BLASTOPORE AND ANUS IN PALUDINA VIVIPARA. 381 



yellow spherical particles which were noticed in the original 

 egg-cell. The endodernial cells are more swollen and glob- 

 ular and contain so much of the finely granular and yellow- 

 coloured food material as to render them nearly opaque and 

 of a dark brown tint in the mass. The commencing invagi- 

 nation is limited by a circular margin (fig. 5, below), which 

 remains throughout the subsequent stages here figured very 

 distinct. The circular form of the blastopore and the com- 

 pletely pyramidal form of the embryo at the period of its 

 full development (fig. 5) are worthy of especial note. In the 

 finished gastrula of Paludina the circular blastopore occupies 

 the base of the cone directly opposite to the apex. It has 

 not, as it has in Limnseus and in Polycera, an eccentric posi- 

 tion nor an elongated form. 



Stage 5. — A slight change of form marks the next stage 

 of development. The blastopore narrows a little, whilst the 

 opposite region of the embryo becomes marked by a ciliated 

 girdle (figs. 7 and 8). - This is the trochosphere stage. The 

 ciliated girdle is placed well forward in Paludina, so that it 

 does not, as in some Mollusca and many Vermes, take an actu- 

 ally equatorial position. Very rapidly now the embryo in- 

 creases in size and the anterior field or upper hemisphere of 

 the trochosphere (the area of the velum) dwindles as com- 

 pared with the posterior field or lower hemisphere. 



With the first changes of shape which mark the develop- 

 ment of the ciliated girdle and the* assumption of the tro- 

 chosphere stage, certain changes in the cells of endoderm 

 and ectoderm are to be detected. Both become more tran- 

 sparent and those of the endoderm have diminished in size, 

 whilst a space separating endoderm and ectoderm traversed 

 by delicate protoplasmic filaments and not to be confounded 

 with the original cleavage-cavity of the blastula (fig. 4, sc) 

 is visible. This new cavity is the mesoblastic cavity or 

 coelom. The filaments which pass across it from endoderm 

 to ectoderm are the first indications of the branched and 

 fusiform corpuscles which subsequently develope in large 

 quantity on its walls and lay the foundations of muscular 

 and connective tissue and hsemolymph. I have described a 

 similar arrangement of the commencing mesoderm in Lim- 

 naeus. At this period of development it is perfectly easy to 

 satisfactorily determine that the only direct continuity of 

 endoderm and ectoderm is at the blastopore, and that this is 

 separated from the position subsequently occupied by the 

 mouth — by the rudiment of the foot (fig. 8). 



Stage 6. — The embryo increases in size and the blastopore 

 narroM'S without any trace of the mouth being visible ex- 



