MOVEMENT OF CELLS 395 



to that condition which we call living. The Uving state is a problem which 

 awaits solution. 



At the period when the process of blastulation comes to an end and the 

 process of gastrulation is initiated, there is an urge directed toward cell move- 

 ment throughout the entire early gastrula. Needham ('42, p. 145) uses the 

 term "inner compulsion" to describe the tendency of the cells of the dorsal-lip 

 area to move inward (invaginate) at this time. Whatever it is called and 

 however it may be described, the important feature to remember is that this 

 tendency to move and the actual movement of the cells represent a living 

 process in which masses of cells move in accordance with the dictates of a 

 precise and guiding center of activity, known as the primary organizer or 

 organization center. 



2. Types of Cell Movement During Gastrulation 



The following types of cell movement are important aspects of the process 

 of gastrulation. 



a. Epiboly 



( 1 ) Extension along the antero-posterior axis of the future embryo. 



(2) Peripheral expansion or divergence. 



b. Emboly 



( 1 ) Involution. 



(2) Invagination. 



(3) Concrescence (probably does not occur). 



(4) Convergence. 



(5) Polyinvagination. 



(6) Delamination. 



(7) Divergence or expansion. 



(8) Extension or elongation. 



(9) Blastoporal constriction. 



Note: While cell proliferation is not listed as a specific activity above, it is 

 an important aspect of gastrulation in many forms. 



3. Description of the Processes Concerned with Epiboly 



Epiboly or ectodermal expansion involves the movements of the pre- 

 sumptive epidermal and neural areas during the gastrulative process. The 

 general migration of these two areas is in the direction of the antero-posterior 

 axis of the future embryonic body in all chordate embryos. In the rounded 

 blastula (e.g., frog, Amphioxus, etc.), the tendency to extend antero-posteriorly 

 produces an enveloping movement in the antero-posterior direction. As a 

 result, the presumptive epidermal and neural areas actually engulf and sur- 

 round the inwardly moving presumptive notochordal, mesodermal, and ento- 



