GASTRULATION. 7 1 



earlier stage (fig. 65), we notice that (a) almost the entire flattened rim of the 

 blastoderm has been added ; that (b) in the posterior portion of this rim the ectoderm 

 is already differentiating the medullary plate of the embryo, m ; that (c) the major 

 growth has taken place backward in witness of this, contrast the distance between 

 the anterior end of the archenteron and the posterior rim of the blastopore in these 

 two stages; that (cT) in this connection the main cell-mass extends itself dorsalward 

 and becomes the plateau-like region of the blastoderm ; that (<?) the germinal wall 

 rising abruptly beside the archenteron in the ear-tier stage becomes excavated in 



b f 



Fig. 66. Sagittal section of gastrula in which ihe embryo is appearing. 



i Archenleron. 



'> Region of outermost margin of ventral lip of blastopore. 



jnn, Posterior mass of cells (in position of ventral lip of blastopore). 



/( /'t Region of blastopore. 



"'. Thickening in medullary plate of embryo. 



*'', Segmentation cavity. 



Fig. 66 A. Section (lateral) parallel to the preceding. 



the later, a portion of its material, at least, being represented by the spongy mass 

 of cells which now forms the floor of the archenteron, a; that finally (/) there is 

 evidence that the posterior rim of the blastoderm is rolling inward, the surface of 

 the blastoderm growing more rapidly in this region than the lower layer with which 

 it is connected. Detailed examination of the sections, however, leads us to the 

 belief that the process of inrolling extends only as far as the point i (fig. 66) ; just 

 above this the inrolled rim of the blastoderm merges with the cells arising from 

 the germinal wall, and from this point inward openings occur between the cells 

 and communicate with the archenteron. The section (fig. 66 A) already referred 



