96 



THE TOPOGENESIS OF THE EMBRYO 



ye^ye/c/ I ^er/77^ff/'. 



Fig. 33. Diagram of 

 an amphibian blastu- 

 la. Anxynale veld = 

 animal field; oerm.gr. 

 =z blastopore furrow; 

 randzone = marginal 

 zone; veg. veld = 

 vegetative field. 



ance ; nevertheless, their importance will 

 appear in the later development. 



The beginning of gastrulation is mark- 

 ed by a small indentation at the dorsal 

 side of the blastula, in the vegetative 

 field, just under the grey crescent which 

 here lies in the marginal zone. This in- 

 dentation is called the blastopore furrow. 

 It is produced by an active change in 

 shape of the cells of that area, which 

 become club-shaped and thereby move 

 inwards. The depth of the indentation 

 soon increases, and in this way the 

 archenteron extends into the egg, along 

 its dorsal side, and in the direction of 

 the animal pole (Fig. 34 a-e). In the 

 course of this invagination, material 

 which was originally on the surface is 

 carried into the interior, so that the wall of the archenteron 

 consists of cells which initially lay at the surface. Material of 

 the vegetative field as well as material of the neighbouring 

 dorsal part of the marginal zone is rolled in over the lips of 

 the blastopore. The vegetative material will form the ventral 

 wall of the archenteron, its so-called floor, whereas the material 

 from the marginal zone will form its roof. The invagination 

 then begins to spread laterally, more or less along the boundary 

 between marginal zone and vegetative field. All along this 

 boundary, marginal zone material begins to roll in, forming 

 a protruding edge, called the Up of the blastopore (Fig. 37 a, b). 

 The invaginating archenteron gradually compresses the blasto- 

 coel. In the end, gastrulation leads to an almost complete dis- 

 appearance of the material of the vegetative field into the 

 interior. Only a small part of this material still protrudes from 

 the blastopore as the yolk-plug (Fig. 34 c, d). Finally this, too, 

 disappears, and the blastopore lips, approaching each other 

 from all sides, close the blastopore. The material of the marginal 

 zone also disappears from the surface by rolling-in over the 

 lips of the blastopore. At the same time, the material of the 



