loS 



DEVELOPMENT OF AMPHIOXUS. 



what smaller than the four lower cells at the vegetative 



pole, since the latter contain a greater quantity of the 



yolk-spherules (Fig. 55). 



The next cleavage giving rise to an embryo of sixteen 



cells is meridional. Then the eight upper and the eight 



lower cells become respectively 

 divided by equatorial cleavages, 

 and so the thirty-two cell stage 

 is reached (Fig. 56). 



The embryo is now known 

 as a blastnla, and consists of a 

 mulberry-like mass of cells sur- 

 rounding a central cavity called 



Fig. 56. Thirty-two cell stage, .... 



consisting of four rows of eight ceils, the segmentation-cavity or bias- 



each ranged around a central seg- fnrrp] 

 mentation cavity (blastocoel). The 



polar body is still visible at the ani- From this point of the de- 

 nial pole. (After HATSCHEK.) 



velopment the blastomeres go 



on dividing with more or less regularity, until the wall of 

 the blastula consists of a great number of cells arranged 

 in a single layer about the central cavity. 



The segmentation of the egg of Amphioxus, however, 

 by no means follows the uniform and stereotyped plan 

 that has been hitherto supposed. It has recently been 

 discovered by Professor E. B. WILSON that Amphioxus 

 presents an example of a polymorphic cleavage. Instead 

 of following one type, it follows three types of cleavage ; 

 namely, a radial type (Figs. 55 and 56), a bilateral type 

 (Fig. 57), and a spiral type (Fig. 58). These three types 

 of cleavage are reducible to a common basis, and are con- 

 nected together by all possible intermediate gradations. 

 Wilson points out that in the bilateral type of cleavage 

 Amphioxus shows a close correspondence with the Ascid- 

 ian embryo. 



