THE DEVELOPMENT OF AMPHIOXUS 13 



consist of two smaller anterior, and two larger ventral cells, 

 and the four macromeres of the vegetal pole, of two smaller 

 dorsal, and two larger posterior cells, the smaller macromeres 

 being larger than the larger micromeres. These relative in- 

 equalities in the quadrants, as well as their bilateral symme- 

 try, continue throughout cleavage. 



Strict synchronism of cleavage is lost in passing from four to 

 eight cells, the smaller cells commencing their division before 

 the larger. In the early stages the intervals between successive 

 cleavages are such that, in spite of lack of synchronism, it is 

 still possible to speak of sixteen- and thirty-two-cell stages, 

 but after this the numerical progression is no longer regular. 



Were the cleavage strictly regular the eight cells would 

 form sixteen by meridional cleavages passing symmetrically 

 through the entire egg, but as a matter of fact, during their 

 formation the spindles of the eight blastomeres change their 

 position so that the cleavage planes of the micromeres are 

 nearly perpendicular to the median plane, while those of the 

 macromeres are rather parallel with this plane. As a result 

 the sixteen-cell stage (Fig. 5, C) consists of eight micromeres 

 arranged in two rows of four cells each, parallel with the median 

 plane and becoming arched in the same direction, and of 

 eight macromeres in two rows of four cells each extending 

 across the median plane, i.e., from right to left, and arched in 

 this direction. The relation between the groups of micro- and 

 macromeres is much like that of the two hands when partly 

 closed and one in the palm of the other. The thirty-two- 

 cell stage (Fig. 5, D) is formed by the horizontal (latitudinal) 

 divisions of each of the sixteen, forming eight meridional rows 

 of four cells each. After thirty-two cells (Fig. 5, E) the 

 divisions become irregular in their appearance. To sum- 

 marize: 



Cleavage 1st 2nd 3rd 4th 5th 



f f 2 small = 4 small = 8 cells 



I 4 micr. < n , . , 



1 2 large = 4 large = 8 cells 

 Ovum = 2 cells = 4 cells = < 



f2 small = 4 small = 8 cells 



I 4 :r ' \ 2 large = 4 large = 8 cells 

 Total number of cells 24 8 16 32 



