THE vertebrates: the amphibia and birds 163 



The streaming movement towards the blastopore begins when the 

 blastopore is quite small; it always goes on fastest in the region in which 

 the blastopore first appears, and the movement here involves a great 

 stretching and elongation of tissue in the direction of the meridian joining 

 the blastopore to the animal pole. This meridian will become the mid- 

 dorsal line of the embryonic axis when this begins to form; and this part 

 of the blastopore is, therefore, known as its dorsal lip ; the ventral lip is 

 the last-formed portion which eventually appears on the diametrically 

 opposed side of the yolk-plug. The material which flows in round the 

 dorsal lip elongates considerably while doing so, and becomes narrower 

 from side to side; this means that material invaginating further laterally 



Figure 9.8 

 The gastrulation movements in a urodele, seen from the side. 



has to move in towards the midline. The invagination streams, therefore, 

 converge from the sides towards the middle, as shown in Fig. 9.8. More- 

 over, as the ring of prospective mesoderm moves into the interior of the 

 egg, its place at the surface has to be taken by the prospective ectoderm. 

 We shall see later that the prospective neural ectoderm also elongates 

 along the midhne, narrowing as it does so ; thus, the whole of the dorsal 

 convergence has to be compensated for by a lateral expansion of the 

 prospective skin. In its crudest outline, therefore, the gastrulation move- 

 ments of the animal hemisphere of the egg can be summarised as, firstly, 

 a great elongation and narrowing along the dorsal meridian (the elonga- 

 tion being so much that the material flows round the blastopore and 

 finishes up as a double layer), and, secondly, to compensate for this, a 

 lateral expansion, in a plane at right angles to the dorsal one, of the pro- 

 spective skin at the opposite side of the egg ; with, of course, one process 



