312 



Embryogenesis: Progressive Differentiation 



invagination in the fish, and that the notion 

 that the process is simple may merely indi- 

 cate our ignorance. 



Gastrulation, then, shifts germinal regions 

 having a tentative or reversible pattern into 

 definitive positions related to the blastopore. 

 The axial ectoderm, whatever its primary 

 pattern, is undoubtedly given its final stamp 

 of independent differentiation tendency by 

 the mesoderm that comes to underlie it. As 

 for the invaginated material, there is no 

 evidence in the fish, and but one case in the 

 chick (heart), to show that any specific 

 differentiation tendency is present before 

 the material undergoes invagination. At this 

 stage of our knowledge, then, most of the 

 embryonic fields appear to originate at gas- 

 trulation. Whatever future investigations 

 will show, it is clear that gastrulation is a 

 critical process, separating and reinforcing 

 if not originating the embryonic pattern of 

 potential areas. 



We know too little of the details of any 

 two of the experimental forms under dis- 

 cussion to compare accurately the course of 

 events whereby any one field moves into 

 definitive position and becomes restricted to 

 the territory of its embryonic anlage. In the 

 teleost nevirula we seem to have a fairly well 

 localized system; defects made in the axis 

 may not even be regulated. In the chick, reg- 

 ulation of some parts remains possible for 

 considerable time. Are these differences real, 

 or an accident of the difference in absolute 

 size and scale of organization between the two 

 embryos, whereby it is difficult to make a 

 really minute defect in the teleost, without 

 much subsidiary damage? Again, in gas- 

 trulation stages, it is clear that Fundulus 

 behaves much as the amphibian embryo 

 does, in that transplants to the embryonic 

 shield are induced to form a variety of struc- 

 tures, somewhat but not exactly correspond- 

 ing to the level of the host axis: a result pre- 

 sumably of the wide imprecise nature of the 

 host induction fields. In the chick, no con- 

 vincing experiments show such histological 

 effect of host on transplant, though polarity 

 may be decidedly affected (Waddington and 

 Schmidt, '33). This peculiarity may be only 

 because the usual transplant tested is a 

 piece of primitive streak, the normal fate of 

 which is not known precisely, or because the 

 chick blastoderm reacts in a special way to 

 transplanted tissue: either by joining with it 

 inextricably in a complex induction, or by 

 walling it off, physiologically speaking, so 

 that it develops as a separate intrusive graft. 



It is obvious that in every way the picture 



we construct of the progressive invisible 

 changes in any embryo, leading to visible 

 differentiation, depends on the experiments 

 possible with that embryo. Slight differences 

 in texture or in chemical surface, perhaps, 

 may determine whether it is possible to make 

 transplants or explants of any given portion; 

 whether defects will heal or an epithelium 

 respond to induction. These factors are only 

 a small part of the equipment of embryonic 

 cells, but they loom very large experiment- 

 ally. If we are to be able to make valid 

 comparisons, our task must be to disentangle 

 these properties from the intrinsic genetic 

 mechanisms, and to discover the role of both 

 in cellular differentiation. This is for the 

 future. 



REFERENCES 



The following list contains only titles actually- 

 cited in the text. Recent comprehensive reviews of 

 the material covered are to be found in Oppen- 

 heimer ('47), Rudnick ('44, '48), and Waddington 

 ('52). 



Agassiz, L., and Whitman, C. 0. 1884 On the 

 development of some pelagic fish eggs. Proc. Am. 

 Acad. Arts & Sci., 20;23-75. 

 Alexander, L. E. 1937 An experimental study of 

 the role of optic cup and overljdng ectoderm in 

 lens formation in the chick embryo. J. Exp. Zool., 

 75;41-68. 

 Bartelmez, G. W. 1912 The bilaterality of the 



pigeon's eggs. J. Morph., 23.-269-329. 

 Brachet, Jean 1944 Embryologie Chimique. 



Masson, Paris. 

 Buno, W. 1951 Localization of sulfhydryl groups 



in the chick embryo. Anat. Rec, 7//.- 123-1 28. 

 Butler, Elizabeth 1935 The developmental capa- 

 city of regions of the unincubated chick blasto- 

 derm as tested in chorio-allantoic grafts. J. Exp. 

 Zool. 70.-357-389. 

 Clapp, Cornelia 1891 Some points in the develop- 

 ment of the toad-fish (Batrachus tau). J. Morph., 

 5.-494-501. 

 Clarke, L. F. 1936 Regional differences in eye- 

 forming capacity of the early chick blastoderm as 

 studied in chorio-allantoic grafts. Physiol. Zool., 

 ^.-102-128. 

 Devillers, Ch. 1947 Explantations in vitro de 

 blastodermes de Poissons (Salmo, Esox). Experi- 

 entia, 3; 7 1-74. 



1948a Le cortex de I'oeuf de Truite. Ann. 



Stat. Cent. d'Hydrobiologie appl., 2:29-49. 



1948b Suppression du material chordal 



dans la gastrula de Truite. Compt. rend. Acad, 

 sci., 227.-141 1-1413. 



1949 Explantations en milieu synthe- 



tique de blastodermes de Truite (Salmo irideus). 

 J. Cyto-embryol. Belgo-Neerland., pp. 67-73. 



1951a Symetrisation et regulation du 



germe chez la Truite. Compt. rend. Ass. Anat. 

 Nancy, XXXVIII Reun., pp. 1-7. 

 1951b Les mouvements superficiels dans 



