No. I.] THE EMBRYOLOGY OF CREPIDULA. 113 



Consequently I have found it impossible to trace with certainty 

 its entire cell origin. Nevertheless the derivation of some of the 

 velar cells can be established with great probability because of 

 their relation to the arms of the cross. I shall describe here 

 merely the preoral portion of the velum, or the prototroch, 

 which is derived in part from the first quartette. 



The first velar row, or prototroch, is derived on the anterior 

 side from the cells immediately surrounding the cross. These 

 cells are: (i) the anterior turrets between the arms of the cross, 

 and (2) some of the second quartette cells at the ends of the 

 arms. These cells are shown in position in Fig. 50, forming 

 a single row of cells surrounding the cross on its anterior side. 

 The turret cells, which, as we have seen, correspond in origin 

 to the trochoblasts of the annelids, form the portions of the 

 prototroch between the arms, while the portions at the ends of 

 the arms are derived from the second quartette (see p. 132). 



In Figs. TJ, jZ, and all the later stages, it can be plainly 

 seen that the velum is divided on the dorsal side of the embryo 

 into a posterior branch, P-B, and an anterior one, A-B. The 

 former runs around the edge of the umbrella, and surrounds all 

 of the first-quartette cells ; the latter runs up on each side from 

 the edge of the umbrella nearly to the apical organ. This 

 anterior branch, therefore, is composed of cells derived from 

 the first quartette. The position of the cells which form this 

 branch of the velum, relative to the large ciliated cells of the 

 posterior cell plate, P-C, Figs, 'j'], "jS, et seq., shows that they 

 are derived chiefly, if not entirely, from cells of the transverse 

 arms of the cross. As the velum belongs largely to the second 

 quartette, we shall consider its origin, structure, and relation- 

 ships more fully in the section devoted to those cells (p. 132). 



V. History of the Second and Third Quartettes of 



ECTOMERES. 



In Crepidula there are no prominent landmarks among the 

 cells of these quartettes, as there are in Nereis, Umbrella, and 

 Unio, and on this account it is difficult to follow the lineage of 

 these cells very far. I have been compelled to use the arms 



