ESTABLISHMENT OF THE ENTODERM 23 



In gastrulation the single cell layer of the blastula is doubled 

 upon itself to form two layers. The outer cell layer is known 

 as the ectoderm and the inner layer as the entoderm. These 

 layers differ from each other in their positional relationship to 

 the embryo and to the surrounding environment. Each has 

 different functional potentiaHties and each will in the course of 

 development give rise to quite different types of structures and 

 organs. It is the importance of their later history rather than 

 any complexity or veiled significance about the way in which 

 they arise that attaches such importance in embryology to the 

 estabhshment of these two layers. 



In the gastrulation of Amphibian embryos (Fig. 6) the yolk 

 forces the invagination of the blastoderm toward the animal 

 pole, but the inpocketing takes place into the blastocoele and 

 the interrelationships of ectoderm, entoderm, and gastrocoele 

 are established in fundamentally the same way as in Amphioxus. 

 Gastrulation in birds is greatly modified by the large amount 

 of yolk present (Fig. 6). Infolding must be effected in a disc 

 of cells resting like a cap on a large yolk sphere. The smallness 

 of the blastocoele sharply restricts the space into which the 

 invagination can grow. Instead of arising as a relatively 

 large circular opening the blastopore appears as a crescentic 

 sHt at the margin of the blastoderm. The crescentic blastopore 

 may be regarded as a potentially circular opening which has 

 been flattened as it develops between the growing disc of cells 

 and the unyielding yolk which underhes them. The invagi- 

 nated pocket of entoderm which grows in from this compressed 

 blastopore is also flattened, conforming to the restrictions 

 of the shape and size of the blastocoele. Moreover the floor 

 of the invagination is represented only by a few widely scattered 

 cells lying upon the yolk. It is as if the lower layer in its in- 

 growth was impeded and broken up by the yolk. The scattered 

 cells representing the floor of the invagination soon disappear and 

 the yolk itself comes to constitute the floor of the gastrocoele. 

 Notwithstanding the great displacement of the blastopore and 

 the gastrular invagination toward the animal pole and the 

 restricted size and incomplete floor of the gastrocoele, the cell 

 layers and the cavity established can be homologized with the 

 corresponding features in forms where the course of develop- 

 ment has not been so extensively modified by yolk. 



