g2 CONK LIN. [Vol. XIII. 



Blochmann ('31) has given a most interesting and complete 

 account of the cross in Neritina. He did not recognize that 

 the four apical cells are in any way connected with this struc- 

 ture, and hence he speaks of it as four cell series ("Zellreihen"), 

 an anterior, posterior, right, and left. He followed the history 

 of these cell series until there were three cells in each one 

 except the posterior, which contained four. Owing to the 

 presence of peculiar shining granules in the terminal cells of 

 the transverse arms, Blochmann was able to trace these cells 

 to a very late stage in the cleavage. He believed that they 

 entered into the formation of the velum, and hence called them 

 " Urvelarzellen." 



In spite of the many minute and wonderful resemblances 

 between the cross in Neritina and Crepidula, the derivation of 

 .the cells composing it is very different in the two animals if 

 Blochmann's account is to be trusted. In Neritina, as in 

 Crepidula, the cross is first recognizable when there are two 

 cells, one basal and one terminal, in each arm. The following 

 scheme shows Blochmann's derivation of the cells of the cross 

 in Neritina, as compared with my account of Crepidula: 



As has been mentioned already (p. 65), Blochmann is cer- 

 tainly wrong in the designations given the outer-belt cells, and 

 consequently wrong in the designation of all cells derived from 

 them. Making allowance for this error, we find that the termi- 

 nal cells are derived from exactly the same source in Neritina 

 and Crepidula. The only other difference concerns the basal 

 cells. In the derivation of these cells Blochmann is certainly 

 in error. Although he expressly states that the basal cells 

 arise from the second quartette, he shows no stages in which 

 the spindles are present, and his figures indicate that the basal 

 cells have arisen exactly as they do in Crepidula and Umbrella, 

 viz., from the apical cells. The position of the cells is the 

 same, and it is highly improbable that their origin is different. 

 As opposed to Blochmann's view, I urge Heymons' careful 



