REPRODUCTION 63 



blastula whose blastocoele is the subgerminal cavity, while its blastoderm 

 is the animal much-less-than-" hemi "-sphere and the yolk mass is the 

 vegetal much-more-than-"hemi "-sphere. This recognition of a blastula 

 stage, comparable to that of Amphioxus, in the development of a reptile or 

 bird would hardly have been possible but for the intermediate condition 

 exhibited by the amphibian with its moderate yolk mass and total cleavage. 



The blastula is to be conceived as an essentially one-layered stage of 

 the embryo, the "layer" being the wall of the blastula, whether one cell 

 thick or more than one cell thick. This stage has two-fold significance. 

 Its immediate importance is in the fact that it gives the embryonic mate- 

 rial increased superficial contact with the environment, thus favoring 

 metabolism. Its prospective significance lies in the fact that further 

 development is to consist, to a large extent, in the manipulation of layers 

 of embryonic material. The adult is hollow. It has a body cavity and 

 other cavities. Most of its organs are hollow. The walls of the hollow 

 structures are constituted of layers — skin, epithelium, endothehum, 

 peritoneum, muscle layers, connective tissue layers. For the construc- 

 tion of such a many-layered thing, the embryo naturally proceeds as early 

 as possible to dispose its building material in the form of layers. 



Gastrula 



In Amphioxus. The blastula stage is briefly transitory. At once 

 changes set in which transform it to a two-layered embryo. In Amphioxus 

 the two-layered gastrula form is attained in a very simple way (Fig. 42). 

 The vegetal hemisphere first flattens, then becomes curved inwards. 

 The infolding (invagination) continues until the material of the original 

 vegetal hemisphere comes into close relation with the inner surface of the 

 wall of the animal hemisphere. The spherical blastula thus becomes an 

 approximately hemispherical embryo whose wall is two layers thick 

 (Fig. 42C). As the process goes on the blastocoele is reduced and finally 

 obhterated. The gastrula is hollow. Its cavity, resulting from the 

 invagination process, at first opens widely to the exterior but the width 

 of the opening is rapidly diminished 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 spherical, 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 (Fig. 4 2D). This region is marked by very rapid pro- 

 liferation of cells, especially at the dorsal edge of the blastopore. Cells 



