No. I.] THE EMBRYOLOGY OF CREPIDULA. 49 



A phenomenon so widespread and so striking cannot be 

 wholly adventitious and without significance. As we have 

 seen, Blochmann explains the formation of the polar furrow 

 in Neritina by the fact that one of the first two blastomeres 

 divides before the other one. This would not explain the 

 constant relation of the polar furrow to the first and second 

 cleavages unless in all the groups mentioned one blastomere 

 divided earlier than the other one, and this of course is not 

 the case. 



Rauber ('82) attributes the formation of the " Brechungs- 

 linie " to a tendency on the part of all the furrows to avoid the 

 pole. This, of course, is not true of the first furrow, and in 

 any case it is no explanation of the phenomenon. Jordan and 

 Eycleshymer ('94) are right when they say (p. 412), "The 

 furrows do not avoid the pole ; but the mechanical cell-stresses 

 are rarely so adjusted that the furrows intersect at the pole. 

 There seems no need for a special term — ' Polflucht ' — to 

 express this fact, since the ' shunning ' of the pole can hardly be 

 a matter of primary significance." But while surface tension 

 is a sufficient causal explanation of such pressure surfaces as 

 the " Brechungslinie," this principle alone is not able to explain 

 the constant position of the polar furrow with reference to the 

 first two cleavages, and this constant position is a matter of 

 primary significance. 



In his classical work on Nereis, Wilson ('92) has carefully 

 described the polar furrows, and has pointed out the fact that 

 they are of great value in the orientation of the G^gg and 

 embryo.^ The position of these furrows is precisely the same 

 in Nereis and Crepidula, except that there is a short polar 

 furrow at the upper pole in Nereis which is generally wanting 

 in Crepidula. In the last section of his paper Wilson points 

 out the significance of the "cross furrow," and although he 

 does not directly explain the cause of its constant relation to 



ones on the dorsal and ventral sides of the egg. As my account is in substan- 

 tial agreement with Kofoid's, and as it touches upon a few points not mentioned 

 by him, I have allowed it to stand as first written. 



1 I had earlier ('91) called attention to the fact that the polar furrow bears a 

 constant relation to the first two cleavages, but had attempted no explanation of 

 this fact. 



