1904.] NATURAL SCIENCES OF PHILADELPHIA, 367 



from the cell identified provisionally as 2b", which lies just beyond 

 the median cells of the first row", and he adds, "I have not been able 

 to determine whether any part of the second velar row arises by y\ih- 

 division of cells of the first; if not this row may include a few of the 

 third quartet (3a"^ and 3b"^, fig. 56) at the points opposite the anterior 

 turrets". It also seems probable (Supplementary Note, page 204) that 

 the cells 2b^"", 2b^-^'^ lie outside the first velar row. Fig. 79 shows 

 two large cells between the first and second velar rows, and they appear 

 to represent the major portion of these cells. Smaller derivatives 

 from them may join 2b^" in forming the median part of the second velar 

 row (V^). Conklin thus finds that the preoral velum arises from " a few 

 cells of the first quartet, many of the second and possibly a few of the 

 third". I do not believe that the third quartet becomes involved 

 in the preoral portion of the velum of Fiona, though doubtless cells 

 from this series are closely connected with it in the stomodseal region 

 and help in the formation of the postoral velum. It will be remembered 

 that in Crepidula secondary mesoblast is derived from the second 

 quartet, while in Fiona it is furnished by the anterior groups of the 

 third, and in this process the large cells of this series, wliich have hith- 

 erto lain well up on the sides of the gastrula, migrate over the under- 

 Ijdng mesoblastic elements and thus become far removed from the 

 region where the A-elum first appears. The formation of secondary 

 mesoderm in the most anterior second quartet group of Crepidula 

 has doubtless the same effect of lessening the external area of the 

 quartet in that region, while the neighboring third quartet cells would 

 lie relatively higher in this form than in Fiona. So when the second 

 velar row forms in Crepidula it will lie relatively lower in the second 

 quartet group (2b-^) and more probably involve third quartet cells, 

 as Conklin states it probably does. 



Regarding the lineage of the velum of Planorhis, Holmes says that 

 "the tip cell (of the anterior arm) divides as far as I can deter- 

 mine, but once, and the two daughter cells become pushed apart by 

 the cell Ib'^", which forms the median cell of the upper row. These 

 cells extend to the anterior trochoblasts on either side, but in later 

 stages they may sometimes be separated from them by cells which 

 wedge in from below". The anterior trochoblasts follow these cells 

 posteriorly, but Holmes states that the tip cells of the lateral arms "do 

 not form a part of the prototroch but enter into the formation of the 

 head vesicle". In this Planorhis differs from Fiona. Blochmann 

 states that the right and left tip cells enter the velum of Neritina. 

 The lower cells in the prototroch Holmes derives from the second 



