EMBRYO FORMATION IN OTHER GROUPS OF VERTEBRATES 23I 



regulation, either when parts of it are removed or when two centres are 

 fused together. But the critical evidence for such a centre is the fact that 

 some blastomeres fail to give embryos. In view of the results of earher 

 work on Fundulus, it is perhaps unsafe to lay much stress on such essenti- 

 ally negative evidence. 



In the stages immediately following this, a phenomenon has been 

 noted which has sometimes been claimed to be a special feature of teleost 

 development with no parallels in other groups. Oppenheimer (1934/', 

 1936a) found that blastoderms of the minnow Fundulus, if removed from 

 the yolk at the i6-cell stage or before and cultivated in salt solution, failed 

 to gastrulate and developed only into featureless balls of cells. A rather 

 similar result was reached for the goldfish by Tung, Chang and Tung 

 (1945). They found that if eggs were divided latitudinally at the i-cell 

 and 2-cell stage, blastoderms could not develop properly unless they 

 remained connected with more than half the total quantity of yolk. From 

 the 4-cell stage the requirement of yolk was less and from the 8-cell stage 

 onwards completely isolated blastoderms could develop into embryos. 

 Devillcrs (1947) found that the blastoderm of the trout was unable to 

 develop in isolation from the yolk even when it had been removed as 

 late as the blastula stage. On the other hand, in the pike Esox similarly 

 isolated blastoderms continued developing quite well. 



The explanation of these facts is not clear. As a matter of fact a some- 

 what similar phenomenon has been reported in the Amphibia. Vintem- 

 berger (1936) showed that if, in the 8-cell stage of the frog, the four animal 

 cells are isolated, they are usually unable to differentiate any axial organs 

 although they contain part of the presumptive posterior region of the 

 notochord rudiment. If however the four cells are placed on a base con- 

 sisting of a mass of endoderm cells from the blastula, they differentiate 

 very much better. There are several different ways in which these results 

 and those in the teleosts could be interpreted. 



(i) It is a fairly general observation that early stages of embryos are 

 more sensitive than older ones to the general injury produced by experi- 

 mental operations. The increase in resistance usually continues at least 

 until the main embryonic organs are laid dovm. It is noticeable, for 

 instance, that it is easier to get good differentiation in tissue culture of 

 chick blastoderms of primitive streak or later stages than of blastoderms 

 taken from eggs in the first few hours of incubation. It may be that we 

 are dealing merely with a general increasing 'tougliness' of the embryos, 

 which varies from species to species. 



(2) Oppenheimer originally considered that the phenomena suggested 

 the passage from the periblast into the blastoderm cells of some substance 



Q 



