530 PATTERNS AND PROBLEMS OF DEVELOPMENT 



The earlier results of chorio-allantoic grafting with definitive primitive- 

 streak, head-process, and somite stages seemed to indicate that only pieces 

 from the node region were capable of extensive development and that this 

 region could give rise to all parts posterior to its level as far as, and includ- 

 ing, mesonephros; but, as the physiological condition indicated by the 

 node moved posteriorly, its development was progressively restricted to 

 more posterior parts; in general, it did not give rise to parts characteristic 

 of levels anterior to its position (T. E. Hunt, 193 1, 1932). Further experi- 

 ments, however, modified this view. Posterior parts of advanced primi- 

 tive-streak stages, containing no more than the posterior two-thirds of the 

 streak, cultivated in vitro, are able to continue differentiation (Wadding- 

 ton, 1935a). Dalton (1935) also found presence of the node region unessen- 

 tial for development of axial structures and regarded the technique of 

 grafting as an important factor in results obtained. Forebrain develops in 

 chorio-allantoic grafts from the head-process stage, even from pieces en- 

 tirely anterior to the head-process and without notochord (Stein, 1933). 

 Heteroplastic grafts between duck and chick of parts of the primitive 

 streak without the node may show more or less reconstitution and may 

 also act as inductors (Waddington and Schmidt, 1933). 



Reconstitution of any part and even of the whole primitive streak after 

 removal occurs in blastoderms explanted to plasma-chick embryo extract 

 after some 20 hours of incubation with development of normal, or almost 

 normal, embryos (Waddington, 1932). The cell movements are regarded 

 by Waddington as the chief factors in this reconstitution, but both the cell 

 movements and the development of normal embryos in these experiments 

 indicate presence and effectiveness of a physiological pattern in relation to 

 which fates of parts concerned in reconstitution are altered. 



Development of ectoderm, mesoderm, and entoderm in small pieces 

 from the different regions of the head-process blastoderm grafted on the 

 chorio-allantois is shown in the maps of Figures 167, 168, and 169 (Rawles, 

 1936). Evidently considerable reconstitution takes place in many of the 

 pieces, for various organ tissues develop in parts which, so far as known, 

 do not normally give rise to them. Many of the potency fields are more 

 extensive than the regions of actual differentiation in normal develop- 

 ment, and Rawles finds that in each field developmental potencies for the 

 part concerned decrease peripherally from a center. On the other hand, 

 each such field is a more or less restricted region or level of the blastoderm. 

 Some organs (heart, liver) develop only in lateral areas; others (notochord, 

 suprarenal, spleen) only in median areas; and still others in both median 



