HORMONES 69 



(1936), on the other hand, obtained very different results with 

 silkworms (Bo77ibyx inori). The corpora allata were removed (by 

 a surgical method to be described by him in a later paper) from 

 larvae in their last instar at times varying from thirty-six hours up 

 to sixteen days after the last moult. As the result of these 

 experiments he concludes that the corpora allata do not influence 

 pupation. Out of forty-two larvae operated upon, twenty-four 

 produced pupae : of the latter, thirteen gave rise to imagines. 

 The time of onset of the " critical period," however, is not stated. 

 The author states that considerable mortality ensued owing to 

 unfavourable weather prevailing at the time ; given better 

 circumstances still more convincing results, he believes, could be 

 obtained. These results appear to be in accordance with experi- 

 ments by Kiihn (1936), who used larvae of Ephestia. By grafting 

 brains of larvae into individuals from which the brains had already 

 been excised, a very high death-rate supervened. Only eight 

 examples out of 150 pupated, and from among these individuals 

 five of them had no corpora allata in the graft. Kiihn, by a 

 series of ligaturing experiments, had previously determined the 

 incidence of a " critical period," but his experiments obviously 

 need repeating owing to the high mortality which prevailed. 



Fraenkel (1935) has brought convincing evidence that it is the 

 action of a hormone which brings about pupation in the fly 

 Calliphom. It is secreted about sixteen hours before pupation at 

 a temperature of 20° C. Fraenkel was unable to locate the actual 

 hormone-producing organ, but showed that it was either the main 

 ganglionic centre or in its immediate neighbourhood. After the 

 hormone is discharged into the blood he found that pupation takes 

 place without the co-operation of the central ganglia. 



In the present state of knowledge, of a very recent line of 

 inquiry, it wo«ld be obviously premature to draw hard and fast 

 conclusions from rather conflicting results. One thing, however, 

 emerges, and that is the existence of hormones which control 

 both ecdysis and metamorphosis. While there is strong evidence 

 in Rhodnius that the corpora allata are the organs secreting the 

 hormones concerned, proof that these bodies perform a similar 

 function in the Holometabola is less convincing. It needs also to 



