432 HOLMES. [Vol. XVI. 



portions of the first tzvo cleavage furrows. As Conklin ob- 

 serves, "no exception is known, either among moUusks or 

 annelids, to the rule that the second and fourth quartettes 

 lie in the future median and transverse planes, and that the 

 first, third, and fifth quartettes lie midway between these planes. 

 The axial differences, therefore, of the first two cleavage planes, 

 which have been mentioned, are differences merely in the axial 

 relations of the four primary entoderm cells, and do not affect 

 the axial relations of the other cells of the ovum, which are 

 always the same among annelids and mollusks." 



The behavior of the first cleavage furrows in Crepidula is 

 very instructive, for it proves that the coincidence of the lower 

 portion of the first cleavage furrow with the transverse axis is 

 not indicative of the axial relations of the first cleavage plane 

 at the time of its formation. In Crepidula, before the formation 

 of the fourth quartette, the two parts of the first cleavage fur- 

 row lie approximately in the same plane. Now the longitudinal 

 axis of the future embryo is definitely marked out at this period 

 by the direction of the anterior and posterior arms of the cross. 

 The first and second cleavage planes at this stage are oblique 

 to these arms of the cross, and hence to the fntiire longitudinal 

 axis of the embryo. Thus, as far as can be ascertained, the 

 original direction of the first cleavage plane is oblique ; subse- 

 quent shiftings of the cleavage furrows are of no concern. It 

 is to be noted that it is the upper parts of the first two furrows 

 that retain very nearly their original direction in relation to the 

 "median axis, while the axial relations of the lower portions 

 change. The shifting of the lower portions of the first cleav- 

 age furrows in no wise alters the fact that the original direction 

 of the first two cleavage planes is oblique. The transverse 

 direction finally taken by the lower portion of the first cleavage 

 furrow in Crepidula is, therefore, due to the fact that it has 

 become shifted in relation to the future median axis of the 

 embryo. 



In Nereis, according to Wilson, the first cleavage plane is 

 transverse, and the second coincides, approximately, with the 

 future median plane. It is evident, as Mead has pointed out 

 ('97, p. 301), that Wilson uses only the portion of the first 



