374 E, RAY LANKESTER. 



Structure representin;^ the phragmacone; and further, it is 

 possible, though little importance can be attached to this sug- 

 gestion, that the Dibranchiate pen-sac, as seen in its earliest 

 stage in the embryo Loligo, «&c., is fused with the surviving 

 remnants of an embryonic shell-gland. When a zoological 

 observatory has been established in the southern seas, and the 

 embryology of Nautilus pompilius worked out, we shall 

 probably know with some certainty the fate of the molluscan 

 shell-gland in the group of the Cephalopoda. By the use of 

 no very great ingenuity it might be possible to conceive of 

 the pro-ostracum alone of the Belemnite as being the plug of 

 the shell -gland, and thus to save the homogeny of the 

 embryonic pen-sac of living Cuttle-fish with the so closely 

 corresponding sac (the shell-gland) of other Mollusca. I will 

 only venture one additional remark of a speculative tendency 

 here, and that is that the siphuncle of the chambered 

 shells of Nautilus and Spirula is so placed as to suggest an 

 inquiry as to whether it may have any relation to this pro- 

 blematical shell-gland. 



The preceding discussions and speculations have been 

 introduced with the object of rendering more clear the points 

 of interest in the facts of the development of Lymneeus 

 stagnalis which are recorded below. 



§ 2. — Development of Lymn^us Stagnalis. 



The well-known egg-jelly of the common pond-snail is to 

 be found on water-plants in most ponds from June to Oc- 

 tober. The jelly encloses a number of tense capsules, each 

 of which contains one, rarely two, eggs. 



Many points of interest in the earliest stages of the de- 

 posited e^^ demand minute investigation with the highest 

 power, and have been entered into in a suggestive rather 

 than a conclusive manner by M. Lereboullet in his extended 

 " Monograph of the Development of LymncRus " (* Annales 

 des Sciences Naturelles,' ser. iv, t. 18, 1862). I shall here 

 only record a few facts tending to show the general disposi- 

 tion of the masses resulting from the segmentation of the 

 primitive egg-sphere, reserving the consideration of the 

 minute structure and relations of the various elements of the 

 yelk for another occasion. The egg-sphere, as laid, has a 

 diameter of about -^^-o- of an inch. By the middle of the 

 third day from its deposition in the warm season it has 

 assumed the form seen in PI. XVI, figs. 8 — 12, and is then 

 -f-i^Tj of an inch in diameter. The intermediate steps are not 

 easy to follow with certainty. It is necessary by sharp 



