Ig4 PRINCIPLES OF EMBRYOLOGY 



always retains its original anterior-posterior axis, whereas the polarity of 

 other regions is more labile. This is an indication that the posterior region 

 is the dominant part of the 'endoderm-field' (Lutz 1952). 



There is not much difficulty in making grafts of pieces of blastoderms 

 of the primitive streak stage, once the method of growing the embryo m 

 tissue culture has been adopted. But the procedure differs slightly from 

 that usual in the Amphibia. In the latter, a hole can be cut in the gastrula, 

 and a graft inserted so that it lies flat with the main surface of the egg. 

 If a similar operation is made in the chick, the edges of the wound often 

 curl back, and fail to heal up with the edges of the inserted graft. It is 

 therefore more effective in bird embryos if the epiblast and endoderm are 

 slightly separated, a pocket formed between them, and the graft mserted 

 into it; this corresponds, more or less, to the method of pushing a graft 

 through the roof of the amphibian gastrula into the blastocoel. 



The only other important difficulty which arises in the chick is con- 

 nected with the interpretation of the results. We saw that in the AmpHbia 

 the conclusive proof of an inducing action by the grafted organiser was 

 given by making the graft of a different species from the host, so that the 

 two sets of tissues could be distinguished in sections, and convincing 

 evidence produced that parts of the secondary embryonic axis had been 

 formed from host tissues. In birds, it has so far been impossible to find 

 two species whose embryonic tissues can be distinguished with certamty ; 

 Waddington and Schmidt (1933) made many grafts between chick and 

 duck embryos and obtained structures which were certainly inductions, 

 but the tissues were similar in appearance, and host and graft could only 

 be recognised in a general way, by the fact that the graft tended to remam 

 a rather separate lump. The final proof of the reality of induction had to 

 be obtained by a different method. This was done by takmg two blasto- 

 derms removing the endoderm from each of them, then placing them 

 with the mesoderm faces together, and with the two primitive streaks 

 not on top of one another. When the whole of such a combmation is 

 grown in vitro, as many as four embryonic axes may appear; one from 

 each of the original primitive streaks, and two more, one induced by each 

 streak in the other epiblast against which it lies (Waddington 1932). The 

 neural grooves are stiU firmly attached to the epiblast from which they 

 arose, and it can be quite definitely seen that the secondary ones have been 

 induced and were not formed from the original streaks. The origins of 

 the mesodermal parts of the embryonic axes is not so certam, and one 

 must guess that they are formed partly from the original streak and 

 partly by induction. 

 The organising powers of the different parts of the streak can be 



