232 PRINCIPLES OF EMBRYOLOGY 



of an organiser-like nature which was necessary for the initiation of gastru- 

 lation (see 1947, where she is aheady somewhat cautious about this hypo- 

 thesis) Tung, Chang and Tung considered their results supported this. 

 The boundaries of the cells, particularly of the periblast but also of the 

 blastoderm, are not very sharply marked in early cleavage stages of tele- 

 osts, and it seems quite possible that the blastoderm contmues to mcor- 

 porate further cytoplasmic material for some time after the cleavages 



have begun. 



(3) It may be, however, that what the yolk provides is not some org^- 

 iser-hke substance but rather a relatively simple essential nutrient This 

 possibility is supported by the fact that DeviUers (1949) found that blasto- 

 derms of the trout, which are unable to differentiate in pure salt solution, 

 will do so when glucose or similar substances are added to the medium. 

 In a short note Trinkaus (1953) reports rather similar results with Fundulus. 



Only if the second of these possibihties is the full explanation ot the 

 facts would we be confronted in the teleosts with a situation which 

 differed radicaUy from that in the Amphibia (since it is improbable that a 

 similar process is the explanation of Vintemberger's results). At present 

 the matter must remain open. 



Experiments on the gastrula stage show that, in teleosts, the invaginat- 

 ing chorda-mesoderm acts as an organiser m a manner extremely similar 

 to that found in the Amphibia and in the chick. This was demonstrated 

 almost simuhaneously by Oppenheimer for the perch and Funduus 

 (1934a, h, i936h) and by Luther (193 5, 193?) for the trout Salmo 

 (Fig. II. 5). The parallel with the amphibian and bird orgamsers is ex- 

 tremely complete (see Review by Oppenheimer 194?)- For mstance 

 there is a similar type of regional determination by the different parts ot 

 the organisation centre, and evocation by dead tissues has been demon- 



strated 



It has been claimed (e.g. by Oppenheimer 1947) that the teleost ecto- 

 derm has some tendency to differentiate neural tissue without the mter- 

 vention of the mesodermal organiser. Oppenheimer tends to mterpret 

 this in terms of an evocation by cytolysis, since in the abnormal embryos 

 which gave evidence of such autonomous differentiation the cells were 

 somewhat unhealthy. It appears however that the neural tissue was not 

 completely without any accompanying mesoderm, but merely without 

 accompanying differentiated chorda-mesoderm. The induction of neural 

 tissue by mesoderm which does not include the chorda, and which may 

 itself differentiate rather poorly, is well known both in the Amplnbia 

 and the chick, and it appears unnecessary to postulate anything more tlian 

 tliis to explain the phenomena in the teleosts. 



