EMBRYONIC INDUCTORS AND ORGANIZERS 483 



the anterior piece induces no neural tissue in the ectoderm, the middle 

 piece induces brainlike parts, the posterior piece chiefly spinal neural 

 structure and occasionally otic vesicle. The middle piece is regarded as 

 ''head organizer," the posterior piece as "trunk organizer" (Eakin, 

 1939^) ; but the experiments do not show that there is anything more than 

 a quantitative difference between them. 



BIRDS 



Following chscovery of induction by the region of the dorsal blastopore 

 lip in amphibians, the question whether a comparable induction occurred 

 in the chick naturally arose. It was suggested that the region of the node 

 at the anterior end of the primitive streak is an "organizer," or at least 

 essential to formation of the embryo. ■'•' However, it was found that por- 

 tions of the streak not including the node can continue development when 

 isolated in vitro, and implantation experiments showed that not only the 

 node region of the streak but more posterior regions without any part of 

 the node can induce neural tissue or plate or a more or less complete em- 

 bryonic axis. Implants from more anterior levels of the streak differenti- 

 ate into notochord, somites, and neural tissue; those from levels farther 

 posterior form only mesodermal structures; but both induce. Both ante- 

 rior and posterior derivatives of the streak, the head process, and the sinus 

 rhomboidalis, and also the neural plate after its formation, can induce 

 neural plate. ^"^ The action is not species-specific; heteroplastic implants 

 between duck and chick are effective. Pieces of primitive streak killed and 

 coagulated by boiling water also induce. The host "individuation field" 

 (Waddington) influences axial orientation of induced parts, as in am- 

 phibians; but sometimes the induced axis is opposed to that of the host 

 and is then supposedly determined by the inductor. Apparently there may 

 be considerable reconstitution of the graft into more anterior parts or a 

 larger part of the longitudinal axis than would have been formed in normal 

 development. Reaction of the host to implanted inductor decreases as de- 

 velopment progresses. Neural plate is induced up to a later head-fold 

 stage, but at this stage only by implantation in anterior regions; at mid- 

 trunk levels only epidermal thickening results. In neural-fold stages only 

 thickening occurs at all levels, and at early somite stages the epidermis 

 shows no reaction (Woodside, 1937). 



33 Wetzel, 1925a, b, 1929; T. E. Hunt, 1929, 1931, 1932; Willier and Rawles, 1931. 

 ^-t Waddington, 1930, 1932, 1933^, 1934, 1935, 1937; Waddington and Schmidt, 1933; 

 Waddington and Taylor, 1937. 



