EMBRYO FORMATION IN OTHER GROUPS OF VERTEBRATES 229 



side, as though the embryo had been spht from tail to head and the two 

 halves pulled apart. These two half embryos were then supposed to come 

 together and fuse in the midHne by a process to which the name concre- 

 scence was given. It can, however, be seen from the map of presumptive 

 areas that this is not the case. There is always complete continuity from 

 one side to the other across the midline. The presumptive areas are wider 

 from side to side than the defmitive organs will eventually be, but the 

 movement from one situation to the other involves only a lateral contrac- 

 tion and longitudinal stretching, and not a moving together and fusion of 

 two originally separate rudiments (Fig. 11.4). 



Figure 11.4 



Gastrulation movements in the trout : 1 is an early stage, showing the whole 

 blastodisc; 2 and 3 are stages in the formation of the embryonic axis. Move- 

 ments taking place in the surface layer shown in solid arrows, those occur- 

 ring below the surface in dotted arrows. (From Pasteels 1940.) 



The mechanism of the gastrulation movements, and in particular of 

 the spread of the blastoderm over the yolk, has recently been extensively 

 studied by Devillers (1951^) and Trinkaus (1951). They both agree, in 

 contradiction to certain earlier authors, that the spread of the blastoderm 

 is not solely due to the contraction of the yolk gel layer. It seems probable 

 that one of the essential factors is the activity of the periblast, which seems 

 to have a spontaneous capacity to spread over the surface of the yolk. 

 The blastoderm also has an autonomous tendency to expand, but it only 

 does so over regions which the periblast has already covered and thus 

 provided with a suitable substratum for it. 



