588 INVERTEBRATA CHAP. 



meres. Of these four, two, which later are shown to be anterior, are 

 rather smaller than the others, so that even at this early period the 

 egg is bilaterally segmented. These four blastomeres in turn divide 

 into an upper and a lower tier, and so the 8-cell stage is attained ; 

 but the four upper cells, termed by Cerfontaine micromeres, are 

 rather smaller than the four lower cells termed macromeres. There 

 are therefore two larger and two smaller micromeres, and two larger 

 and two smaller macromeres. 



In attaining the 16-cell stage each cell divides into right and 

 left daughters by radial planes, so that two tiers of eight cells 

 should be formed ; but four macromeres move downward and four 

 micromeres upwards, so that four tiers of four cells each are formed. 

 Each cell then divides into upper and lower halves, and in the 



an 



FIG 428. Stages in the segmentation of the egg of Amphioxus lanceolatus. 

 (After Cerfontaine. ) 



A, 32-cell stage seen from the side. B, optical sagittal section of a young blastula. an.p, 

 animal pole of the egg ; pb z , second polar body ; veg.p, vegetative pole of the egg. 



32-cell stage we have actually eight tiers of four cells each. But, 

 as is shown in Fig. 428, this unstable arrangement does not persist. 

 The blastomeres glide on one another, and we get a circle of four cells 

 at both the animal and the vegetative pole, and three intervening 

 tiers of eight cells each. The tier at the animal pole consists of the 

 smallest cells, that at the vegetative pole of the largest, and in the 

 intermediate tiers we have a gradual passage from the one size to 

 the other. 



At the next period of cleavage the planes of division are no 

 longer parallel to one another in all the cells. The equatorial cells 

 divide into right and left daughters, but the planes of division in the 

 polar cells are oblique. After the next cleavage the egg has divided 

 into 128 cells, and these commence to flatten out against each other 

 and to take on the character of a columnar epithelium which is 

 ciliated. 



At the next stage, when there are 256 cells, the embryo is a 



