250 OUTLINES OF CHORDATE DEVELOPMENT 



laterally as well as anteriorly (Fig. 91). During these early 

 stages the endoderm cells are considerably scattered, and are 

 arranged as a solid mass or definite layer only in the region of 

 the blastoporal margin. As the endoderm cells gradually ex- 

 tend across the segmentation cavity, two other processes become 

 apparent (Fig. 92, C). First, the free posterior extremities 

 of the germ wall approach, and finally meet and fuse; this proc- 

 ess, is known as the closure of the blastopore, the blastoporal 

 opening being present only virtually, and represented by the 

 region between the extremities of the germ wall. The second 

 process is the beginning of the extension of the blastoderm, or 

 gastrula, from every side save the region of the blastoporal 

 margin. The endoderm cells soon extend out to the margin of 

 the segmentation cavity in every direction except anteriorly. 



The closure of the blastopore produces a small thickened 

 region, representing the contracted blastoporal margin, which 

 is left just within (anterior to) the germ wall, where this now 

 becomes continuous posteriorly. The process of extension of 

 the blastoderm now involves this region and leaves the con- 

 tracted blastoporal margin well within the area pellucida. 



Normally the egg is laid twenty-two to twenty-three hours 

 after fertilization, in approximately this condition, with gastru- 

 lation not quite completed. The structure of the unincubated 

 blastoderm may therefore be described as follows. Three 

 general regions are to be distinguished. The original area pel- 

 lucida is surrounded by a complete area opaca, and beyond this 

 is a narrow margin where the blastoderm cells are extending 

 over the surface of the yolk. The pellucid area is formed by a 

 layer of ectoderm cells, slightly thickened toward the middle 

 of the area; posteriorly it includes also a sheet of endoderm 

 cells. The endoderm is in the form of a definite layer only 

 toward the posterior margin, elsewhere the endoderm cells are 

 scattered through the segmentation cavity, in the process of 

 migrating toward its anterior margin. A narrow space between 

 the endoderm and yolk, really the remains of the segmentation 

 cavity there, is the rudiment of the archenteron. The area 

 opaca, save in its posterior region, is formed by the thickened 



