REPRODUCTION 43 



The blastula is an essentially one-layer 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 that it gives the embryonic material increased superficial 

 contact with the environment, thus favoring metaboHsm. 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, endothelium, peritoneum, muscle layers, 

 connective tissue layers. For the construction 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. 38). 

 The vegetal hemisphere first flattens, then becomes curved inward. 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 approx- 

 imately hemispherical embryo whose wall is two layers thick (Fig. 38C). 

 As the process goes on the blastocoele is reduced and finally obliterated. 

 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. This region is marked by very rapid proliferation 

 of cells, especially at the dorsal edge of the blastopore (Fig. 38!)) . 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 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 primary invagination but by the 

 secondary growth process. 



