MOVEMENT OF CELLS 397 



thus represents a small but extremely important step in the migration of cells 

 from the exterior to the interior during gastrulation. 



b. Invagination 



The phenomenon of invagination, as used in embryological development, 

 implies an infolding or insinking of a layer of cells, resulting in the formation 

 of a cavity surrounded by the infolded cells (figs. 189, 190, the entoderm). 

 Relative to gastrulation, this process has two aspects: 



( 1 ) mechanical or passive infolding of cells, and 



(2 ) active inward streaming or inpushing of cells into the blastocoelic space. 



In lower vertebrates, the dorsal-lip area of the blastopore is prone to exhibit 

 the active form of invagination, whereas the entoderm of the lateral- and 

 ventral-lip regions of the blastopore tends to move in a passive manner. The 

 notochordal-canal, primitive-pit area of the primitive streak of higher verte- 

 brates is concerned especially with the active phase of invagination. 



c. Concrescence 



This term is used in older descriptions of gastrulation. The word denotes 

 the movement of masses of cells toward each other, particularly in the region 

 of the blastopore, and implies the idea of fusion of cell groups from two 

 bilaterally situated areas. It probably does not occur. (However, see develop- 

 ment of the feather in Chap. 12.) 



d. Cell Proliferation 



An increase in the number of cells is intimately concerned with the process 

 of gastrulation to the extent that gastrulation would be impeded without it, 

 in some species more than in others. Cell proliferation in Amphioxus, for 

 example, is intimately associated with the gastrulative process, whereas in the 

 frog it assumes a lesser importance. 



e. Polyinvagination 



Polyinvagination is a concept which implies that individual or small groups 

 of cells in different parts of the external layer of the blastula or blastodisc 

 invaginate or ingress into the segmentation (blastocoelic) cavity. That is, 

 there are several different and separate inward migrations of one or more cells. 

 This idea recently was repudiated by Pasteels ('45) relative to the formation 

 of the entodermal layer in the avian blastoderm. It applies, presumably, to the 

 ingression of cells during the formation of the two-layered blastula in the 

 prototherian mammal, Echidna (see p. 364). 



/. Ingression 



The word ingression is suitable for use in cases where a cell or small 

 groups of cells separate from other layers and migrate into the segmentation 



