376 E. RAY LANKESTER. 



it were, in the point of intersection of the sectors of the large 

 yelk-masses. It is possible that this is not the same cor- 

 puscle as that seen in figs. 1 and 2. If it be the same we 

 have this to observe — that whereas in Aplysia the Rich- 

 tungsblaschen escapes from the paler pole of the unsegmented 

 egg, where the smaller enveloping cells are formed, in 

 Lymnaus the pole from which the Rich tun gsblaschen is 

 detached does not exhibit the more active, but the less active, 

 segmentation. Accordingly, the small cells in Lymnaus 

 would appear not to correspond with the small cells in 

 Aplysia ; they are not advancing, in the case of Lymnmus, 

 to enclose the four larger masses as they do enclose the two 

 large spheres of Aplysia, but are growing in the opposite 

 direction. In fig. 7, taking the position of the Richtungs- 

 blaschen and the general shape again as a guide, we find the 

 larger cells still left unenclosed by the smaller, which are 

 now sinking in on the lower surface to form the primitive 

 alimentary canal of the gastrula-form seen in the sub- 

 sequent figures. This interpretation depends upon the 

 assumption of the constancy of the position of the Richtungs- 

 blaschen, and also on the marked agreement in form of the 

 embryos when placed as drawn in figs. 4 and 7. If we might 

 U disregard this, and invert fig. 6, we should have what would 

 appear to be a much more intelligible mode of formation of 

 the primitive in-pushing of the gastrula of Lymnceus. Fig. 

 4 being inverted, we should, looking at it in the light of fig. 

 7, and disregarding Richtungsblaschen, see in this stage the 

 gradual extension of the smaller cells over the larger, so as 

 to enclose them, just as certainly does occur in Aplysia, 

 and the in-pushing in the base of fig. 7 would be the final 

 result of the growing over and approximation of the circum- 

 ferential border of the cap of enclosing cells. Unfortunately 

 the embryo or segmented egg-mass in the stage seen in 

 fig. 7 is too opaque to allow of our obtaining evidence on 

 this point from its actual structure. The question as to the 

 precise mode of formation of this gastrula, and, indeed, of 

 all gastrula-forms, is one of such very great interest at 

 present that I have not kept silence about the difficulties 

 which this has presented to rae, though a little more time 

 and care than I have given to this part of the developmental 

 history of Lymnceus would settle the point.^ In figs. 8, 9, 



' M. LerebouUet's account does not help one very much in this part of 

 the history. He figures one embryo as perfectly spherical and composed of 

 " twenty equal spheres." I did not come across such embryos, but they 

 would clearly be later than the stage given in fig. 1, and intermediate 

 necessarily between it and the youngest gastrula-phase, namely fig. 7. In 



