EMBRYO FORMATION IN OTHER GROUPS OF VERTEBRATES 247 



(cf. p. 135). The comparative study of the epigenetic systems in different 

 groups of vertebrates gives an opportunity to see the kinds of effects which 

 such gene changes produce. 



The first noteworthy epigenetic event in the development of vertebrates 

 is the fixing of the plane of bilateral symmetry. In the Amphibia, as we 

 have seen, the future organisation centre normally becomes located soon 

 after fertilisation and is sometimes visible as the grey crescent. In the other 

 primitive non-yolky type of vertebrate egg, that of the cyclostomes, 

 Montalenti and Maccagno claim that the situation is very similar. In the 

 protocordates (ascidians and Amphioxus) the dorso-ventral plane is also 

 fixed definitely at an early stage. In both the ascidians and amphibians 

 there is indeed some evidence that the dorso-ventral plane is at least fore- 

 shadowed before fertilisation occurs. In the latter the position of the 

 plane is at first labile and only gradually becomes definitely deter- 

 mined. By the time cleavage is completed and the blastula stage reached, 

 the dorsal plane is firmly determined and it is difficult to cause a new dorsal 

 region to appear elsewhere when the egg is divided in two. Such powers 

 of regulation are, however, not completely extinguished until consider- 

 ably later (p. 177). The results obtained by Bytinski-Salz on the cyclostomes 

 suggest that the same is true in that group. 



In the blastoderms of both the teleosts and the birds the situation seems 

 to be rather different. We have some evidence (Vakaet 1953) that the 

 growing oocyte of the teleost has a bilateral structure, but in the develop- 

 ing embryos the position of the dorso-ventral plane is quite easily alter- 

 able until a late stage of the blastula, as is shown by the results of Luther 

 in teleosts and Lutz in birds. It seems probable that the original bilaterahty 

 in the oocyte is swamped by the great flood of yolk which is laid down, 

 and that a bilateral structure is only gradually and slowly re-established. 

 It is true that Tung and Tung (1944) argued that, in the goldfish, an organ- 

 iser-like region beomes located on one side of the egg shortly after fertili- 

 sation, and thus endows it with a bilateral symmetry, but it is doubtful 

 if their experimental material was adequate to sustain this conclusion 

 (p. 230). It seems likely therefore that we have to accept a difference in 

 timing as between the holoblastic and blastodermal eggs, the dorso- 

 ventral plane being determined much later in the latter. 



In the Ampliibia the agent wliich determines the dorso-ventral plane 

 was the inner yolk mass. In the teleosts it seems to be the periblast. It is 

 difficult to say how far these two can be considered as in any way com- 

 parable, either in their anatomical derivation or their mode of action. In 

 both groups what is determined is, in the first place, the invagination of 

 endoderm; and hard on the heels of this comes the mesoderm, which 



