XIII 



421 



After a pause a smaller cell III is given off from the large cell I 

 on its ventral side ; this cell then slides back towards the posterior 

 end and pushes the cell b, which till now has occupied that position, 

 on to the dorsal surface. Then II divides into II X and II 2 by an 

 oblique cleavage-plane ; but this plane rotates till it is transverse 

 and IIj is in front of II 2 . About an hour later a divides into 

 a v and a. 2 , and b into b l and b. 2 , by planes of division parallel to the 

 long axis of the egg. Then I gives off, just as it did before, a cell on 

 the ventral side. This is denominated IV, and it slides back in the 

 same way as did III to the posterior end of the egg ; & 2 and \ are 

 now both dorsal, and a t has rotated till it is in front of a.,, and \ is 

 in front of b. 2 (Fig. 331). 



In this 9-cell stage, therefore, the large cell I occupies the 

 anterior pole of the egg, and behind it are four longitudinal rows of 

 two cells each, making up the rest of the 



I 



egg, thus % IV H! b 1 . The large cell I gives 



a 2 III II 2 & 2 



off still another small cell V on the ventral 

 side, which passes backwards, pushing III 

 back so that it occupies the posterior pole 

 of the egg. Then the three lateral and 

 dorsal rows of cells, i.e. II : - II 2 , a l - .,, 

 and \-bfr divide by cleavage -planes, 

 transverse to the long axis of the egg, into 

 rows of four cells each. 



Soon III divides into I'il l in front, and 

 III 9 behind, and this, raises the ventral row 



-V, IV, 



III to the number of four 



Flo. 332. Sixteen-cell stage in 

 the segmentation of the egg 

 of Callidina russeola. (After 

 Zelinka.) 



Where a cell has divided into 

 right and left sisters these are 

 marked with the suffixes r and 7. 



cells. The large cell I and the three upper- 

 most cells of the three dorsal rows, viz. a v 

 b v IIj, are distinguished from the rest by 

 the greater abundance of yolk granules 

 in them. 



The next change is the doubling of the ventral row by cleavage- 

 planes parallel to the long axis of the egg. We have now five 

 longitudinal rows of four cells each, two of these being ventral and 

 three dorsal. 



But soon the dorsal rows begin to be doubled in the very same 

 manner ; the granular cells, however, usually remain undivided, but 

 occasionally become divided into right and left sisters as shown in 

 Fig. 333, while the cell I gives off, for the last time, a cell VI on the 

 ventral side ; the latter slides back in the usual manner and presses 

 the daughters of III! and III 2 on to the dorsal surface. The effect 

 of this is to shove forward the capping cells of the dorsal rows, which, 

 as we have already seen, are distinguished from the rows to which they 

 belong by being more granular. Thus, pushed forward, this crown of 

 three granular cells begins to overarch the front end of the large cell I. 



