200 PRINCIPLES OF EMBRYOLOGY 



itself but also in the nature of the evocators and in the conditions which 

 convert gastrula ectoderm into organiser. 



c. The metabolism of the organiser 



It will be convenient next to discuss the latter problem (Reviews: 

 Brachet 1944, Needham 1942, Boell 1948). Child's theory of axial 

 gradients would suggest that the blastopore region, which is un- 

 doubtedly of extreme biological activity, should have a higher rate of 

 respiratory metabohsm than the rest of the gastrula; and we have seen 

 that methylene blue, a well-known stimulant to respiratory processes, 

 can cause the release of evocating power in presumptive ectoderm. There 

 are, obviously, considerable technical difficulties in measuring directly the 

 respiration of pieces of tissue as small as the blastopore region, and the first 

 attempts to compare its activity with that of other parts of the embryo led 

 to rather contradictory results. There is no doubt that the consumption of 

 oxygen rises fairly rapidly in the dorsal region of the neurula, when the 

 tissues of the embryonic axis are differentiating. If one wants to assess the 

 metabohsm of the organisation centre at the time it exerts its main in- 

 ductive effect, it is necessary to have an instrument which is sensitive 

 enough to give an accurate reading of the oxygen uptake within the 

 short period of gastrulation. It was not until Needham adapted the 

 Cartesian diver technique of Linderstrom-Lang that this requirement was 

 fully met. 



Using this instrument Needham and his co-workers (see Boell, Need- 

 ham and others, 1939) found that in most series of experiments there was 

 no appreciable difference between the rate of oxygen uptake by the blasto- 

 pore region and by a piece of tissue from a similar position on the ventral 

 side of the egg. This confirmed the conclusion of earlier work with a less- 

 sensitive instrument by Waddington, Needham and Brachet (1936), but 

 there were other experiments, by Brachet (1936), Brachet and Shapiro 

 (1937), Fischer and Hartwig (1938) which seemed to show a higher acti- 

 vity in the blastopore region. The situation was cleared up by Boell (1942) 

 and Barth (1942), who measured the respiratory rate of a series of isolated 

 fragments from all different parts of the gastrula. They found that there is 

 indeed a gradient in respiration. Its high point, however, is not at the 

 blastopore but at the animal pole, and it falls off from there to reach its 

 lowest in the yolky endoderm. The actual figures given by BoeU are: 

 Q'og (= ni^l, O2 per [xg nitrogen per hour) 49 for presumptive neural 

 plate near animal pole, 21 for dorsal hp, 28 for posterior presumptive 

 ectoderm, 1-3 for endoderm. There is, of course, more yolk in the endo- 

 derm cells than in ectoderm, and since this is a relatively inert material 



