348 GERMINAL ORGANIZATION INDUCTION PHENOMENA 4 



In this zone, there is a continuity of properties or activities which are still labile 

 but tentative towards specialization. We can imagine, as the simplest explanation 

 a purely quantitative variation, with a tendency to preserve the level reached 

 by each territory, differences in physiological age providing simultaneously 

 various stages of one process; or, if a more complex explanation is necessary, it 

 may be two or more processes varying at a different rate, or even in opposite 

 directions, or a variation in the relative number of several organelles. 



{e) Some biochemical information 



Another clue toward the understanding of developmental kinetic is the search for 

 biochemical differences between more and less active parts of the germinal system, 

 mostly considered during gastrulation. Only a few of these important investigations 

 need be mentioned here. 



In amphibians especially, the impressive bulk of results (see Gregg and 

 Ornstein, 1952, 1953; Lovtrup, 1955) must be reduced here to essential infor- 

 mation. Numerous instances of regional differences concerning the general trend 

 of metabolism have been brought to light. There have been discovered variations 

 of enzymatic activities along the main axis (Barth and Sze, 1953), and differences 

 between dorsal and ventral half in the respiratory quotient (J. Brachet, 1939; 

 Sze, 1953), in their capacity to reducing brillant cresyl blue (Piepho, 1938), in 

 their adenosine triphosphate and apyrase content (Fujii et al., 1951), in their 

 protease activity (D'Amelio and Ceas, 1956), and in capacity to incorporate 

 tagged amino-acids (Eakin et al., 1951 ; Waddington, 1954; Flickinger, 1954) and 

 labelled CO2 (Tencer, 1958). 



An extensive chromatographic study of free amino acids in Xenopus embryos 

 and larvae has been published by Deuchar (1956). For the early stages which 

 interest us here, it is remarkable that the whole content in free aspartic and 

 glutamic acids is higher in the morula than in the gastrula; also that in the early 

 gastrula the dorsal lip, compared to the presumptive neural area and to the 

 ventral parts, has the highest content in free amino acids, the neural area being 

 second in quantity; similar differences are found for both aspartic acid and valine, 

 while glutamine is more concentrated in the whole dorsal than in the ventral 

 half. Several of these amino acids definitely vary during cleavage: glutaminic acid 

 increases from the 2-cell stage to the 32-cell stage, glutamic acid only from 2- to 

 8-cell stage, while the aspartic acid content falls from the 8-cell stage to the 

 blastula (Deuchar, 1958a). 



Other measurements have yielded valuable information, although not to be 

 correlated at this point with morphochoresis. Changes in tyrosinase activity occur 

 very early in the frog egg. They seem to be a direct consequence of fertilization. 

 This enzymatic activity increases up to the beginning of gastrulation, then drops 

 slowly (Stearns and Goldstein, 1956). 



^ For an accurate study of the cytological changes taking place in the cells of the noto- 

 chordal area, the progressive increase of their mutual contact, their secondary arrangement 

 in disks and coins, the conversion of their content into cell membranes containing large sap 

 vacuoles, their behavior in disaggregating media, see Mookerjee, Deuchar and Waddington, 

 (1953)- 



