THE vertebrates: the amphibia and birds 



171 



posteriorly, leaving a gap (Fig. 9.13). The same phenomenon can be seen 

 even in smaller fragments when these are grafted into abnormal situations. 

 Grafts taken from the region of presumptive mesoderm in the newt and 

 placed in some other part of the gastrula, usually succeed in moving below 

 the surface into the mesodermal layer, often forming a small blastopore 

 of their own to do so. Moreover, the direction in which this invagination 

 occurs is more or less defmitely implicit in the fragment. Some of the most 



■ , .fl!^:^^__ Q 





Figure 9.13 



Tissue movements in parts of gastrulae: a shows two vegetative half- 

 gastrulae of the newt placed together so that the blastopores point towards 

 one another. The form which develops (b) is a'duplicitas cruciata'; the two 

 streams of mesoderm from the two blastopores have met head-on, and been 

 forced to spread out to each side, so that each head region is derived half 

 from one egg and half from the other. (From Schleip, after Spemann.) 

 Figure c shows the 'tail' developed from the anterior portion of a chick 

 blastoderm transected just behind the node. (After Waddington 1932.) 



Striking examples of this have been described by Waddington (1941) in 

 the anuran Discoglossus, in which the gastrulation movements are very 

 rapid and perhaps for this reason seem to be rather definitely determined 

 in the tissues ; if a small part of the dorsal mesoderm of this form is re- 

 moved and grafted back, reversed in direction, it starts pushing out a 

 tongue of tissue which moves in the opposite direction to that of the main 

 stream of mesoderm which surrounds it. 



The inherent movement-tendencies of the parts of the gastrula are, 

 however, at first by no means unalterable, as many experiments have 



