HOMOLOGY, ANALOGY AND PLASIS 



165 



acinar formation, their disposition and 

 vascular arrangements suggesting an endo- 

 crine function. In teleost fishes the islet 

 tissue is particularly distinct, being encap- 

 suled and occasionally present in masses 

 of considerable size, which permit of 

 ready isolation from the rest of the 

 pancreas; in other vertebrates the islet 

 tissue is intermingled with the alveolar. 

 In its topographical arrangement, in its 

 structural characters, and in its embry- 

 ology, this islet tissue is similar or 

 homologous throughout the group. 



The functional attributes of this tissue 

 are also distinctive and specific. No 

 matter from what vertebrate it is obtained, 

 an extract of the material lowers the 

 percentage of sugar in the blood, promotes 

 storage of glycogen in the liver of a 

 diabetic and allows of utilization of 

 glucose by the tissues, in virtue of which 

 actions it can temporarily restore to 

 normal the defective carbohydrate metab- 

 olism in cases of diabetes mellitus. At 

 one time it was claimed that the islet 

 tissue is merely a functional phase of the 

 alveolar epithelium of the pancreas, that 

 it can be transformed into alveolar 

 epithelium and vice versa; all the available 

 evidence goes however to show that while 

 the two varieties of tissue owe their 

 ultimate origin to one and the same 

 mesoblastic Anlage, an early differentia- 

 tion occurs, the one form becoming quite 

 distinct from the other, with no possibility 

 of reciprocal transformation (see Bensley). 

 The two varieties of tissue are structurally 

 and developmentally distinct and inde- 

 pendent, and their functional attributes 

 are similarly distinct and independent. 

 The foregoing examples of endocrine 

 organs have been cited, partly because of 

 the precision with which we can charac- 

 terise their respective activities, partly 

 because of the abundance, unusual in a 

 physiological question, of comparative 



experimental data. Other endocrine 

 examples, less definite in detail but all 

 pointing in the same direction, might have 

 been adduced. It will be noted that the 

 criterion of similarity in respect of Func- 

 tion B on which we have relied is identity 

 of a highly specific chemical or pharma- 

 cological product. The criterion itself 

 is not beyond cavil; from industrial 

 chemistry one might furnish endless 

 illustrations of identity of product based 

 on entirely different manufacturing proc- 

 esses, i.e., on entirely different intrinsic 

 mechanisms; furthermore, we know that 

 closely similar biochemical products, e.g. 

 diastatic and proteolytic ferments, may be 

 elaborated by very different types of 

 organism, vegetable and animal, and 

 therefore quite conceivably by different 

 intrinsic mechanisms. In a difficult and 

 involved scientific question it is sometimes 

 possible however to err in the direction of 

 undue caution. Most people would prob- 

 ably be willing to concede that in the 

 cases cited the test of identity as regards 

 Function B is appropriate and adequate. 

 The poison-glands of snakes might also 

 be cited as organs whose mechanism is in 

 a way particularised by the peculiar 

 pharmacological products they yield. 

 The active constituents of a venom are 

 bodies allied to proteins, any given 

 venom being a complex mixture of different 

 albuminous toxins, which, roughly sep- 

 arable by physical or chemical means, are 

 distinguished one from another as yet 

 only by biological test. Two main cate- 

 gories of snake-poison, colubrine and 

 viperine, are recognized by their action, 

 but the pharmacological distinction be- 

 tween them is not hard and fast like the 

 naturalists' distinction between the group 

 of viperine and the group of poisonous 

 colubrine snakes from which they are in 

 each case derived. Apart from other 

 poisons of animal origin, it may be said 



