94 CONK UN. [Vol. XIII. 



zellen vz und vz^ in den seitlichen Reihen erkennbar war." 

 Truly such a fact is "especially noteworthy" when it is found 

 reproduced in another very different animal ; and, standing as it 

 does in direct relation to the origin of bilateral symmetry, it is 

 a fact of profound significance. 



The further history of the cross in Neritina is not given in 

 detail. An ectodermal invagination is described as occurring 

 at the apical pole, and Blochmann's Fig. 59 shows that this 

 invagination includes the four apical cells and all the cells of 

 the transverse arms, except the terminal ones. Judging by the 

 figures the transverse arms seem to have been drawn into this 

 invagination, while the turret cells and the anterior and posterior 

 arms lie outside of it. If such a thing really happens, the trans- 

 verse arms of the cross must first be wholly separated from all 

 their connections with surrounding cells, and then drawn into 

 the invagination ; in fact, the figures mentioned show that this 

 has happened, for the terminal cells of the transverse arms (the 

 " Urvelarzellen ") have been drawn inward until they immedi- 

 ately adjoin the turret cells. Concerning this invagination I 

 have already spoken (p. 31), and I need only repeat here that 

 I believe it is not a normal formation. 



The two granular tip cells are the only ones which Bloch- 

 mann was able to trace farther than the stage already men- 

 tioned (his Fig. 59). These cells he calls the Urvelarzellen, 

 and he states that the velar cells first appear between them, 

 on the dorsal side of the embryo, and then, apparently by the 

 division of the Urvelarzellen, they extend ventrally around the 

 anterior end of the embryo. It is almost certain that these 

 same cells form part of the velum in Crepidula (see p. 132). 



Heymons ('93) figures the cross plainly enough in Umbrella, 

 as is shown by Diagram \2, c and d, taken from his Figs. 14 and 

 20, and yet he does not appear to have recognized this structure. 

 To be sure, he speaks of a cross of ectoblast cells being pres- 

 ent, and refers repeatedly to the cross in Neritina and Crepid- 

 ula, but the cross which he points out in Umbrella is a wholly 

 different thing from that in either of the other forms. It is 

 composed entirely of cells of the second quartette (a", b", c", 

 d"), does not reach the centre of the ectodermal field, and has 



