358 GERMINAL ORGANIZATION INDUCTION PHENOMENA 4 



perspicacity of workers. It remains difficult to interpret normal development in 

 the light of the numerous experimental results. Therefore, an attempt to sum- 

 marize actual data and ideas concerning neurogenic induction must be under- 

 taken as a heuristic enterprize. In order to give it the best objectivity, I think it 

 advisable not to follow a chronological order, but to group at least the larger part 

 of the relevant data according to the methods by which they have been obtained : 

 first, the observations sensu stricto; second, the strictly microsurgical experiments, 

 with a distinction between those carried out on the whole egg and those concerning 

 isolation, or combination of fragments or large pieces, e.g., explantations, sand- 

 wiches; third, the modification of development by use of physical and chemical 

 agents; fourth, the recourse to foreign (xeno-) inductors. A more limited inquiry 

 will be devoted to secondary and minor inductions. 



When we have gathered all this information, we should, if this field of research 

 has already been sufficiently explored, be able to perceive a clue towards an 

 explanation of the neurogenic induction. A restriction however, must be im- 

 mediately brought to this extensive program. It is neither advisable nor required, 

 to present here an extensive review of the five categories of enquiries. The results 

 of the pre-war period have been summarized in several books (Huxley and 

 De Beer, 1934; Spemann, 1938; P. Weiss, 1939; Dalcq, 1935b, 1938b, 1941b), and 

 are largely to be considered as classical knowledge. More recently, comprehensive 

 reviews have been written by Pasquini (1949), Bautzmann (1951), Raunich 

 (1953), Holtfreter and Hamburger (1955) on amphibians in general, by Mangold 

 (1958) on the head of Urodeles, and by Rudnick on fish and birds (1955); 

 moreover, extensive reports have been written by Woerdeman (1952, 1955a, b) on 

 the use of serological methods, while Waddington (1952b) has devoted a book to 

 experimental results concerning bird eggs. Therefore it will suffice to consider 

 only research which is relatively recent and apparently significant for the under- 

 standing of neurogenic induction. 



Before entering this inventory, let us add first that we intend to consider for 

 each methodological category the results available not only concerning amphib- 

 ians but any vertebrate, second, we are not interested in unravelling all kinds 

 of subtleties relative to any particular method of investigation, but only in 

 gathering all the available information which may help to answer the following 

 question : how does neurogenic induction work during the development of a vertebrate embryo'? 



A. Observations 



How the cells of the ectoblast progressively change during induction with respect 

 to their shape, the structure of their cytoplasm and their mutual arrangement 

 (Fig. 34) has been observed a long time ago by embryologists, using classical 

 methods. From sections, one gets the impression of a shape-giving growth; the 

 occurrence of numerous mitoses is always observed and also the active utilisation 

 of the yolk. In the young Pleurodeles gastrula, the ectoblastic cells are rich in 

 yolk platelets, and the dorsal region is thicker than the ventral (Fig. 34, A). 

 In the young neural plate, already made distinct by a pigmented hem, the upper 

 part of the cells is largely free of platelets; in the corresponding thinner ventral 

 epiblast, the yolk resorption is less pronounced. This difference is made more 



