l82 PRINCIPLES OF EMBRYOLOGY 



the blood supply of the chorio-allantoic membrane of much older chick 

 embryos. Neither of these methods enabled one to investigate the eJEFect 

 of one part of the embryo on its neighbours, which Spemann had showed 

 to be of fundamental importance in the Amphibia. And, owing to the 

 way the blastoderm is built up of three superposed and closely adherent 

 layers, even the isolated fragments contained a not-well-defined mixture 

 of tissues and were not comparable to the specific gastrula pieces culti- 

 vated by Holtfreter in his salt solutions. As a result, very few definite 

 conclusions could be drawn from work which relied on these techniques. 

 Such theories as were suggested were cast in the old terms of 'potencies' 

 and 'embryonic segregation'. 



New possibilities were opened up when the technique of tissue culture 

 was adapted to the task of keeping alive the entire blastoderm after 

 removing it from the egg and cleaning it of the adhering albumen and 

 yolk. In such blastoderms, the three germ-layers can be separated from 

 one another, at least in certain regions, and fragments can be grafted 

 into abnormal places where their influence on the surrounding tissues 

 can be studied (Waddington 1932). 



We have seen that the morphological changes going on during gastru- 

 lation are more complex in the birds than in the Amphibia, and so are the 

 organiser phenomena, probably because the morphological and physio- 

 logical processes are intimately connected. The first stage in bird gastru- 

 lation is the formation of the endoderm, and this is the earliest stage at 

 which an experimental attack has proved possible. In the young blasto- 

 derm, in which the primitive streak is just beginning to be indicated, the 

 endoderm may be peeled off, rotated about a vertical axis, and replaced 

 either head to tail, or so that its longitudinal axis makes a right-angle 

 with that of the epiblast (Waddington 1933^). It is found that such a 

 rotation has a powerful influence on the elongation of the primitive streak 

 and of the embryo which eventually develops. If the rotation has been 

 through a right-angle, the head end of the embryo is curved round to- 

 wards the new position in which the head end of the endoderm has been 

 placed. With a complete head to tail rotation the usual result is that the 

 embryo is greatly shortened, and does not extend fully across the area 

 pellucida. It is clear that the elongation of the epiblastic part of the embryo 

 tends to proceed in the posterior-to-anterior direction of the endoderm. 

 In some cases of head-to-tail rotation the influence of the endoderm is 

 more far reaching, and a new primitive streak and embryo are induced, 

 running from the posterior part of the endoderm to meet the original 

 embryo head-on in the centre of the area pellucida. The endoderm is 

 therefore in some sense an organiser, since it can call forth the develop- 



