THE QUARTERLY REVIEW OF BIOLOGY 



of the neck, we know of no other gland 

 that exhibits the same biochemical char- 

 acteristics. Within the limits of distribu- 

 tion aforesaid we may therefore say that 

 the thyroid gland displays a striking ac- 

 cordance of homology and Function B. 



Comparative anatomy has however 

 pushed the "homology" of the thyroid 

 beyond the limits of the Pisces into the 

 lower Craniata, or cyclostomes, into the 

 Acrania (Amphioxus) and into the Uro- 

 chorda or tunicates. Adult cyclostomes 

 have a true thyroid; in the larval cyclos- 

 tome, Ammocoetes, the thyroid is repre- 

 sented by a complicated glandular struc- 

 ture of unknown function, the endostyle 

 organ, which opens into the pharynx by 

 a wide duct, and from which at meta- 

 morphosis the thyroid follicles of the 

 adult are derived. In close details of 

 structure as well as in its topography the 

 endostyle organ of Ammocoetes corresponds 

 with organs of similar name in Amphioxus 

 and in tunicates (cf. D. Marine). The 

 greater elaboration of structure of the 

 endostyle organ, coupled with its free 

 communication to the pharynx, undoubt- 

 edly indicates a difference in its intrinsic 

 mechanism as compared with that of the 

 thyroid. 



Dismissing from our minds the unusual 

 interest and suggestiveness of this partic- 

 ular morphological finding, let us confine 

 ourselves to the purely formal question 

 with which we started. It is plain that 

 while the endostyle organ of these three 

 types may be said to be homologous one 

 with another (and then chiefly in the 

 sense of positional homology), one cannot 

 assert that the endostyle organ itself is, 

 in any received sense of the word, homol- 

 ogous with the thyroid. We should 

 rather phrase the matter by saying that the 

 thyroid of the adult lamprey finds its 

 Anlage in some part of the endostyle 

 organ of Ammocoetes, or simply that the 



thyroid is a derivative of the endostyle 

 organ. Just at the point, therefore, 

 where the word homology proves inappro- 

 priate to express the nature of the relation, 

 a difference in respect of Function B makes 

 its appearance. 



The medullary tissue of the adrenals. The 

 adrenal glands of mammals, consisting 

 of two parts, cortex and medulla, have a 

 complex mode of development. The cor- 

 tex, homologous with the interrenal 

 body of elasmobranchs, arises from meso- 

 derm cells of the genital ridge. The 

 medulla, represented in fishes by the 

 segmen tally arranged "paired bodies," 

 is derived from cells which belong to the 

 same neuroblast masses as give rise to the 

 nerve cells of the sympathetic ganglia. 

 Throughout the group of the Vertebrata 

 this medullary tissue presents the appear- 

 ance of an endocrine organ. It strikes a 

 brownish yellow color on treatment with 

 salts of chromic acid (chromaffin reaction) 

 and on extraction yields a hormone, 

 adrenaline, whose chemical constitution 

 has been fully determined. The hormone, 

 from whatsoever vertebrate derived, exer- 

 cises in mammals very specific pharma- 

 cological effects, which are the same as 

 those produced by electrical stimulation 

 of the nerves of the thoracico-lumbar 

 sympathetic outflow. Here, therefore, we 

 have a case of morphological or embryo- 

 logical homology with (so far as tests 

 have hitherto gone) complete similarity 

 of intrinsic physiological mechanism 

 throughout the vertebrate group. 



Islet tissue of pancreas. Among the 

 secreting alveoli of the pancreas of 

 vertebrates, and derived from the same 

 endodermal Anlage as the pancreas, are 

 multitudinous patches of a peculiar variety 

 of tissue, the so-called islets of Langer- 

 hans. The cells of this tissue, which 

 show affinities for particular stains and 

 are of two recognisable types, have no 



