334 GENERAL EMBRYOLOGY 



ring, though this is not until the blastula has become converted 

 into the gastrula. This specialized group of cells may be 

 termed the germ ring, for it is evidently equivalent to the 

 structure already known by that name in the Fishes, Amphibia, 

 and other forms. At the time this rapid proliferation com- 

 mences, the vegetal hemisphere becomes flattened. The large 

 cells of this region then arch up slightly into the blastocoel and 

 soon begin to fold, or swing, inward about their postero- ventral 

 margin as a relatively fixed point (Fig. 151, B, C, D). This 

 motion is made possible by the rapid extension of a sheet of 

 cells which come off from the germ ring and which are thus 

 drawn in, to line the inside of the animal hemisphere. Without 

 going into details here, we may say that finally the inturning 

 of the vegetal cells becomes complete (Fig. 151, E) and the 

 blastula is converted into a cup-like structure, widely open 

 toward one side (the posterior or postero-dorsal). 



The wall of the organism is now composed of two layers or 

 epithelia, the original blastocoelic cavity is nearly or quite 

 obliterated, and a new cavity is formed, lined by the inturned 

 cells, and widely open to the outside. This structure is the 

 didermic gastrula. The two cell-layers composing its wall are 

 the primary germ layers. The newly established gastrular 

 cavity is the archenteron or primitive gut cavity. The super- 

 ficial layer of cells, including the original animal hemisphere of 

 the blastula and also some cells derived from the proliferating 

 area, is known as the outer germ layer, or ectoderm, or ectoblast, 

 or epiblast; the layer lining the archenteron, partly the cells of 

 the vegetal hemisphere of the blastula and partly the cells 

 derived from the proliferating region, is known as the inner 

 germ layer, or endoderm, or entoblast, or hypoblast. The opening 

 of the archenteron to the outside is the blastopore; the periphery 

 of the blastopore is spoken of as its margin or lip, and we have 

 seen that it is the region largely occupied by the germ ring: 

 it is here that the two primary germ layers are directly continuous 

 with one another. 



The process of infolding, such as is carried out here by the 

 vegetal cells which come to line the ventral region of the archen- 



