42 2 MORGAN. [Vol. X. 



portion of the disc. The middle of this region of more rapid 

 growth marks the middle line of the later embryo. 



The formation of the embryonic portion is due to two pro- 

 cesses. The tongue of cells growing forward beneath the 

 surface contributes to the greater opacity of this region, but 

 the result is more especially due to a relatively greater thick- 

 ness of the wall over the region where the embryo forms. 

 Here the blastoderm retains almost the same thickness that it 

 had in an earlier stage ; while in the extra-embryonic region a 

 rapid process of thinning takes place, until only a single layer 

 of columnar cells is left beneath the outermost layer (PI. 

 XXIV, Fig. 14, A). I have not been able to picture to myself 

 clearly the cell-migrations that bring about or are involved in 

 this process. The phenomenon is a most important one, and I 

 regret exceedingly that I have not mastered the situation. 



A few of the changes involved seem to be the following : The 

 area of the blastoderm enlarges somewhat while these changes 

 are taking place, as will be seen by comparing Figs, i and 2, 

 PI. XXIII. The larger surface of the crescentic extra- 

 embryonic region will give room only for the cells already at 

 the surface of that region. These superficial cells will have 

 also increased their surface areas, and will then occupy the 

 larger surface. It seems not improbable that, in addition, 

 some of the lower cells may push up and come to the surface. 

 If, however, all three layers of cells found in this region at 

 the earlier stage (Fig. i) were to come to the surface, a much 

 greater area would be necessary to accommodate them than 

 is actually present. I therefore infer that from this extra- 

 embryonic region the lower cells must actively migrate beneath 

 the surface to pass on the one hand into the embryonic 

 portion to help to keep it thick while it enlarges, and on the 

 other hand into the germ-ring, producing there a similar result. 



In the stage shown in Fig. i the thickness of the blastoderm 

 is formed by four vertical layers of cells (excluding the outer- 

 most flattened layer), PI. XXIV, Fig. 13, C. When the embry- 

 onic region forms, it is at first composed of four vertical layers 

 of cells (with the same exception), and is almost as thick as the 

 early blastoderm of the preceding stage (Fig. 14, E). But 



