5 o CONK LIN. [Vol. X 1 1 1 . 



the first two cleavages, yet that explanation lies so near the 

 surface that I should not take the trouble to enter upon that 

 subject here were it not for the fact that I have a few sug- 

 gestions to make which are not found in his work. 



The polar furrows are in all cases the result of spiral cleav- 

 ages, and the direction of the polar furrows relative to the 

 first and second cleavages is always dependent upon the 

 direction of the spirals. Because the second cleavage is 

 laeotropic, the vegetative polar furrow bends to the right in the 

 first cleavage and to the left in the second ; in Physa, in which 

 the direction of the spirals is reversed, the direction of the 

 polar furrows is reversed. 



The cause of these relations can be made plain by means of 

 the accompanying diagram. In Diagram 2, a, the macromeres 

 B and D lie at a slightly lower level than A and C, and have 

 given off A and C by a laeotropic division. It is seen in this 

 figure that there is but one polar furrow, and that it turns to 

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

 seen in the second. This is the state of affairs which prevails 

 in Crepidula, Neritina, Umbrella, Urosalpinx, etc. Let us 

 suppose, however, that the passage from the two to the four- 

 cell stage had taken place in the reverse direction as it does 

 in Physa, and as is shown in Diagram 2, b, where A and C lie at 

 a somewhat lower level than B and D, and have given off the 

 latter by a dexiotropic division. There is here but one polar 

 furrow, and when seen in the first furrow it turns to the left ; 

 when in the second furrow, to the right. It is evident, there- 

 fore, in all those cases where there is but one polar furrow 

 which turns to the right when seen in the first furrow, and 

 to the left when seen in the second, that the second cleavage 

 was laeotropic. 



As a rule when there is but one polar furrow, it is somewhat 

 shorter at the formative than at the vegetative pole. Diagram 2, c. 

 Yet as an extreme case there are found ova in which the single 

 polar furrow is almost equal in length at the two poles ; this is 

 admirably illustrated by the Qg% of C. convexa. Diagram 2, a, 

 which is laden with a large quantity of yolk, and in which the 

 macromeres A and C, while lying at a slightly higher level than 



