Ill PRIMARY INDUCTION EXPERIMENTAL DATA 367 



become customary to stain it in vivo, usually with Nile blue, or sometimes with 

 neutral red. In a vast majority of operations, this tagging is absolutely innocuous 

 and without appreciable effect on the morphological fate of the material. How- 

 ever, a few recent results obtained on eggs of fishes and birds show the contrary. 

 In the exchanges of ventral and dorsal halves of trout blastodiscs performed by 

 Devillers {cf. p. 347) staining of the ventral half by neutral red causes it to 

 gastrulate if the dorsal half is removed; the vacuolizing effect of the dye can be 

 observed in vivo. In the chick blastoderm at the primitive streak stage, the unstained 

 Hensen node, when isolated and grafted, yields ectoblast and eventually a neural 

 tube, while when stained with Nile blue it develops into notochord and mesoblast 

 (Grabowski, 1956, 1957), or even into a whole minute embryo (Grabowski, 1958). 

 It could well be suspected that in amphibian eggs also, the staining would either 

 raise or lower the morphogenetic potential in some special cases. However, a 

 critical examination of the various experiments in which this useful method was 

 applied does not make it appear objectionable. 



Among the many modifications obtained by operation on blastulae or young 

 gastrulae, the impressive phenomenon remains, more than 35 years after its 

 discovery, the evocation of a secondary embryo. In amphibians, this can be 

 realized either by the famous graft of a blastoporal lip into the ventral side of a 

 host; or by insertion of the inducing piece into the blastocoele (Mangold's 

 Einsteckungsmethode) ; or, at least in anurans, by the 180° rotation of the animal 

 half on the vegetal half, i.e., a subequatorial translocation. Induction can also 

 be obtained in fish (trout, lamprey) by similar methods. Among amniots, only 

 birds have been properly investigated, although technical difficulties have limited 

 the enquiry. Waddington (1933a, b), working in vitro, obtained a secondary 

 primitive streak by superimposing two ectophylls (primary ectoderms) in opposite 

 directions; Woodside (1937) confirmed this result by the direct insertion of a 

 primitive streak between the two layers. Also, rotating the ectophyll 180° relatively 

 to the endophyll, produces induction of a secondary streak (Waddington, 1933a, 

 b). A few attempts by this author have shown that a rabbit ectophyll reacts to 

 the contact of a chicken primitive streak, and vice versa. 



These operations, as such, demonstrate only the existence of a neurogenic 

 induction; its causal analysis must be approached by other techniques. Beyond 

 the crude fact of induction immediately appears the problem of how the new 

 system is organized, and especially, why it shows the distinction between the head 

 and the rest of the body. The answer to this query could only be searched for in 

 amphibians, and since Spemann (1931) recognized the possibility of inducing 

 separately a head, a trunk, or a tail, it has remained a central topic of experi- 

 mental embryology. For sake of brevity, it must be stated beforehand that it 

 has been admitted, during the last ten years, that the relevant distinction is not 

 between head and body, but between the prechordal and chordal parts of 

 the organism. This assumption has made necessary the use of new terms e.g. 

 archencephalon, deuterencephalon, spino-caudal structures (Lehmann, 1941, 1942a, b), 

 and acrencephalon, chordencephalon (Dalcq, 1947b) but they are no longer sufficient to 

 describe easily and without ambiguity the various situations. To face this termino- 

 logical difficulty, I have proposed (1957a) the solution shown in Fig. 39, which 



Literature p. 483 



