FORMATION OF THE LAYERS. 



When laid the egg is about O'iO5 mm. in diameter. It is in- 

 vested by a delicate membrane, and is somewhat opaque owing 

 to the presence of yolk granules, which are however uniformly 

 distributed through it, and proportionately less numerous than 

 in the ova of most Chordata. Impregnation is external and the 

 segmentation is nearly regular (fig. i). A small segmentation 



FIG. i. THE SEGMENTATION OF AMPHIOXUS. 



A. Stage with two equal segments. 



B. Stage with four equal segments. 



(Copied from Kowalevsky. ) 



C. Stage after the four segments have become divided by an equatorial furrow 

 into eight equal segments. 



D. Stage in which a single layer of cells encloses a central segmentation cavity. 



E. Somewhat older stage in optical section. 

 sg. segmentation cavity. 



cavity is visible at the stage with four segments, and increases 

 during the remainder of the segmentation ; till at the close (fig. 

 I E) the embryo consists of a blastosphere formed of a single 

 layer of cells enclosing a large segmentation cavity. One side 

 of the blastosphere next becomes invaginated, and during the 

 process the embryo becomes ciliated, and commences to rotate. 

 The cells forming the invaginated layer become gradually more 

 columnar than the remaining cells, and constitute the hypoblast; 

 and a structural distinction between the epiblast and hypoblast 

 is thus established. In the course of the invagination the seg- 



