JEXNIXGS: DEYELOPMEXT OF ASPL.VXCHNA HERPJCKIL 23 



axis of the precediug spiudle. An optical section of the egg along its 

 chief axis, showing the asters in the cells B^ and D'^, is given in Plate 

 2, Fif. 10. Spindles are formed in all four cells nearly or quite in 

 the position indicated by the direction of separation of the asters. 



The tendency of the karyokinetic processes in the posterior half of 

 the egg to gain upon those in the anterior half, shown during the last 

 division, is continued and accelerated. Spindles appear in C^ and D^, 

 while the nuclei in A^ and B^ are still spherical and have distinct mem- 

 branes. Figure 9 gives a view of this stage from the right side ; the 

 large spherical nucleus of B^ is represented by a broken outline. The 

 spindle in C^ has a dorso-ventral direction, and its middle coincides with 

 the middle of the length of the cell ; the two asters are of equal size- 

 In Z^ tlie spindle is nearer the dorsal side of the egg, and is inclined, 

 passing from dorsal and anterior to ventral and posterior. The ventral 

 aster is the larger. 



Cleavage takes place first In the larger cell B^, separating a large 

 ventral blastomere, d*-^, from a smaller dorsal one, d*-K At the same 

 time the two cells (considered as a whole) elongate dorso-ventrally. 

 In so doing, the ventral blastomere, d*'^, remains nearly stationary, 

 while d*-^ moves in the direction of the animal pole of the egg. (Com- 

 pare Figure 12, a sagittal section of a five-cell stage, with Figure 

 10, the corresponding section of a four-cell stage, observing the posi- 

 tion of the cells in relation to the general form of the egg.) As a 

 result of this, the dorsal end of the cell B^, and, to a less -degree, the 

 ends of A^ and C^, are displaced in the same direction ; that is, the 

 whole animal pole moves toward the micromere end of the egg. At 

 the same time the cells A^, B^, and C^ are slightly compressed dorso- 

 ventrallj'. This is the beginning of that peculiar rotation of the blasto- 

 meres in the eggs of Eotifera, described by Zelinka ('91) and others, 

 which eventually results in the process of gastrulation. 



C^ divides next, the cleavage being equal; the products are c*'^ 

 and c^ -^ 



Before the cleavage is finished in B^ and C^, spindles have been 

 formed in B^ and A^, division taking place in them in the order named. 

 The cleavage is equal, as in C^. 



The order of cleavage, then, for the four cells, is as follows : B, C, B, A. 

 This rhythm reappears in later cleavages. 



The third cleavage is therefore equatorial, dividing the egg into two 

 layers of four cells each. The ventral cells are a*'^-d*-^, the dorsal cells 

 a^-^-d*'"^. The egg is still slightly unsymmetrical. 



