2 FORMATION OF THE LAYERS. 



Chordata. Impregnation is external and the segmentation is nearly 

 regular (fig. 1). A small segmentation cavity is visible at the stage 



FIG. 1. THE SEGMENTATION OF AMPHIOXUS. (Copied from Kowalevsky. ) 



A. Stage with two equal segments. 



B. Stage with four equal segments. 



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. 



with four segments, and increases during the remainder of the seg- 

 mentation ; till at the close (fig. 1 E) the embryo consists of a blasto- 

 sphere formed of a single layer of cells enclosing a large segmenta- 

 tion 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 hypo- 

 blast ; and a structural distinction between the epiblast and hypo- 

 blast is thus established. In the course of the invagination the 

 segmentation cavity becomes gradually obliterated, and the embryo 

 first assumes a cup-shaped form with a wide blastopore, but soon 

 becomes elongated, while the communication of the archenteron, or 

 cavity of invagination, with the exterior is reduced to a small blasto- 

 pore (fig. 2 A), placed at the pole of the long axis which the sub- 

 sequent development shews to be the Jiinder end of the embryo. 

 The blastopore is often known in other Chordata as the anus of 

 Rusconi. Before the invagination is completed the larva throws off 

 the egg-membrane, and commences to lead a free existence. 



Up to this stage the larva, although it has acquired a cylindrical 

 elongated form, has only the structure of a simple two-layered gas- 

 trula; but the changes which next take place give rise on the one 



