72 Swale Vincent and F. D. Thompson, 



the fact that tliis differentiation and that of the cells into islet and 

 zymogenous is never so complete as in higher vertebrates. The trans- 

 formation of one of these varieties into the other is more easy in the 

 viper, under the influence of slight functional changes. 



This author finds, in fact, variable islets disseminated throughout 

 the whole pancreas. These are the „seconiary islets", derived from 

 acini and destined to reform acini. He describes further a large mass 

 of islet tissue at the splenic end of the pancreas, the „primary islets", 

 of the type which occurs in the mammalian embryo, derived directly 

 from the primitive embryonic pancreatic tubules, and not from acini. 

 They have little or no tendency to be converted into secreting tubules. 

 He points out that, while in the sheep these primary islets for the 

 most part atrophy at a certain stage of developement, in the viper 

 they persist to adult age. So far as we have been able to find, 

 however, he gives no histological distinctions between the two kinds 

 of islet, and his drawings show numerous small groups or isolated 

 cells of the „primary" („troubles") type scattered throughout the 

 zymogenous tissue of the pancreas. 



So far the only reptiles we have been able to examine are two 

 species of Cheloiiia, viz. Chrysemys pietà, and Kinosternon pennsylvanwum. 

 We have studied carefully diiferent regions of the pancreas in these 

 two animals, fixing and staining the preparations in many diff'erent 

 ways. At first we were astonished at not finding in either animal 

 any islets resembling those we were familiar with in higher animals, 

 but ultimately we discovered in certain regions of the organ large 

 masses and columns of solid structure staining more deeply Avith all 

 reagents tjian does the zymogenous tissue. These are in all respects 

 identical with the similar structures in the bird (see PI. V, fig. 12, 

 hath, t, and compare with PI. IV, fig. 7, hath. t.). They form large 

 masses in certain regions of the pancreas, insinuate themselves in the 

 form of networks and branching columns into various regions of the 

 zymogenous tissue, and small groups of cells or even individual ele- 

 ments are found widely scattered in some parts of the organ. 'JMiis 

 tissue obviously corresponds to what, we have described under the 

 iiauie of „baMiy(;hi(>iiie" in mammals and birds. 



