DEFINITION OF THE MAJOR PROCESSES OF GASTRULATION 107 



The blastula stage is held to a spherical aggregate by the presence 

 of this surface coating. However, since the coating is not itself divided 

 with each cleavage, but simply folds into the furrows between blas- 

 tomeres, the inner boundaries of the cells do not have the same domi- 

 nating force that is present on the outer surface. Such cells are found 

 to be interconnected by slender and temporary processes. The forma- 

 tion of cells, once thought to be the cause of gastrular movements, is 

 now thought to be merely a convenience for the execution of such 

 movements. 



Epiboly in the frog is the concern of this surface coat rather than 

 of individual cells. It is due to the expansive nature of the surface 

 coating while it is in contact with a suitable substratum. The potential 

 ectoderm (animal hemisphere cells) and the potential endoderm 

 (vegetal hemisphere cells) are, in fact, competitors in the tendency to 

 cover the surface area. The driving force is this surface coat, and for 

 some reason the potential of the animal hemisphere portion of it is 

 greater than the potential of the surface coat of the vegetal hemisphere. 



Definition of the Major Processes of Gastrulation 



Gastrulation is now recognized as an extremely complicated but 

 highly integrated and dynamic change in the embryo, brought about 

 by a combination of physical and chemical forces arising intrinsically 

 but subject to extrinsic factors. We do not yet fully understand the 

 process in any animal, particularly because of the elusive nature of the 

 forces involved. Gastrulation is concerned with cell movements, 

 changes in physical tension, and in the metabolism of carbohydrates, 

 proteins, and possibly even the lipids. 



Before attempting to describe the process as it occurs in the frog, 

 it would be well for us to have a clear-cut understanding of the mean- 

 ing of the various terms often encountered in such a description. The 

 student is advised to consult frequently the Glossary at the end of this 

 text, not only while reading this section but also throughout the dis- 

 cussion of embryology as a scientific subject. 



(Continued from opposite page.) 

 pore showing the accumulation of pigment and the stretching of the cells toward 

 the lines of invagination. (Bottom) A. piinctatum, blastula. Section of the pro- 

 spective blastoporal region. Note the black streaks of condensed coat material, 

 and the flask cells attached to it. (Courtesy, Holtfreter, 1943, J. Exper. ZooL, 

 94:261.) 



