THE EPIGENETICS OF THE EMBRYONIC AXIS 183 



ment of a new embryo. But we do not know how far it can be said to 

 determine the developmental fate of the epiblast; probably it only deter- 

 mines that a new primitive streak shall be formed, leaving the later events 

 undecided. As we shall see, a second organiser action has to go on witliin 

 the streak before the definitive embryo appears. The organising action 

 of the endoderm is perhaps connected with the posterior-anterior stream- 

 ing which it seems to undergo itself (Fig. 10.5). 





Figure 10.5 



On the left is a chick embryo, cultured in vitro, in which the anterior- 

 posterior axis of the endoderm was reversed in the early streak stage. Two 

 embryos have developed, one (with tail end towards the bottom of the page) 

 in the original direction of the epiblast, the other in the opposite direction; 

 the latter must have been induced by the endoderm. (From Waddington 



I933-) 



On the right the small diagram shows a blastoderm of the duck, at the time 



of laying, transected in the dorso-ventral plane; twin embryos develop. 



After Lutz 1949.) 



The early endoderm, in the stages when the primitive streak has not 

 yet appeared, is in a very labile condition. Lutz (1949) has shown that if a 

 duck's blastoderm is cut across at this period, both parts may form a 

 complete embryo, and this is true whatever the orientation of the line of 

 section (Fig. 10.5). The endoderm, then, can regulate very well and its 

 inducing capacity is present, in some degree, throughout the whole of it. 

 If one studies the orientation of the embryos which are formed in tliis 

 way, it is found that that developed in the posterior part of the blastoderm 



