JEXNIXGS: DEVELOPMENT OF ASPLAXCHNA HEREICKII. 33 



which are of the greatest importance, have been taking place, partly as 

 a consequence of these cleavages. As the constriction of the cyto- 

 plasm in d^-'^ and d^'^ occurs, these cells show in a most pronounced way 

 the tendency to become of a rounded form. The inner portions of the 

 cell are withdrawn still more from the centre of the egg, until the an- 

 tero-posterior measurements are no greater than their dorso-ventral 

 dimensions. This is shown in the section. Figure 38. In this egg, a 

 surface view of which is given in Figure 37, the cleavage of d""-'^ is 

 finished, and the products, d^-^ and c?^ *, have already passed into the 

 " resting stage," so that they take whatever form is impressed upon 

 them by the surroundings. But d^-^ is just dividing into d^-'° and rf®•^ 

 and the form shown in section by d^-^ in Figure 38, as compared with 

 the form of d^^ in Figure 32, shows the change which I have been 

 describing. 



At the same time the cells of the other quadrants, A, B, and 0, are 

 entering upon the stages preparatory to karyokinetic division. As a 

 first step they also retract their deeper parts and bring their protoplasm 

 into a more compact mass, as shown by a comparison of quadrant B 

 in Figure 34 (Plate 4) with the similar quadrant, A, in Figure 38 

 (Plate 5). 



As a consequence of this withdrawal of material from the inner parts 

 of the egg, the large ventral cell d^-^, which has now passed into the 

 resting stage, moves inwai'd to occupy the space which would otherwise 

 be vacant, — being forced to do so, of course, by the greater dorso-ventral 

 extension of all the other cells. The result is shown in Figure 38. 

 This partial enclosure of d^-^ by the other cells is of course a stage in 

 the process of gastrulation. 



Before the cleavage of all the cells of quadrant I) is finished, the 

 karyokinetic processes have begun in tlie other three quadrants. (See 

 Plate 5, Figs. 39-42.) The first cells to show the characteristic nu- 

 clear phenomena are those of the fourth or dorsal layer, a^-*-c^-*. As 

 previously stated, the asters at first take up such a position in these 

 cells as would lead, if unchanged, to a meridional cleavage. But in these 

 three cells, as in c?^•^ there is a revolution of the asters and nuclei, 

 resulting in a dorso-ventral position of the spindles. At any given period 

 the three cells are not in exactly the same phase of division, though very 

 nearly so ; the order, beginning with the most advanced, is c^-*, b^-'^, a^-*. 

 The sequence is thus the same as in previous cleavages. The division is 

 equatorial and the dorsal product is the smaller, as in the cleavage of 

 the corresponding cell (c?^"*) of the left posterior quadrant. In the three 



VOL. XXX. — NO. 1. 3 



