396 GASTRULATION 



dermal areas. (Study fig. 190A-H.) In flattened blastulae the movements of 

 epiboly are concerned largely with antero-posterior extension, associated with 

 peripheral migration and expansion of the epidermal area. (See fig. 202.) The 

 latter movement of the presumptive epidermal area is pronounced in teleost 

 fishes, where the yolk is engulfed as a result of epidermal growth and expan- 

 sion (figs. 210B; 21 ID). 



The above-mentioned activities, together with cell proliferation, effect 

 spatial changes in the presumptive epidermal and neural areas as shown in 

 figures 189, 190, 191, 198, and the left portion of figure 202A-I. It is to be 

 observed that the epidermal crescent is greatly expanded, and the area cov- 

 ered is increased; also, that the neural crescent is changed into a shield-shaped 

 area, extended in an antero-posterior direction (figs. 192A; 2021). 



4. Description of the Processes Involved in Emboly 



While forces engaged in epiboly are rearranging the presumptive neural 

 and epidermal areas, the morphogenetic movements concerned with emboly 

 move the presumptive chordamesodermal and entodermal areas inward and 

 extend them along the antero-posterior axis of the forming embryo. This in- 

 ward movement of cells is due to innate forces within various cell groups; 

 some apparently are autonomous (i.e., they arise from forces within a par- 

 ticular cell group), while others are dependent upon the movement of other 

 cell groups.) (See p. 447.) We may classify the types of cell behavior during 

 this migration and rearrangement of the chordamesoderm-entodermal areas 

 as follows: 



a. Involution and Convergence 



Involution is a process which is dependent largely upon the migration of 

 cells toward the blastoporal lip (e.g., frog, see heavy arrows, fig. 192) or 

 to the primitive streak (e.g., bird, see arrows, fig. 204C-E). The word involu- 

 tion, as used in gastrulation, denotes a "turning in" or inward rotation of 

 cells which have migrated to the blastoporal margin. In doing so, cells located 

 along the external margin of the blastoporal lip move over the lip to the inside 

 edge of the lip (see arrows, figs. 191C-E, H; 192B, C). The inturned or in- 

 voluted cells thus are deposited on the inside of the embryo along the inner 

 margin of the blastopore. The actual migration of cells from the outside surface 

 of the blastula to the external margin of the blastoporal lip is called con- 

 vergence. In the case of the primitive streak of the chick, the same essential 

 movements are present, namely, a convergence of cells to the primitive streak 

 and then an inward rotation of cells through the substance of the streak to 

 the inside (arrows, fig. 204; black arrows, fig. 202). If it were not for the 

 process of involution, the converging cells would tend to pile up along the 

 outer edges of the blastoporal lip or along the primitive streak. Involution 



