444 



PRINCIPLES OF EMBRYOLOGY 



refraction which would indicate the appearance of a fibrous structure of 

 the cytoplasm (Waddington 1940). The neck is certainly very contractile, 

 and its tendency to contract almost certainly plays a part in pulling the 

 elongating dorsal material inwards and thus beginning the gastrulation 

 process (Fig. 20.13). 



A movement of one piece of tissue towards the interior of another tissue 

 mass can, however, occur with little sign of the participation of flask cells 



Figure 20.14 

 Engulfment by the endoderm. Row i, a small fragment of vmcoated endo- 

 derm becomes incorporated into an endodermal substratum. Row 2, a 

 fragment of blastoporal cells, partly covered by coat, behaves similarly but 

 also forms a groove. Row 3, if the endodermal fragment is covered by ecto- 

 derm, the latter at first spreads but then rounds up and becomes isolated. 

 (From Holtfreter 1943-44.) 



in the procedure. Holtfreter (1943 -1944) has described how groups of 

 blastoporal or endodermal cells, placed against a larger mass of endoderm, 

 become as it were engulfed into it (Fig. 20.14). He has shown that im- 

 portant factors in this situation are the adhesiveness between the mem- 

 branes of different types of cells, and what he speaks of as the 'surface 

 tensions' developed along such cell-to-cell or cell-to-medium interfaces. 

 By placing in contact small groups of cells from different tissues, he was 



