No. I.] THE EMBRYOLOGY OF CREPIDULA. 139 



As to the origin of other portions, no comparisons can be 

 drawn since Blochmann has made no further observations on 

 this point. The turret cells in Neritina are very small, and 

 have not divided up to the last stage in which they can be 

 recognized, and one cannot tell from Blochmann' s figures 

 whether they form any part of the velum or not. His figures 

 would indicate, though they would by no means establish 

 this point, that the velum does not branch dorsally as in 

 Crepidula. 



Heymons did not observe the origin of the velum in Umbrella. 

 However, the following statement quoted from his work (p. 278) 

 shows that in this animal the velum is the same in its general 

 appearance, and must occupy essentially the same position as 

 in Crepidula: "Die vorderste Partie des Eies, die dem friiheren 

 animalen Pol entspricht, wird von hellen grossen Ektodermzellen 

 bekleidet. Dieser ganze Theil setzt sich bald noch scharfer als 

 in friiheren Stadien von der iibrigen Masse des Eies ab, und 

 zwar geschieht dies besonders durch das Auftreten des Velums. 

 Letzteres beginnt sich gleich nach dem Verschluss des Gastru- 

 lamundes zu zeigen und besteht anfanglich aus einigen hellen 

 und kornchenreichen Ektodermzellen, die sich spater aneinander 

 legen und dann in einer kontinuirlichen Reihe rings um den 

 Embryo herum zlehen. Der von ihnen umschlossene Bezirk 

 ist als Velarfeld zu bezeichnen. Die Mitte desselben fallt mit 

 dem friiheren Centrum des animalen Poles zusammen, welches, 

 wie oben erwahnt wurde, mitsammt den Richtungskorpern an 

 das Vorderende des Embryonalkorpers gelangt war." Hey- 

 mons observed the first division of the four turret cells, but he 

 did not follow them to their destination. 



There is no velum or prototroch in Unio, and consequently 

 we need not be surprised to find certain cells which enter into 

 the velum in Crepidula diverted to other uses in that animal. 

 Thus Lillie finds that the cells 2a'-', 2b'', and 2c"S which are 

 velar cells in Crepidula, assist in forming the larval mantle of 

 Unio. He records two divisions of each of the turret cells, but 

 did not determine their destiny. 



Wilson, in his work on Nereis, first established the exact cell 

 origin of the prototroch among the annelids. In this animal it 



