60 Proceedings of Royal Society of Edinburgh. [sess. 
latter becomes adenoid in its most highly differentiated form, while 
the processus digitiformis is not known to he thus constituted, 
would appear to he of minor significance hy analogy with Weldon’s 
discovery that the suprarenal body in the Icthyopsida (Bdellos- 
toma) probably represents a metamorphosed excretory blastema.” 
The theory seems a plausible one, but as Howes nowhere refers to 
any actual work upon the structure of the organ, it is conceivable 
that he may not have thoroughly appreciated the distinct histo- 
logical difference of the rectal gland from the vermiform appendix. 
The case of analogy he cites seems scarcely conclusive, and he 
seems to take for granted that the gland is secretory and not 
excretory, a view which is upheld by no direct evidence. 
On taking a general view of these suggestions, none of them are 
entirely satisfactory. It seems unlikely that the gland is concerned 
in reproduction, as Hyrtle supposes. If, as Leydig thinks, it is of 
a nature resembling that of the glands of Brunner, its glycerin 
extract might be expected to show some digestive action. 
The rich blood supply, the character of the secreting cells, 
resembling so closely as they do the cells of the kidney, and the 
occurrence of urea in considerable amount in the secretion, all 
point to the structure having an excretory function, and playing 
the part of a supplementary kidney. 
When the peculiar richness of the blood and tissues of the 
elasmobranchs in urea is remembered, this action of the rectal 
gland becomes of very considerable interest. 
REFERENCES. 
1. Monro. — Structure and Physiology of Fishes , 1785. 
2. Ev. Home. — Lectures , PI. XCVII. 
3. Owen. — Lectures , PI. LXXV. 
4. Dumeril. — Poissons i i. p. 157. 
5. Leydig. — Beitr. Mikros. Roch ., p. 56. 
6. Howes. — Journ. Linn. Soc. ( Zool .), xxiii. p. 393. 
7. Blanchard. — Mittli. ub. d. Bau und Entw., etc. ; Mitth. 
aus d. Embry. Inst. Wien, 1880, Bd. i. 
