394 GASTRULATION 



with external conditions. One of the important intrinsic factors involves the 

 so-called organization center referred to in Chapter 7. However, before con- 

 sideration is given to this center, we shall define some of the major processes 

 involved in gastrulation. 



There are two words which have come into use in embryology relative to 

 the process of gastrulation, namely, epiboly and emboly. These words are 

 derived from the Greek, and in the original they denote motion, in fact, two 

 different kinds of motion. The word emboly is derived from a word meaning 

 to throw in or thrust in. In other words, it means insertion. The word epiboly, 

 on the other hand, denotes a throwing on or extending upon. These words, 

 therefore, have quite opposite meanings, but they aptly describe the general 

 movements which occur during gastrulation. If, for example, we consider 

 figure 169, these two words mean the following: All the presumptive organ- 

 forming areas below line a-b in (C) during the process of gastrulation are 

 moved to the inside by the forces involved in emboly. On the other hand, 

 due to the forces concerned with epiboly, the presumptive organ-forming 

 materials above line a-b are extended upon or around the inwardly moving 

 cells. 



Associated with the comprehensive molding processes of epiboly and emboly 

 are a series of subactivities. These activities may be classified under the fol- 

 lowing headings: 



( 1 ) morphogenetic movement of cells, 



(2) the organization center and its organizing influences, and 



( 3 ) chemodifferentiation. 



C. Morphogenetic Movement of Cells 



1. Importance of Cell Movements During Development 

 AND IN Gastrulation 



The movement of cells from one place in the embryo to another to establish 

 a particular form or structure is a common embryological procedure. This 

 type of cell movement is described as a morphogenetic movement because it 

 results in the generation of a particular form or structural arrangement. It is 

 involved not only in the formation of the blastula where the movements are 

 slow, or in gastrulation where the cell migrations are dynamic and rapid, but 

 also in later development. (See Chap. 11.) In consequence, we may say that 

 cell migration is one of the basic procedures involved in tissue and organ 

 formation. 



The actual factors — physical, chemical, physiological, and mechanical — 

 which effect cell movements are quite unknown. However, this lack of knowl- 

 edge is not discouraging. In fact, it makes the problem more interesting, for 

 cells are living entities utilizing physicochemical and mechanical forces peculiar 



