104 GASTRULATION 



when the nuclear to cytoplasmic volume relationship of the con- 

 stituent cells becomes most efficient, differentiation begins, and we can 

 observe the movements and surface changes attendant upon gastrula- 

 tion. 



Until recently the yolk hemisphere of the early cleavage stages of 

 the amphibian embryo was considered to be relatively inert. It was 

 even considered a deterrent to morphogenetic movements of gastrula- 

 tion. The vegetal hemisphere becomes cellular, but more slowly than 

 does the animal hemisphere. The cells are larger and always contain 

 abundant yolk. Nicholas (1945) wrote: "The concept of the inert- 

 ness of the yolk mass probably inhibited our realization of its possible 

 import." This attitude is now subject to change. 



By applying vital dyes to the yolk hemisphere of the early and late 

 blastula stages, Nicholas has found positive activity on the part of these 

 yolk-laden vegetal pole cells, suggesting that they are concerned with 

 the formation of the blastocoel, with the process of ingression, and 

 also with the changes in water balance and later rotation. It now 

 seems that the yolk assumes a dynamic rather than a passive role both 

 in the process of blastulation and in anticipating the changes pre- 

 requisite to gastrulation. 



In an exhaustive series of studies, Holtfreter and Schechtman have 

 attempted independently to analyze the morphogenetic movements 

 both before and during the gastrulation process in the amphibia. It is 

 very probable that the formative influences so evident at the time of 

 gastrulation are present long before the initial involution of cells to 

 form the dorsal lip of the blastopore. The following description is 

 based largely on the studies of these investigators and is supported by 

 direct observation, available to anyone. 



There is a protein-like surface coating or film on amphibian eggs, 

 present from the beginning. It has plastic elasticity, is not sticky on 

 its outer surface, but is an integral syncytial part of the living egg. This 

 coat persists and its strength increases during development. It is in- 

 volved in matters of cellular elasticity, cell aggregation, cell polarity, 

 cell permeability, resistance to external media, osmotic regulation, and 

 tissue affinity — all physical phenomena which are important in gas- 

 trulation movements. 



This surface coating is divided only superficially by the segmenting 

 blastomeres of the early embryo and yet (Holtfreter): "the syncytial 

 coat thus represents at least one, and perhaps the most significant one. 



