No. I.] THE EMBRYOLOGY OF CREPIDULA. 53 



by figures a to dm. which the polar concentration of protoplasm 

 and nuclei is very marked. In all eggs in which there is but 

 one polar furrow there is decided polar differentiation of the 

 yolk and protoplasm ; where two polar furrows are present this 

 segregation is less pronounced. 



The statement that the polar furrow turns to the right when 

 seen in the plane of the first cleavage is true only when there 

 is one polar furrow, and that the one at the lower pole. When 

 there are two polar furrows, as in Diagram 2, ^and/, the lower 

 one still preserves this same relation when seen from the 

 animal pole, while the upper one bends to the left when seen 

 in the first furrow, and to the right when seen in the second. 

 Of course, if these were viewed from the vegetal pole, the 

 relations would be reversed. 



The fact that in very many cases the first cleavage is dexio- 

 tropic and the second cleavage laeotropic is a profoundly 

 important and significant one, determining as it does, not only 

 the direction and relation of the polar furrows, but also influ- 

 encing more or less the character and directio7i of every succeeding 

 cleavage. They are the first of a long series of spiral cleavages 

 which take place alternately to the right and to the left, each of 

 which, except the first, finds the sufiicient cause of its direction 

 in the direction of the preceding cleavage. 



4. The Axial Relations of the First Two Cleavages. 



Throughout the course of segmentation the four macro- 

 meres remain very much larger than the cells to which they give 

 rise, and as they do not change their relative position, at least 

 until about the time of the closure of the blastopore, it becomes 

 very easy to orient all the future furrows and cells with 

 reference to the first two cleavages. If we examine one of 

 the later stages, such as Figs. 61 and 64, in which the 

 antero-posterior axis of the embryo is well marked by the 

 elongated blastopore, we find that the four macromeres and 

 the polar furrow are still recognizable, and that the cleavage 

 line in which the polar furrow bends to the right, i.e., the first 

 cleavage, is transverse to the antero-posterior axis of the 



