EMBRYONIC DEVELOPMENT OF GASTEROPODS. 613 



greatly: thus in the common Lymnceus stagnalis or 'Water-Snail' 

 of our ponds and ditches, it is nothing else than a mass of soft 

 jelly about the size of a sixpence, in which from 50 to 60 eggs are 

 imbedded, and which is attached to the leaves or stems of aquatic 

 plants; in the Buccinum undatum, or common "Whelk, it is a 

 membranous case, connected with a considerable number of similar 

 cases by short stalks, so as to form large globular masses which 

 may often be picked-up on our shores, especially between April and 

 June ; in the Purpura lapillus, or Rock-Whelk, it is a little flask- 

 shaped capsule, having a firm horny wall, which is attached by a 

 sort of foot to the surface of rocks between the tide-marks, great 

 numbers being often found standing erect side by side ; whilst in 

 the Nudibranchiate order generally (consisting of the Doi-is, Eolis, 

 and other ' Sea-Slugs ') it forms a long tube with a membranous wall, 

 in which immense numbers of eggs (even half a million or more) 

 are packed closely together in the midst of a jelly-like substance, 

 this tube being disposed in coils of various forms, which are usually 

 attached to Sea-weeds or Zoophytes. — The course of Development, 

 in the first and last of these instances, may be readily observed 

 from the very earliest period down to that of the emersion of the 

 Embryo ; owing to the extreme transparency of the Kidamentum 

 and of the egg-membranes themselves. The first change which 

 will be noticed by the ordinary observer, is the ' Segmentation ' of 

 the Yolk-mass, which divides itself (after the manner of a cell 

 undergoing binary subdivision) into two parts, each of these two 

 into two others, and so on, until a mulberry-like mass of minute 

 yolk-segments is evolved (Fig. 323, a — f). Soon after this ' Mul- 

 berry mass ' has been formed, it commonly begins to exhibit a 

 very curious alternating rotation within the egg, two or three turns 

 being made in one direction, and the same number in a reverse 

 direction : this movement, which is due to Ciliary action, is often 

 extremely transitory in its duration; but in the Lymnaus it con- 

 tinues almost up to the escape of the embryo, and, when several 

 Ova are brought into view at once under a low magnifying power, 

 the spectacle is a very curious one. 



477.. A separation is usually seen at an early period, between 

 the anterior or Cephalic portion, and the posterior or Visceral por- 

 tion, of the Embryonic mass ; and the development of the former 

 advances with the greater activity. One of the first changes 



side at l ; m, still more advanced Embryo, seen at n as retracted 

 within its shell : — a, superficial layer of Yolk-segments coalescing 

 to give origin to the Shell ; c, c, Ciliated Lobes ; d, Foot ; g, hard 

 plate or operculum attached to it ; h, Stomach ; i, Intestine ; 

 m,n, masses (glandular) at the sides of the oesophagus ; o, Heart'?:; 

 s, Retractor Muscle (?) ; t , situation of Funnel ; v, membrane 

 enveloping the body ; x, Auditory Vesicles ; y, Mouth. 



