38 EMBKY'OLOGY OF THE LOWEE VEETEBEATES ch. 



though not to the extent seen in Polypterus. The segmentation 

 cavity remains, therefore, relatively capacious so as to permit of a 

 considerable amount of invagination of the yolk mass into it. 



The gastrulation process so far as can be judged seems to consist 

 mainly if not entirely as in Amphioxus of (1) invagination and (2) 

 overgrowth, only in this case the relative importance of the former 

 has been lessened and that of the latter increased. 



Amphibia. — It will be convenient to consider first the gastrula- 

 tion-phenomena as seen in the common frog (Bana temporaria), this 

 animal having been more exhaustively studied than has any other 

 Amphibian. 



The first sign of the onset of gastrulation is the appearance of a 

 short latitudinal linear involution (Fig. 24, a) of the surface of the blas- 



Fig. 23. — Gastrulation in Petromyzon : based on Goette's figures (1890). 



The individual sections are orientated in the same way as the corresponding sections in 

 Pigs. 18 and 21. 



tula considerably on the abapical side of the equator (about 25°, Kopsch) 

 and just at the boundary of the large-celled region. It appears on 

 that side of the egg on which the blastula roof is commonly rather 

 thinner than it is elsewhere. This involution groove, as seen in a 

 surface view of the egg, extends laterally and as it does so assumes a 

 crescentic curvature (Fig. 24, h). The extension of the groove in 

 length continues while its radius of curvature diminishes until finally 

 it forms a closed circle (Fig. 24, b, c, d). 



The groove at its first appearance lies close to the boundary 

 between the small cells of the apical region — characterized in the 

 frog by their dense black pigment — and the large pale-yellow yolk- 

 cells. During subsequent stages the groove continues to mark the 

 boundary between the two types of cell, so that in the last stage 

 mentioned when the groove fornis a complete circle the mass of almost 

 white yolk-cells within it (yolk-plug) stands out in striking contrast 

 with the coal-black cells covering the rest of the egg surface. 



