44 CONK LIN. [Vol. XIII. 



2. The Second Cleavage. Figs. "J— 10. 



The spindles usually appear simultaneously in the two blast- 

 omeres, Figs. 7, 9, though occasionally earlier in one than the 

 other, as shown in Fig. 8. The axes of the two spindles are 

 almost parallel to each other, and at right angles to that of the 

 preceding spindle. The two spindles are not quite parallel, how- 

 ever, as is shown in Fig. 7, where the spindles are laeotropic, 

 the left pole in each case being at a higher level in the egg 

 than the right one. Thus the axes of the spindles, when viewed 

 from the side, cross each other at a slight angle. It will also 

 be noticed in Fig. 7 that the entire spindle in each blastomere 

 lies somewhat to the left of the median plane of the blastomere. 

 The position and direction of the spindles in this case indicate, 

 before the division occurs, that the cleavage will be laeotropic. 

 The spiral character of the preceding cleavage could be ob- 

 served only after the division had occurred. 



The first cleavage furrow is at first a straight line as seen 

 from the animal pole. Fig. 6, but as the second cleavage comes 

 on, this line becomes bent slightly to the right when placed in 

 the line of vision, Figs. 7, 9. From the angles where this 

 bent portion joins the rest of the first furrow, the two halves 

 of the second cleavage run outward toward the periphery, Figs. 

 9, 10. The second cleavage really consists of two quite 

 independent furrows ; their ends never meet at the centre, and 

 one of them may appear somewhat earlier than the other. Fig. 

 8. These furrows begin to form near the animal pole and run 

 out around and through the blastomeres until they reach the 

 vegetal pole, completely dividing the two blastomeres into four, 

 which are approximately equal in size. 



3. The Origin and Significance of the Polar Fiirrozvs. Figs. 

 ^-12, Diagravi 2. 



The bent portion of the first furrow included between the 

 central ends of the second cleavage is a feature of considerable 

 practical as well as theoretical importance. It is a well-known 

 fact that there is, in the eggs of many animals, a furrow at the 

 intersection of the first and second cleavage planes, which does 



