96 BLASTULATION 



Either on the surface of the whole blastula or in sections of the 

 blastula, one can determine the margins of the downward-moving 

 pigmented coat and active animal hemisphere cells and the conse- 

 quent slight thickening of the equatorial band. This margin is known 

 as the marginal zone, marginal belt, or germ ring. Such a marginal 

 region of activity, where yolk is most actively being transformed into 

 cytoplasm, is found in Amphioxus and chick, and probably in all 

 vertebrate embryos. It will have much to do with the subsequent 

 formation of the lips of the future blastopore. 



The marginal zone is approximately equatorial in the blastula 

 stage, except for the slight reduction in pigmented cells at the side 

 where the original gray crescent was located. Opposite to this gray 

 crescent region the wall of the blastula appears to contain relatively 

 more layers of cells and is therefore somewhat thicker. These changes 

 occur in anticipation of the next process in embryonic development, 

 namely gastrulation. 



There have been numerous attempts to explain blastocoel forma- 

 tion and the movements anticipating gastrulation. These have been 

 based largely on purely physical phenomena. An attempt has been 

 made recently to summarize these concepts, as follows (Holtfreter, 

 1947: Jour. Morph., 80:42): 



If present in large numbers, the cells at the periphery of the body tend 

 to establish a semipermeable layer, while the cells of the interior become 

 separated from each other by secretion fluid. Thus the aggregation may be 

 transformed into the configuration of a blastula or, if the amount of internal 

 fluid increases further, into an epithelial vesicle. The occurrence of cylin- 

 drical stretchings and of migrations of the peripheral cells into the interior 

 of the vesicle has been interpreted as possibly being caused by a surface 

 tension lowering action of the inner fluid. Even in the absence of a gradient 

 of surface tension, however, isolated embryonic cells tend to elongate and 

 to migrate in one direction, with their original proximal pole leading the 

 way, and possibly invagination movements may be partly due to this in- 

 herent dynamic tendency of the individual cell. Other factors influencing 

 shape and movements of the cells in aggregate, or in a normal embryo, 

 appear to be cell-specific differences of adhesiveness arising in the course 

 of differentiation and making for a sorting out, grouping and aggregation 

 of the various ceil types into tissues and organs. With the appearance of 

 fibrous and skeletal structures, of local differences in mechanical stress and 

 hydrostatic pressure, of differential growth and a variety of metabolic proc- 

 esses, so many new factors are introduced that the principle of interfacial 

 tension loses more and more of its original primacy as a morphogenetic 

 agent. 



