to the typical teleost, where they are scattered along the 

 postcardinals (Figure 10-26). The interrenal, or cortical 

 tissue, of the teleost forms a sheath of acidophylic cells 

 around the posterior cardinal veins in the cranial region of 

 the kidney. 



In the shark or holocephalan the chromaffin or supra- 

 renal tissue is completely separated from the interrenal. The 

 former lies anterior to the interrenal as masses associated 

 with the sympathetic ganglia. The interrenal tissue forms a 

 compact mass between the opisthonephric kidneys which is 

 distinct enough to be removed. Experimental work indicated 

 that the secretions of these two parts are suggestive of those 

 of the adrenal of higher forms. However, there is consider- 

 able variation in details of function. 



In cyclostomes the suprarenal and interrenal tissues are 

 separated. The latter is represented by clusters of cells along 

 the posterior cardinals throughout the length of the body 

 cavity. The suprarenal tissue is arranged as strands along 

 the dorsal aorta (Figure 10-26). 



THE GONADS 



The gonads of all of the vertebrates are secondarily endo- 

 crines; they contain interstitial cells which secrete steroid 

 compounds. These interstitial cells may be of splanchno- 

 pleure origin like those of the adrenal cortex, which are also 

 capable of secreting sex hormones. The sex hormones func- 

 tion not only in the production and maintenance of sex cells 

 but also in the appearance and continuance of secondary sex 

 features which are necessary for reproductive success. Sex 

 reversals, sometimes functional and involving many mor- 

 phological changes, can be produced by use of the hormone 

 of the opposite sex early in development. 



ISLETS OF LANGERHANS 



Within the pancreas of higher forms and .sharks, islets of 

 Langerhans develop from the tubules of this otherwise di- 

 gestive gland. Islet tissue in the actinopterygian fishes tends 

 to be concentrated. There is usually one islet or sometimes 

 two large islets in the region of the bile duct (Figure 9-13). 

 In the agnath fishes pancreatic islets have been observed in 

 the lamprey (Figure 9-26) but not the hagfish. In the lam- 

 prey there are three masses of cells in the wall of the gut 

 just behind the junction of the pharynx and the midgut. 

 These masses have been shown experimentally to affect the 

 sugar concentration of the blood. Whether or not they actu- 

 ally produce insulin is another thing. 



CORPUSCLES OF STANNIUS 



The corpuscles of Stannius, which occur in the kidney of 

 many actinopterygian fishes, have often been viewed as 

 potentially endocrine. These arise as many (40 or more) 

 diverticula from the nephric ducts in the anterior part of 

 the opisthonephros (segments 9 to 12) of Arma (Figure 10- 



23) and Lepisosteus or, in teleosts, as a few or even a single 

 pair of diverticula in the posterior region of the kidney. 



These corpuscles have not been observed in other verte- 

 brates and are thus assumed to have evolved within the 

 actinopterygian group. They are not observed in the chon- 

 drosteans (Acipenser). It has been suggested that they are 

 homologous with the Miillerian ducts, but such an homol- 

 ogy is unlikely. Furthermore, these corpuscles are not a 

 part of the adrenal system. Experimental work has revealed 

 no function for them. They appear to be only modified 

 kidney tubules, perhaps related to the Leydig's glands of 

 chondrichthians. The number of these appears to have 

 undergone reduction in the teleosts. There are none in some. 



THE PINEAL ORGAN 



The pineal organ, an outgrowth from the roof of the 

 brain (Figure 12-1, see next chapter), is recognized as a 

 photoreceptor in fishes (it is vestigial in the hagfish), but its 

 glandular structure suggests endocrine activity. Removal of 

 the pineal organ of the guppy (Lebisles) is followed by re- 

 duced growth rate, skeletal abnormalities, and marked 

 stimulation of both pituitary and thyroid glands. Increased 

 activity was also noted in the corpuscles of Stannius and in 

 remnants of the pronephric tissue. It is suggested that the 

 action of the pineal is secretory and its action is mediated 

 through the pituitary and thyroid glands. The presence of a 

 pineal nerve also indicates a more direct (nervous) as- 

 sociation. 



In the mammal the pineal is glandular and richly sup- 

 plied with blood. There are no nervous elements in it al- 

 though the parenchymal cells have processes. No endocrine 

 function has been demonstrated and the structure is looked 

 upon as rudimentary and, in the adult, degenerate. 



GENERAL OBSERVATIONS 



The presence of a system of distinct endocrine glands 

 characterizes the vertebrates, but does not subdivide this ar- 

 ray. Presumbably these glands can be followed back to 

 some stage in phylogeny at which they disappear as discrete 

 structures. It can be assumed that like the mammals the 

 lower vertebrates have endocrine-producing cells or tis- 

 sues, similar to those producing secretin (doudenal wall) 

 and rennin (kidney), which are not organized into distinct, 

 separate glands. 



The protochordates lack apparent endocrines. Hatschek's 

 fossa of Amphioxus and the stomochord of the hemichor- 

 dates may be divergent developments of the Rathke's pouch 

 of the vertebrate. The urinary vesicles and solenocytes of 

 Amphioxus may represent the thymus diverticula of the 

 vertebrate, while the endostyle and thyroid are definitely 

 divergent products of a single structure. The aggregation of 

 specialized secretory cells into endocrine glands is but 

 another of the evidences of the increasing complexity of the 

 vertebrates as contrasted to invertebrates. 



382 



THE ENDOCRINE GLANDS 



