86 Royal Society : — 



epidermic cells at the aboral pole, which develops a deep furrow, 

 groove, or pit iii its centre almost amounting to a sac-like ca\lty 

 opening to the exterior. The first (chitinous) rudiment of the 

 shell appears as a disk on the surface of this gland ; but also, in 

 some cases, the ca^■ity or groove is filled by a chitinous plug. 



Let the walls of the sac close and the activity of its lining cells 

 continue, and we have the necessary conditions for the growth 

 of such a " pen " as that of the Decapod Cephalopoda. 



At present the details of the development of the "pen" in the 

 Cephalopoda are not fully knowTi ; but the author has evidence 

 that it is formed in an enclosed sac-like diverticulum of the epi- 

 dermis, but he has not yet ascertained the earliest condition of 

 this sac. The history of its development becomes surrounded with 

 additional interest in relation to the shell-gland of the other 

 Mollusca. 



The position of the groove of the shell-gland in Pisidium 

 suggests a possible connexion of its chitinous plug with the liga- 

 ment, which it will be worth inquiring into in other developmental 

 histories of Lamellibranchs. 



The internal shells of other Mollusca besides the cuttlefish are 

 certainly not in some cases (e. g. Aphjsia) primitively internal, but 

 become enclosed by overspreading folds of the mantle. But in the 

 case of Limax and its allies, it is possible, though the matter 

 requires renewed investigation, that the shell is a primitively in- 

 ternal one representing the shell-plug. 



There is yet one more possible connexion of this shell-gland and 

 plug : this is the chitinous secretion by which Terehratula and 

 its allies fijc themselves to rocks &c. The position of the peduncle 

 exactly corresponds to that of the shell-gland ; and an exami- 

 nation of "Professor Morse's recently published account of the 

 development of Terebratulhia leaves little doubt that at the pole o£ 

 attachment, which very early develops its function and fixes the 

 embryo, an in-pushing occurs, and a kind of shallow gland is 

 formed which gives rise to the homy cement. The author's own 

 observations on the development of Terehratula vitrea do not extend 

 to so early a period as this. 



It is perhaps scarcely necessary, in conclusion, to point out the 

 close resemblance of shell-gland and plug to the byssal gland and 

 its secretion. They are closely similar structures ; but there does 

 not appear to be any reason for considering them " serial homo- 

 logues," or more closely related than are, say, the hairs on the 

 head of a man \vith the hairs on his chest. 



April 16, 1874. — Joseph Dalton Hooker, C.B., President, in the 

 Chair. 



" Studies on Biogenesis." 

 By William Eobeets, M.D., Manchester. 



The object of the investigation is to inqiure into the mode of 



