the Diptera, for example, there are several 

 species in which young larvae or pupae produce young. The genus 

 Miastor is a classic case. Here the young while still within the mother 

 become sexually mature. 



The prolongation of larval life was noted for the Ammocoetes 

 larva of the lamprey, which requires from three to four years to reach 

 the time of metamorphosis. The larva is much like Amphioxus, and 

 if psedogenesis should occur in this species we would have a case quite 

 parallel, I believe, to that seen in the perennibranchiate urodeles. A 

 permanent Ammoccetes would be classed as an extremely primitive 

 chordate not very distantly related to Amphioxus; in fact, before 

 Ammoccetes was discovered to be the larva of Petromyzon, it was so 

 considered. May it not be possible that Amphioxus itself is a perma- 

 nent larva of some cyclostome more primitive than the myxinoids or 

 the petromyzonts? Possibly it is, but such a view would seem to dis- 

 credit the Amphioxus theory of the ancestry of the vertebrates, a 

 view dear to our hearts and one that we should be reluctant to aban- 

 don. 



The causes of psedogenesis are obscure, but we are at least justified 

 in attributing the foreshortening of development of the soma to cer- 

 tain growth-inhibiting agents or to the absence of certain stimuli to 

 metamorphosis. In general it may be said that low temperatures re- 

 tard development and produce defective organisms; and the perenni- 

 branchiate Amphibia live in water that is low in temperature. Pos- 

 sibly these animals are merely senescent and have lost so much 

 growth momentum that they are unable to push through to com- 

 plete adult differentiation. 



Certain experiments on anuran larvae seem to throw some light 

 on this matter. It has been shown by several writers, in the case of 

 certain species of frog which have a prolonged larval period, that 

 prompt metamorphosis may be induced by thyroid feeding. It is also 

 possible to induce precocious metamorphosis in other species by sim- 

 ilar methods. This suggests that the underlying cause of paedogenesis 

 may have something to do with the failure of the thyroid to function 

 or to a deficiency in its secretion. The recent experiments of B. W. 

 Allen and his pupils are of interest in this connection. It was shown 

 that the early extirpation of the hypophysis in tadpoles prevents the 

 development of the thyroid gland and that the operated individuals 

 remain permanent larvae. 



