138 PRINCIPLES OF EMBRYOLOGY 



The two hormones which are most fully authenticated are, firstly, a 

 'growth and moulting hormone', which has the effects suggested by its 

 name, and secondly a juvenile' hormone. The effect of the latter is to 

 prevent the moulting larva from developing into the pupa or adult; meta- 

 morphosis is inhibited until the concentration of juvenile hormone, which 

 falls throughout larval life, has sunk low enough. The growth and moult- 

 ing hormone seems always to be secreted by a gland located in the 

 thoracic region, usually known as the prothoracic gland. The activity of 

 this gland is, however, itself stimulated by an 'activating' substance. Tliis 

 is, in many cases, formed by certain neurosecretory cells in the brain; it is 

 sometimes transmitted along the nerves, and in particular into an annexe 

 of the brain known as the corpus cardiacum, from which it may be re- 

 leased into the haemolymph to reach eventually the prothoracic gland. 

 The juvenile hormone is secreted from another organ, known as the cor- 

 pus allatum; and this again may be activated by influences from the brain, 

 which in this case are probably nervous in nature. 



In the higher Diptera, such as Drosophila, the interactions between these 

 organs are made more confusing by the fact that they all He very close 

 together. The main hormones are produced in an organ known as Weiss- 

 man's ring, or the ring gland. This is closely attached to the upper part 

 of the brain, that is to the region of the neurosecretory cells. The ring 

 gland itself is complex; the part nearest the brain corresponds to the corpus 

 cardiacum, that furthest away to the corpus allatum, while the lateral 

 parts probably function as the prothoracic gland (Fig. 8.11). 



The growth and moulting hormone is produced periodically towards 

 the end of each instar throughout the whole of larval life. If the source of 

 the juvenile hormone is removed from a young larva (e.g. by extirpating 

 the corpus allatum) a premature metamorphosis occurs, giving rise to a 

 dwarf pupa or adult (Fig. 8.12). On the other hand, if corpora allata from 

 young larvae are transplanted into a larva ready to metamorphose, it is 

 caused to undergo an extra larval moult instead and only finally meta- 

 morphoses a stage later, forming a giant. Moreover, by removing the 

 source of metamorphosis hormone when it has begun but not completed 

 its secretion, or by implanting corpora allata from earlier stages, it has been 

 possible to obtain abnormal balances between the two hormones and thus 

 to provoke partial metamorphoses, which produce hemipteran individuals 

 intermediate between nymph and adult, and lepidopterans intermediate 

 between larva and pupa. 



In some insects, the life-cycle includes not only moulting and meta- 

 morphosis, but also a period of complete standstill, a so-called diapause. 

 It is often in the form of such a resting stage that the animal passes the 



