314 CELLS, TISSUES, AND ORGANISMS 



and sexual maturity. Consequently the metamorphosis of the marine 

 invertebrates follows closely on the heels of embryonic development. 



It is only among the higher insects and amphibia that one rou- 

 tinely encounters larval stages of prolonged duration. Therefore the 

 provisions for metamorphosis, akin as they are to the developmental 

 mechanisms of the embryo, must be held latent long into the post- 

 embryonic period and then reactivated. 



It is easy to see that metamorphosis, whether early or late, chal- 

 lenges our understanding of developmental physiology. Especially in 

 the more advanced forms of insect metamorphosis, larva and adult 

 seem to have little in common— except for their genes. 



My present purpose is to emphasize that the metamorphosis of 

 the animal as a whole is the over-all expression of a mosaic of meta- 

 morphoses at the level of the individual cells. Moreover, what we see 

 at the cellular level is a coordinated and controlled interplay between 

 nuclear information and cytoplasmic acts. 



The molting and metamorphosis of insect integument 



The skin of insects proves to be an extremely favorable object for 

 examining these relationships. In all insects it consists of a single layer 

 of living epidermis sandwiched between an overlying cuticle and an 

 underlying basement membrane. The cuticle is synthesized and se- 

 creted by the epidermal cell. Each cell contributes a Httle zone of 

 overlying cuticle. And since the cuticle shows a distinctive structure at 

 each stage in metamorphosis, the cells signal their state of difiFerentia- 

 tion in terms of the cuticle they secrete. 



The secretion of cuticle, as well as virtually all other aspects of 

 insect development, is promoted by a certain hormone which circulates 

 in the blood. This growth hormone is the product of an endocrine 

 organ in the anterior end of the insect— the so-called "prothoracic 

 glands." The hormone itself has been isolated and crystallized by Bu- 

 tenandt and Karlson (1954). Its empirical formula is C18H30O4. The 

 hormone is termed "ecdyson," because one of its early and conspicuous 

 eflForts is to induce a molting, or ecdysis, of the cuticle. 



When acted upon by ecdyson, the epidermal cells throughout the 

 body synchronously detach themselves from the old cuticle and pro- 

 ceed to secrete a new one. In the case of a larval molt, the new cuticle 

 shows only minor differences from the one that is shed. Therefore, in 

 terms of the activity of the epidermal cells, it seems necessary to con- 

 clude that a larval molt in response to ecdyson corresponds to a re- 

 newed bovit of activity on the part of synthetic mechanisms already 

 present in the cytoplasm of the larval cells. And it also seems clear 



