346 GENERAL EMBRYOLOGY 



the posterior margin of the blastoderm, where the germ ring 

 becomes elevated above the yolk as the endoderm is folded 

 under the ectoderm. In the Teleosts little or no indication of 

 invagination can be found. In these forms the germ ring 

 extends rapidly over the yolk, reaches and passes an equator 

 of the egg, and then as it continues to advance, gradually nar- 

 rows, and finally closes completely, having passed over the 

 entire yolk mass (Fig. 160). This overgrowth of the blasto- 

 derm occurs more rapidly in the anterior and lateral directions 

 than in the posterior direction, so that the blastopore finally 

 closes in nearly the same relative position as in the frog and in 

 Amphioxus, i.e., postero-ventrally. As the germ ring extends 

 around the yolk, only a single, and very thin, layer of cells is 

 left behind it as a covering layer. In the posterior and postero- 

 lateral regions alone, is the involution of an inner layer well 

 marked. It should be noted that in the Teleosts the endoderm 

 is largely replaced functionally by a specialized protoplasmic 

 region on the surface of the yolk, known as the periblast, which 

 contains free nuclei derived originally from those of the margin 

 of the blastodisc (Figs. 150, C; 157). 



During the later stages of the overgrowth of the germ ring, 

 as it contracts after passing the equator of the egg, its sub- 

 stance is payed into its more slowly advancing posterior region, 

 where it forms a longitudinal median thickening (Fig. 160). 

 This thickened region of the blastoderm is the primitive streak, 

 the earliest rudiment of the essential parts of the embryo, which 

 gradually differentiate out of its anterior end. 



In such a gastrula as this the endoderm forms a flat median 

 plate of cells lying directly upon the surface of the periblast 

 (yolk), and the archenteron is present only virtually as a narrow 

 space between the endoderm and periblast (Fig. 157). In such 

 a case the formation of a true gut cavity is independent of the 

 formation of the inner layer, and occurs later by a process of 

 folding. 



The mesoderm is differentiated at a comparatively early stage, 

 and the distinction between peristomial and gastral mesoderm 

 is very clear. The peristomial mesoderm appears as a small 



