THE EPIGENETICS OF THE EMBRYONIC AXIS 187 



primitive streak there are rather vaguely deHmited. areas v^hich exhibit 

 tendencies to produce specific organs, such as heart, eye, hver, etc. 

 (Reviev^s: Rudnick 1944, 1948). It is probably safe to assume that these 

 rough locahsations are, in the first place, characteristics of the mesoderm, 

 and that a tendency to form a defmite ectodermal organ such as the eye in- 

 dicates the localisation of a mesodermal eye inductor rather than of the eye 

 itself (Fig. 10.8). If that is so, the phenomena are essentially similar to the 



0-7 mm. 



A Liver 



• Heart 

 ®Chorda 

 ^Thyroid 

 ©Nephros 

 X Intestine 



• Erythrocytes 

 "■"Melanophores 

 ■* Skeletal muscle 



Figure 10.8 



(a) Map showing regions of the definitive primitive streak blastoderm from 

 which various tissues differentiate in chorio-allantoic grafts; ectodermal 



tissues on left, mesodermal on right. (After Rudnick 1948.) 



(b) Regions of the head process blastoderm from which heart muscle differ- 

 entiates in chorio-allantoic grafts ; the closeness of the hatching indicates the 



relative frequence of heart differentiation. (After Rawles 1943.) 



vague localisations of different capacities within the amphibian mesoderm 

 described by Holtfreter (p. 177). There is little evidence that the ectoderm 

 in general has any capacity to form neural tissue in the absence of an 

 inducing stimulus from the mesoderm, but there is a possibility that the 

 region which develops into the forebrain may be able to do so even when 

 isolated from mesoderm (cf. Waddington 1952^, p. 109). 



Other evidence of regionality within the organiser emerges clearly 

 from the results of intra-blastodermal grafts. As in the Amphibia, there is 

 a tendency for grafts of the anterior part of the streak to induce heads, 



