52 CONKLIN. [Vol. XIII. 



fore the polar furrow, while running in the same direction at 

 both poles, is distinctly shorter at the animal than at the vege- 

 tal pole. Diagram 2, d, represents the condition of the polar 

 furrow in C. plana ; it shows that in this egg, which has less 

 yolk than that of C. fornicata, the blastomeres A and C overlie 

 B and D still more than in the case last mentioned, and that 

 they meet in a point at the animal pole. There is here no polar 

 furrow at all at the animal pole, though the one at the vegetal 

 pole is well developed. In Diagram 2, e, which is a diagram- 

 matic representation of the &g^ of Discocoelis as described by 

 Lang, the macromeres A and C not only overlie B and D, but 

 they meet in a line, which forms a polar furrow at the animal 

 pole lying at right angles to the one at the vegetal pole. These 

 furrows may or may not be equal in length ; generally the one 

 at the animal pole is the shorter, though Lillie has found that 

 it is the longer in Unio, which is due to the fact that in 

 this case the cells at the animal pole are larger than those 

 at the vegetal. Finally, in Diagram 2, f, which represents 

 the 0.^^ of Botryllus, we find the greatest degree of com- 

 pactness of the blastomeres ; the polar furrows at the upper 

 and lower poles are nearly equal in length, and the individual 

 blastomeres no longer preserve independence of outline, but 

 are rounded into a nearly perfect sphere. Two or more of 

 these different forms may be found at different stages in the 

 cleavage of the same o.^^. At the moment of cleavage the 

 blastomeres are generally more independent and less compact 

 than during the " resting stages " between cleavages. Thus 

 in many ova the blastomeres at the moment of cleavage are 

 like those represented in Diagram 2, e, while during the 

 "resting period " they become much more compact, like those 

 shown in Diagram 2,/, 



Two types of ova are represented in the diagram given above, 

 one in which there is scarcely any polar differentiation, the other 

 in which it is well pronounced. The former is represented by 

 figures e and/, and in such cleavage forms polar concentration of 

 protoplasm and nuclei is impossible, the nuclei in fact lie near 

 the centres of the blastomeres and the yolk is uniformly distrib- 

 uted throughout the protoplasm ; the latter type is represented 



