404 HOLMES. [Vol. XVI. 



stages ; the lower cells resulting from these divisions are some- 

 what larger than the upper ones. When the ^gg contains about 

 seventy-five cells, the lower cell in each quadrant divides, gen- 

 erally in a laeotropic direction, but the direction of the cleavage 

 does not appear to be constant. PL XIX, Fig. 32, shows that 

 the division of 20"'^ was probably dexiotropic, but shifting of the 

 position of cells has of course to be allowed for. The cells 

 2a^''^, 2<5'^■^ and 2t''■^ previous to their division, have become 

 more and more flattened in a direction transverse to the ver- 

 tical axis of the ^gg. After their division their daughter-cells 

 become flattened still more in the same direction ; one of these 

 usually lies above the other, which alone comes in contact with 

 the entomeres ; sometimes they lie obliquely, especially in the 

 a and c quadrants, the outer cell touching the entomeres by a 

 small portion of its boundary. These cells form the anterior 

 and lateral boundaries of the blastopore, the cells of the third 

 quartette lying at the angles. It will be convenient to apply 

 the term "stomatoblasts" to these cells, 2^'"", 2U'''', and 2^", and 

 their derivatives, as they form a part of the margin of the blas- 

 topore, and later take part in the formation of the stomodaeum. 

 The term " stomatoblasts," however, was used by Wilson to 

 designate the cells 2rt^ 2<5'^ and 2c^, and their derivatives, which 

 in Nereis form a ring of cells around the blastopore. In Pla- 

 norbis the cells 2«''', 2^"', and 2C'''' never come in contact with 

 the blastopore, and probably do not take part in the formation 

 of the stomodaeum, so that the term "stomatoblasts" would 

 hardly seem applicable to them. The term " stomatoblasts " 

 is used, therefore, to designate only the lower product of the 

 division of the cells which correspond to the stomatoblasts of 

 Wilson. In Nereis the division of the cells 2rt^ 21^, and 2c^ 

 is transverse to the polar axis and all of the products of the 

 cleavage border on the blastopore. In Planorbis these cells 

 divide in nearly the opposite direction, so that 2^^'', 2F'^, and 

 2c^''^ lie above the ventral cells and never come into relation 

 with the entomeres. The same cells divide into an upper and 

 a lower moiety in Unio, Neritina, Umbrella, Crepidula, Limax, 

 and Ischnochiton. In none of these forms, except Ischnochiton, 

 has the cleavage of 2«^'^ 2^'''', 2c'''' been described. The cell 



