326 E. RAY LANRESTER. 



in consequence of its dilatation by liquid has a much paler 

 appearance than has the solid mass of granular cells grouped 

 in the opposite hemisphere. It is at first difficult in many 

 specimens to make out the cells whicli form the wall of the 

 vesicular region — and in my former paper I made the mis- 

 take of regarding' this part of the embryo as consisting of 

 four big cells. The four big cells -which I thus misplaced 

 really belong to the opposite — the anti-klastic hemisphere. 

 They are seen in pi. xvi, fig. 5, of my former paper (where 

 the directive corpuscle E. is quite erroneously introduced), 

 and are also seen in fig. 5 of the present plate. 



In fig. 8, an embyro, with its more diaphanous vesicular 

 hemisphere and its darker granular hemisphere, is repre- 

 sented as seen from the surface. In fig. 7 the same embryo 

 is focussed to the median plane and the cavity traversed by 

 the processes of the paler cells is seen. 



The exact number and arrangement of the cells in the 

 dark antiklastic hemisphere is difficult to ascertain. The 

 four which are seen by a view of the antiklastic pole un- 

 doubtedly become invaginate and give rise by division to the 

 primitive enteron. They take up this position partly in con- 

 sequence of the circumcrescence of the surrounding cells which 

 are seen to the number of eight in figure 5 of the accom- 

 panying plate encroaching upon them. Partly, however, 

 their own growth and division is in such a direction as to 

 form a basin-like area Avith its concavity facing the anti- 

 klastic pole (see figs. 7 and 10 of pi. xvi of my former 

 paper). Thus the mode of formation of the diploblastic 

 Limnseus is intermediate between epibole and embole. The 

 widely open blastopore with its shallow basin-like archen- 

 teron (or gastrula-stomach) now narrows rapidly and assumes 

 the elongated slit-like form represented in fig. 9. 



This elongated form of the blastopore is important in con- 

 nection with the question of the persistence of the blastopore 

 as either anus or mouth. Where the blastopore actually per- 

 sists as, or is identical in position with, the anus, it has a 

 circular margin which narrows to a minute passage as in the 

 Echinoderms and Paludiua (see on the Plauulaof Paludina, 

 Lankester, vol. xv of this Journal, p. 159). In Limnaeus, 

 and in some other Gasteropods the blastopore is elongated. 

 Its posterior end (marked Red. in fig. 9 of the present plate) 

 closes up, and thus is formed the rectal peduncle which I 

 previously figured, and of the existence of which I do not 

 feel any doubt in spite of the failure by Rabl and others to 

 detect it. I have preparations (prepared with osmic acid 

 and glycerine) which ^how it clearly. The anterior end of 



