Endocrine Mechanisms 



763 



Another active principle from the duodenum, and one which has been 

 separated from crude secretin extracts, is cholecystokinin a powerful agent in 

 producing gallbladder contraction. 



A fourth factor, enterogastrone, has been isolated from the duodenal mu- 

 cosa. Crude extracts containing this factor inhibit gastric secretion and gas- 

 tric motility, but, after purification, the extract largely loses the latter ca- 

 pacity. The activity of enterogastrone therefore appears to be principally 



Fig. 289. Position of the retinal pigments in the ommatidium of the eye of the shrimp, 

 Palaemonetes, L, in the Hght-adapted state, D, in the dark-adapted state, and E, after 

 injection of eyestalk-extract into a dark-adapted specimen kept in darkness. C, Cornea; 

 DP, distal retinal pigment; PP, proximal retinal pigment; RP, reflecting pigment; RH, 

 rhabdome; and BM, basement membrane. From Kleinholz.'"^ 



antisecretory, and since in the presence of enterogastrone the gastric juice 

 produced under the influence of gastric stimulants such as histamine is low 

 in acid content and rich in pepsin it would appear that enterogastrone pref- 

 erentially inhibits the acid-secreting parietal cells of the stomach. 



Other gastrointestinal chemical coordinators have been proposed on the 

 basis of brief experiments but these appear to have a more questionable ex- 

 istence than the aforementioned ones. 



HORMONES AND PIGMENT-CELL ACTIVITIES 



Integumentary Chromatophores. The roles of hormones in the control 

 of the integumentary color changes in cephalopods, crustaceans, insects, fishes, 

 amphibians, and reptiles have been dealt with at some length in Chapter 

 21 and will not be reviewed here. 



Retinal Pigment Migration. The movements of pigments within the eyes 

 of many animals such as vertebrates, insects, .and crustaceans contributes im- 



