Endocrine Mechanisms 737 



although the animal usually survives for about a year.^'^^ If a pupa is chilled 

 by exposure to a temperature of 3-5° C. for Wz months metamorphosis will 

 then occur in a little over one month after the pupa is restored to the higher 

 temperature. When a chilled pupa is united parabiotically with an unchilled 

 one, both will metamorphose synchronously in about Wi months (Fig. 281, 

 A and B). Implants of brain from a chilled pupa will induce metamorphosis 

 in an unchilled pupa. 



A reconciliation of the observed influences on metamorphosis of the pro- 

 thoracic gland, on the one hand, and of the brain, on the other, has recently 

 been effected. ^'^^' ^^^ In Platysaniia cecropia the active fraction from the 

 brain arises in the inner mass of each cerebral lobe of the brain, within 

 which are found two groups of neurosecretory cells, a median and a lateral 

 group. The median group of cells corresponds with the pars intercerebralis 

 cells of the hemipterans.^^^ To induce metamorphosis of a brainless pupa a 

 portion of a brain containing both groups of neurosecretory cells must be 

 implanted, suggesting that these two groups collaborate in the production 

 of the material involved. Killed brain, or brain extract, has thus far proved 

 ineff^ective as a substitute for living brain tissue in inducing metamorphosis. 



If one transects pupae of Platysamia just anterior to the sixth abdominal 

 segment and then seals a plastex cover slip over the cut end of each por- 

 tion one obtains preparations which are viable for eight months or more 

 and development can be readily observed in them. Chilled anterior por- 

 tions, or brainless anterior portions which have received an implant of a 

 chilled brain, undergo normal metamorphosis. The posterior portion will 

 not undergo metamorphosis even when a number of chilled brains are im- 

 planted into a single abdomen. Such abdomens will metamorphose, however, 

 when grafted to metamorphosing anterior portions. An endocrine factor in 

 addition to that from the brain is obviously essential. That the source of the 

 second factor is the prothoracic gland can be shown by the induction of 

 pupation of isolated abdomens by the implantation of both chilled brains and 

 prothoracic glands (Fig. 281, C and D). The latter may come from either 

 chilled or unchilled pupae. Prothoracic glands by themselves are not ade- 

 quate. It would therefore appear that metamorphosis normally depends on 

 the production of at least two factors. The first, produced from the brain, 

 is apparentlv required to acti\'ate the prothoracic glands (Fig. 281, E and F). 



Other experiments^ "^"^ suggest that the prothoracic gland principle influ- 

 ences termination of pupal diapause and consequent metamorphosis through 

 action on the cytochrome svstem of the larval tissues. 



Molting in Crustaceans. The decapod crustaceans in their development 

 typically undergo a number of molts, passing through a series of character- 

 istic larval stages, and after having achieved the body-form of the adult 

 they continue to grow through periodic molting of the exoskeleton. Little 

 or nothing is known regarding the integrating factors concerned in the larval 

 development, although it seems probable that the endocrine factors oper- 

 ating here diff^er little from those known to be operating in latet molting and 

 growth. 



The molting process is a complex one. The molting cycle may be divided 

 into four periods: (1) premolt, a period of active preparation for molt, in- 

 cluding gradual thinning of the cuticle and storage in the gastroliths or 



