414 - Multicellular Animals, Especially Man 



indicate that melatonin may also be impor- 

 tant in determining pigmentation processes 

 in mammals, including man. 



For many years it was believed that the 

 thymus gland might be an endocrine organ 

 but recent studies indicate that this is not 

 the case (p. 335). 



Neurosecretion." Nerve cells release hor- 

 monelike compounds at the distal extremities 

 of their axons, and these potent excitatory 

 substances play a key role in the transmission 

 of nerve impulses across the synaptic gaps of 

 the nervous system (p. 453). Acetylcholine, 

 the excitatory agent of the parasympathetic 

 nervous system (p. 465), and noradrenalin, 

 of the sympathetic system (p. 467), are excel- 

 lent examples of neurosecretions. A more de- 

 tailed consideration of these important com- 

 pounds will be undertaken later, in connec- 

 tion with a discussion of the nervous system 

 (Chap. 25). 



Specialized nerve cells and glandular de- 

 rivatives of nerve tissues are known to pro- 

 duce neurohormones. The hypothalamus and 

 the posterior lobe of the pituitary gland 

 (which is an embryonic outgrowth from the 

 brain) provide good examples, as does the 

 pineal gland (see above). Among insects, one 

 finds a group of specialized neurosecretory 

 cells in the brain (cerebral ganglia). These 

 cells transmit excitatory substances to asso- 

 ciated glands (the corpora cardiaca), and 

 jointly these structures exert an important 

 influence upon development and metamor- 

 phosis (p. 415). 



There are some cases in which the hormonal 

 product appears to be synthesized in the 

 centron of the nerve cell and then to be con- 

 ducted along the axon toward the tip. It is 

 interesting to note, therefore, that a wavelike 

 sort of "peristaltic flow" has been observed 



6 The line of distinction between a rwnmhuiiioi 

 and a neurohormone is not very sharp. However, gcn- 

 erallv the term neurohumor (for example, acetyl- 

 choline) is used to designate a neurosecretion that 

 lias a restricted local action upon certain synaptic 

 membranes, whereas a neurohormone (for example, 

 noradrenalin) in addition to such local action can, 

 as a result of being carried in the blood stream, exert 

 widespread effects throughout the bod). 



in the axon fibers of many living nerves — as 

 is shown in the beautiful time-lapse cine- 

 matic records of Paul Weiss at the Rocke- 

 feller Institute in New York and of Cecil 

 Taylor at New York University. 



Hormones of Invertebrate Animals. Stud- 

 ies on the endocrine systems of insects, pio- 

 neered by V. B. Wigglesworth in England and 

 Carroll M. Williams in the United States, 

 have shown that the complex cycle of devel- 

 opment in these animals is definitely con- 

 trolled by hormones. Some work has also 

 been done on other arthropods, but generally 

 speaking invertebrate endocrinology has not 

 yet received an adequate share of attention. 



Differences are found in the endocrine sys- 

 tems of various insects, especially when there 

 are differences in the life cycles. Many in- 

 sects, such as moths and butterflies, undergo 

 complete metamorphosis, in which case dis- 

 tinct larval, pupal, and adult stages can be 

 recognized (Fig. 22-9). But others, such as 

 bugs and grasshoppers, display incomplete 

 metamorphosis. In this case, the larval, or 

 rather the nymphal stages, gradually merge 

 from molt to molt until finally, at the last 

 molt, a winged adult emerges. Only one of 

 the insect systems will be discussed, however. 

 This deals mainly with the life cvcle of the 

 Cecropia moth (Fig. 22-10), which exempli- 

 fies the complete type of metamorphosis. 



An important component in insect sys- 

 tems is the juvenile hormone. The juvenile 

 hormone is produced by a small pair of 

 glands, the corpora allata, which lie in the 

 head, behind the cerebral ganglion, or brain 

 (Fig. 22-11). The juvenile hormone inhibits 

 development in the tissues, while other hor- 

 mones (see below) continue to induce molt- 

 ing. The larva grows and molts through 

 several stages and pupation is not initiated. 

 Bttt if the corpora allata are removed pre- 

 maturely, pupation and metamorphosis soon 

 begin. In some insects (e.g., silkworms) this 

 results in the formation of a very small, 

 though perfectly formed adult. Conversely, if 

 extra glands are implanted, pupation is de- 

 layed until the larva has grown enormously, 



