88 J. Thos. Patterson. 



I have said above that invagination takes place by a turning or 

 rolling nnder of the free margin. It is important to show that there 

 is a plain rolling under, and the following facts are offered as 

 proof. First, as regards the morphological evidence ; I think it is 

 clear from the above description that this line of proof strongly 

 supports the conclusion. There is no other explanation for the ap- 

 pearance of a cavity just beyond the posterior margin (Fig. 15, c) 

 than that it was brought about as the result of the rolling under of 

 the edge and of the simultaneous forward growth of the involuted 

 cells. 



This conclusion can be tested experimentally; for an injury made 

 on the edge of the thin posterior margin (Fig. IX) just previous 



Fig. IX. Scheme for the operation in Experiment I. 



to gastrulation ought to be carried down beneath the blastoderm 

 during the course of further development, that is, it ought to be 

 found in the entoderm. 



Experiment I. 



The operation was made thirty-three and one-fourth hours after 

 fertilization, and the egg was then incubated for three and three- 

 fourths hours. The result of the injuiy is shown in the posterior 

 end of the median section (Fig. 66). There is a distinct dorsal 

 lip, in which the deeper portion shows the cells affected by the opera- 

 tion. In the vitelline membrane, a short distance posterior to the 

 dorsal lip, is the break made by the operating needle (at op). All 

 of the injured cells are found in the entoderm, while the ectoderm is 

 well differentiated almost back to the end of the section. Just ante- 



