Reproduction 253 



widely to the exterior, but the width of the opening is rapidly dimin- 

 ished by inbending of the wall about it and it is soon reduced to a 

 narrow blastopore. In consequence of this contraction of the wall 

 around the blastopore, the form of the entire gastrula tends at first 

 to become spheric, but before the contraction is completed the gastrula 

 begins to elongate in the direction of the axis which passes through 

 the blastopore. 



An important accessory activity attends this process of narrowing 

 the blastopore. The blastoporal rim is a region of transition from the 

 outer to the inner layer. This region is marked by very rapid prolifer- 

 ation of cells, especially at the dorsal edge of the blastopore (Fig. 

 210D). Cells produced within this growth zone or germ-ring are 

 added, some to the outer layer and some to the inner layer. This growth 

 process, then, is concerned both in the narrowing of the blastopore and 

 in the elongating of the embryo. A direct consequence of it is that the 

 material of a certain region of the inner layer immediately adjoining 

 the blastopore attained its internal position, not as result of the pri- 

 mary invagination, but by the secondary process of growth. 



At the close of the gastrula period (Fig. 210D) the embryo is an 

 elongated ovoid, the slightly larger end being anterior while the now 

 very narrow blastopore marks the posterior end of the long axis. So 

 rapid is development that this stage is attained about seven hours 

 after fertilization. 



In Amphibians. In the amphibian the vegetal wall of the blastula 

 (Fig. 207E-G) is so thick that the vegetal hemisphere is, in effect, solid. 

 It consists of large cells heavily laden with inert yolk. Such a wall 

 cannot readily bend inward as does the corresponding thin and labile 

 layer of the Amphioxus blastula. 



Three processes going on simultaneously effect gastrulation. The 

 beginning of gastrulation is seen when a crescent-shaped groove (Fig. 

 211 A, I) forms at a certain place on the surface of the blastula. It lies 

 just on the vegetal side of the equator determined by the animal and 

 vegetal poles and extends transversely to the median plane determined 

 by the first cleavage. The equator and a zone extending superficially 

 somewhat into the vegetal hemisphere are marked by especially rapid 

 cell-proliferation. It is in this particularly active region, the germ- 

 ring, that the groove appears. Figure 211A' represents a section in 

 the median plane of an embryo at this stage. The groove (I) is the 

 result of an invagination which occurs near where the upper thin wall 

 and lower thick wall of the blastula join. The outer layer bounding the 

 invagination consists of smaller cells which have moved inward from 

 the superficial germ-ring region; the deeper wall of the invagination 

 consists of yolk-cells. The groove, initiated as a slight invagination, 



