Y4 J. Thos. Patterson. 



In order to understand fully the process of gastrulation, it will 

 be necessary to consider somewhat in detail a series of stages cover- 

 ing a period of at least thirteen hours preceding the involution of 

 the margin. Indeed, a knowledge of the entire history of cleavage 

 is necessary ; for all these early stages may be said to be preparatory 

 to gastrulation. It does not fall within the scope of this paper, 

 however, to consider these earlier periods. They have been studied 

 and described by Miss Blount ('07). According to her account 

 the supernumerary sperm nuclei disappear between ten and twelve 

 hours after fertilization, and the marginal cells then "open peripher- 

 ally and the periblast becomes organized with nuclei derived from 

 the cleavage nucleus." From this time on the blastodisc increases in 

 diameter by the addition of cells from the marginal and central peri- 

 blast, cells which are "individualized" about the periblastic nuclei. 



In the study of surface views of the developing egg, the changes 

 observed between twenty and twenty-eight hours after fertilization 

 are not very noticeable, for during the greater part of this period the 

 blastoderm appears as a white opaque disc, there being no differentia- 

 tion into areas opaca and pellucida. The disc, however, is not of 

 equal opacity in all places, for the central region is more opaque 

 than the marginal zone, these two parts gradually merging into each 

 other. From the twentieth to the twenty-fifth hour^ the margin 

 of the disc is very irregular and gradually fades out into the sur- 

 rounding zone of white yolk, which, for the most part, constitutes 

 the "marginal periblast." From the twenty-fifth to the twenty- 

 eighth hour the margin gradually becomes more regular and distinct, 

 and at the same time the central opaque region increases rapidly, 

 almost doubling its diameter. By the twenty-ninth hour the margin 

 is still more regular and distinct, and the circumference of the disc 

 is almost a circle (Fig. V, A). 



Between the twenty-ninth and thirty-first hours the entire disc 

 becomes more uniformily opaque, that is, the marginal region becomes 

 thicker. This condition lasts but a few minutes, for almost im- 

 mediately a small area lying just posterior to the center of the disc 



'Throughout this paper the age of the egg will be designated by the number 

 of hours that have elapsed after fertilization has taken place. 



