No. I.] THE EMBRYOLOGY OF CREPIDULA. 59 



and they therefore form a valuable landmark for orientation. 

 From their peculiar position and shape I shall call them the 

 "turret cells"; their further history will be considered in 

 another place. 



After the division of the nuclei, and even after the cell body 

 has divided, the turret cells continue to rotate in a clockwise 

 direction until they lie at the ends of the furrows separating 

 the four apical cells. In this case, therefore, as in every other 

 which I have observed, the spi^-al character of the cleavage is 

 much more pronounced after the nuclear division than during 

 that division. It seems to be a phenomenon belonging to and 

 caused by the cytoplasm rather than the nucleus. 



In Discocoelis (Lang, '84), Nereis (Wilson, '92), and Limax 

 (Kofoid, '95) the first quartette divides at the time the second is 

 being formed, and before the third quartette is formed the first 

 has divided twice. In Planorbis (Rabl, '79), Neritina (Bloch- 

 mann, '81), Unio (Lillie, '95), and Crepidula the first quartette 

 divides once before the third is formed; while in Umbrella 

 (Heymons, '93) and Urosalpinx the first does not divide at all 

 before the third is formed. In general the rate of development 

 of the upper hemisphere is indicated by these facts ; in Nereis 

 the development of the upper hemisphere is very precocious ; it 

 is very tardy in Umbrella and Urosalpinx; while Planorbis, 

 Neritina, Unio, and Crepidula occupy an intermediate position 

 in this respect. 



In most gasteropods so far studied the turret cells have 

 essentially the same peculiarities of size and position as in 

 Crepidula, so that during the early stages of cleavage they can 

 be recognized at a glance. In Nereis Wilson has found that 

 these cells form the prototroch, and he therefore calls them the 

 trochoblasts. Mead ('94) also has found that they form a part 

 of the prototroch in Amphitrite and Clymenella. In Crepidula 

 at least two of these cells, probably all four, form a portion of 

 the velum ; but because I am not certain as to the destiny of 

 the two posterior ones (ic^ and id2), I prefer to call the group 

 for the present by a non-committal name. Their destiny has 

 not been determined in any other form. 



