132 CONKLIN. [Vol. XIII. 



same cell. I do not doubt that this posterior region of telo- 

 blastic growth has essentially the same origin and destiny in 

 Nereis, Unio, and Crepidula ; and, if so, it follows that the 

 cell 2d in Crepidula is really like the " first somatoblast " 

 in Nereis and Unio, although its earlier divisions are very 

 different. This difference, as I have already explained, is 

 probably in the main a time difference, being due to the 

 shortening of the history of this cell in the annelid and lamel- 

 libranch. 



(c) The Vehtm. — The velum can first be distinguished 

 as a row of small polygonal cells running across the ventral 

 surface of the embryo immediately posterior to the apical plate 

 and some distance in front of the blastopore, Vi, Figs. 65-67. 

 These cells can be traced out to the sides of the embryo, where 

 they turn forward and dorsalward, and finally become wholly 

 indistinguishable from the surrounding cells. This row can be 

 recognized in the velum throughout all the further development. 

 It forms the most anterior row of velar cells, and ultimately 

 becomes the most important part of the velum. 



Just posterior to this first row is a second, V^, which is com- 

 posed of larger cells and is less distinct than the first row. The 

 median portion of this second row can be recognized in all the 

 older stages, Figs. 'j6, 79, 81, 82, as two or more large cells 

 with clear protoplasm and vesicular nuclei ; its lateral portions 

 are not clearly marked. 



The cells of these two rows are not ciliated at first, and can 

 be traced only by their form and position. The long velar 

 flagellae which they afterward bear do not develop until a late 

 period, but from the time when the blastopore closes until these 

 flagellae appear the embryo swims about in the &gg capsule by 

 means of the short cilia which cover the cells of the apical, 

 dorsal, and pedal cell plates. 



The median portion of the first row arises from the cells 

 which lie just beyond the ventral end of the apical plate. These 

 cells are in all probability 2b'---^-'' and 2b'■-■^•'■^ One of these 

 cells is shown dividing in Fig. 71. In Fig. 72 a transverse row 

 is formed from these cells which is plainly the first row of velar 

 cells. It will be remembered that the apical plate is formed 



