THE ANATOMY OF THE THYROID GLAND 191 



which broad venous spaces to a certain extent encircle the quasi- 

 lobiile. The veins thus lie at the periphery while the artery on 

 reaching the group promptly breaks up into a plexus of broad 

 capillary spaces — venae lymphaticae — which surround the folli- 

 cles within the quasi-lobule. The number of follicles in the group 

 varies from four or five to several score. 



In Carcharias the condition is similar to that in Mustelus, there 

 being no indication of lobular groups except about the occasional 

 irregular deficiencies in the thyroid mass. Except for the anatom- 

 ical disintegration of the gland in one fish there was similarly no 

 indication of lobulation in Squalus, but as none of my specimens 

 from this species were prepared as total mounts I can not speak 

 with the same certainty as in the other species. 



The form of the thyroid folhcles is subject to considerable 

 variation, but, in general, they may be said to be of ovoid shape, 

 and, as pointed out for the mammalian thyroid by Streiff ('97), 

 they present frequent diverticula. The Elasmobranch thyroid 

 differs from those described by Streiff in that they show very little 

 tendency to branch and no indication of a tubular character when 

 the whole follicles are examined in total mounts of the gland (fig. 

 13) . In cut sections diverticula are of frequent occurrence and are 

 apparently the result of pronounced infoldings of the follicular 

 wall rather than of any protuberance, or of any tendency of the 

 follicle to branch. Fig. 13 shows characteristic follicles from all 

 four species; the figures are of whole folhcles and differ from the 

 cut sections in that only the largest infoldings of their wall are 

 visible. As already indicated, diverticula are more apparent in 

 sections than in the preparations (total mounts) from which the 

 drawings have been made. The particular follicles drawn from 

 Carcharias present rather greater infoldings than those from the 

 other species. I have not, however, observed that this is charac- 

 teristic of Carcharias. In the figure the magnification is the same 

 for the several Selachian species but less by one half for Raia; the 

 follicles of Raia are, therefore, relatively about twice as large as 

 shown. - 



The size of the folhcles is subject to considerable variation as 

 regards the individual follicles, the different thyroids, and the 



