398 PEOF. G. B. HOWES ON THE 



this passage (whicli is in reality a wide funnel-shaped prolonga- 

 tion of the gut) and the cjecum coli of the higher Vertebrata. 

 Comparison of PI. II. figs. 9 to 14 shows unmistakably that that 

 prolongation (be its original significance what it may) has become 

 converted by constriction into the " duct " of the processus 

 digitiformis of the Sharks ; and suggests that the Batoidei in all 

 probability retain a more primitive condition thereof than do the 

 Selachoidei of to-day. Attenuation of the " duct " has been seen 

 {ante, p. 397) to be most marked in Notidanus {dv, fig. 14). 

 Thanks to the generosity of Prof. Huxley, I have been enabled 

 to examine a foetal Notidanus * measuring 15 centim. in total 

 length. In it the duct is much shorter and relatively wider than 

 in the adult, and in no way bound down to the intestinal wall ; 

 it stands out from this in the manner indicated in fig. S {Zygcena 

 malleus). In Zygcena this condition is retained; audit will be 

 observed that it is just such as would result from elongation of 

 the intestinal diverticulum of Baia (fig. 9) vdth accompanying 

 constriction. Squatina {^BUna\, fig. 15) is, in respect to this 

 constriction, transitional between Baia and Zygcena. That the 

 Selachoidei, in which this structure becomes most modified, pass 

 through a stage such as is here represented is clear from the 

 condition of the young Notidanid ; and in the absence of further 

 embryological data I can only conclude that the Batoidei do 

 present us with the least modified condition of the parts, and 

 that the duct-like base of the appendix digitiformis seen in them 

 is the representative of a structure closely comparable to, if not 

 homologous with, the csecum coli. Whether that portion of this 

 " duct " which, in the SelacJwidei, skirts the wall of the gut is a 

 superadded passage formed from behind by a duplicature of the 

 mucous membrane, as Blanchard supposes (2. p. 182), has yet 

 to be proved ; from the facts herein recorded, I incline to the 

 belief that it is not, but that the apparent superaddition may have 

 resulted from adhesion. 



The processus digitiformis is, as Dumeril has remarked (7. 

 p. 158), " a true secretory organ." Its secretory glands have 

 been compared by Leydig (21. p. 57) to those of Brunner, met 

 with in the mammalian intestinal wall ; and he calls attention to 

 the " dirty yellow " nature of their product. Blanchard has 

 attempted to institute (2. p. 181) comparisons between the secre- 



* That alhided to by him in P. Z. S. 1876, p. 44. 



