ON THE DEVELOPMENT OF THE POND-SNAIL. 375 



pressure or by needles to remove the egg- from its envelope, 

 to avoid anything like actual contact with it, and to study 

 it with high powers (250 to 400 diameters) by both trans- 

 mitted and reflected light. The egg is not a transparent 

 one, and is very easily distorted by manipulation. Osmic acid 

 solution of one per cent, is useful in the earlier but more 

 especially in the later stages of the investigation, and en- 

 abled me to preserve specimens permanently. 



Formation of the Gastrula. — With the first contractions of 

 cleavage one or two pellucid drops are extruded from the 

 brown yelk-mass, and remain adherent to the axial point of 

 the egg, as in many other molluscs and worms ; they are the 

 well-known "Richtungsblaschen," and disappear, becoming 

 detached at a later stage of development. They may serve a 

 useful purpose for the embryologist if they enable him to re- 

 cognise at any subsequent period when they are present the 

 original pole at which they made their appearance. But it must 

 be borne in mind that such droplets of albuminous matter are 

 occasionally extruded from eggs of the same character as those 

 oi Lymnceus at other points during later stages in the process 

 of segmentation of the egg-sphere. In PI. XVI, figs. 2 and 3, 

 lateral and polar views of the egg when exhibiting four 

 divisions are given. In figs. 5 and 6 a series of smaller 

 segmentation cells is seen extending itself so as to surround 

 four larger spheres. The stage intermediate between this 

 and the simple quadripartite form I have not yet observed, 

 nor is it clear from M. Lereboullet's figures whence precisely 

 these smaller cells arise. He figures an egg consisting of 

 four large cells with four little ones surmounting them, but 

 does not demonstrate Avhence these four smaller cells have 

 originated. I have not seen the egg in this state. If we 

 compare the case of Aplysia we find there a series of smaller 

 cells growing over and enclosing two larger segmentation 

 spheres, but the origin of these smaller cells is clear from the 

 beginning; even in the unsegmented egg the pale trans- 

 parent portion of the egg from which they are formed is 

 distinguishable from the more granular opaque mass which 

 forms the two large enclosed spheres. This is the first point 

 of obscurity in the transition from fig. 3 to fig. 10. It 

 can, no doubt, be readily cleared up by painstaking observation 

 of a large number of eggs. In fig. 4 we have a lateral view 

 of the same egg as that of figs. 5 and 6. The drawing is so 

 placed that the smaller cells are below the large spheres 

 above. This is for comparison with the succeeding fig. 7. 

 At the pole of fig. 4 is seen a clear albuminous corpuscle, 

 uiuloubtedly of the nature of Richtungsbliischen, sticking, as 



