228 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



observer. lu Figure 19 the process of maturation is seen to be com- 

 pleted, the polar globules lying in a slight depression on the dorsal surface 

 of the egg. The 2-cell stage is shown in Figure 20. The 4-cell stage is 

 seen in Figure 21 to be approaching, and has been reached at the stage 

 shown in Figure 22. The two blastomeres on the side toward the 

 observer appear to be of equal size, the other two are hid from sight. 

 A view of the egg immediately after the next division is shown in Figure 

 23 ; the appearance nine minutes later is shown in Figure 24. These 

 both represent the 8-cell stage, and show that the four cells which lie 

 nearest the polar globules are smaller than those more remote. They also 

 show that division has occurred in such a manner that the pair of cells 

 occupying the upper right-hand corner of the figure is in contact with 

 the diagonally opposite pair of cells in the lower left-hand corner of the 

 figure, whereas the pair of cells in the upper left-hand corner is entirely 

 separated from that diagonally opposite it. This arrangement is due to 

 no accidental shoving of cells one over another, but is found invariably 

 occurring at the 8-cell stage. The diagonally opposite cells which are in 

 contact form respectively the posterior dorsal and anterior ventral por- 

 tions of the embryo. This arrangement of the cells of the 8-cell stage 

 has up to the present time been overlooked by all writers on tunicate 

 embryology except Ciiabry ('87). He both distinctly recognized and 

 clearly figured it. (See his Planche XVIII. Fig. 9.) But, as I pointed out 

 in a previous paper ('94), that hemisphere of the egg which he, following 

 Van Beneden et Julin, called dorsal, was really the ventral hemisphere, 

 so that he wrongly calls the cells in contact the anterior dorsal and pos- 

 terior ventral. If we correct his naming of the hemispheres, his obser- 

 vations on Ascidiella are brought into complete agreement with mine on 

 Ciona regarding this point. In both cases the posterior dorsal and ante- 

 rior ventral cells of the 8-cell stage are in contact. Though Seeliger ('85) 

 apparently overlooked the fact, his figures (Taf I. Figs. 7, 8, and 10), 

 when their orientation is corrected as I ('94) have shown to be necessary 

 for other reasons, pi'esent precisely the same arrangement of cells in the 

 8-cell stage of Clavelina. This condition is therefore probably of general 

 occurrence among the simple and social Ascidians. 



The 16-cell stage immediately after its formation is shown in Plate IV. 

 Fig. 25, and half an hour later in Figure 26. In the stage represented 

 by Figure 26, spindles, directed as indicated by the arrows, were already 

 visible in the large cells, occupying the lower half of the figure, though 

 none had yet appeared in the smaller cells composing the upper half of 

 the figure. This fact foreshadows an earlier division on the part of the 



