No. I.] THE EMBRYOLOGY OF CREPIDULA. 47 



There are also objections to the word <' Brechungslinie," 

 proposed by Rauber ; it is not a breaking line, nor a broken 

 portion of a line, and the name indicates nothing with 

 regard to its position. Moreover the fact that the " Brechungs- 

 linie " is not constant in position indicates that it is not the 

 result of a determinate series of spiral cleavages, as is the 

 case among annelids and mollusks, but that it is merely a 

 " pressure surface," the result of surface tension, and it there- 

 fore has no reference to the character of the cleavage, which 

 might be radial or bilateral or neither. This term, therefore, 

 even if unobjectionable for the purpose for which it was employed 

 by Rauber, ought not to be applied to the furrow in question. 



The expression " polar furrow," however, is open to none of 

 the objections mentioned ; this furrow is found only at the two 

 poles of the egg, and so far as the name is descriptive at all, it 

 is quite accurate. I shall, therefore, use it exclusively here- 

 after to designate that portion of the first furrow which lies 

 between the central ends of the second furrow, both at the 

 animal and vegetal poles. Although always and entirely a 

 part of the first furrow, it seems to lie in, and form a part of, 

 both the first and second furrows. 



Although in different animals the polar furrow may bear no 

 constant relation to the embryonal axes, it does in all known 

 cases of spiral cleavage bear a very constant relation to the 

 first and second cleavages. In Crepidula, for example, if the 

 first furrow be placed in the line of vision, the polar furrow 

 always bends to the right, in the second furrow it bends to 

 the left, and this is true whichever end of the furrow is nearer 

 the observer. These relations are true only when the Q.g^ is 

 viewed from the animal pole ; obviously they would be reversed 

 if seen from the vegetal pole, i.e., the polar furrow would bend 

 to the left when in the first furrow and to the right when in 

 the second. This relation is of great practical importance, 

 since it enables one to distinguish at a glance the first furrow 

 from the second, even up to an advanced stage, and it thus 

 forms a ready means of orientation. In Fig. 10 and all suc- 

 ceeding stages it is impossible to distinguish between the first 

 and second furrows except in this way ; in Figs. 8 and 9, how- 



