EMBRYO FORMATION IN OTHER GROUPS OF VERTEBRATES 233 



It appears probable that there is a considerable difference of a quantita- 

 tive, if not of a qualitative, character between the teleosts and the Amphi- 

 bia in the extent of regulation which can go on in portions of the blastula 

 and gastrula separated from the main centre of the organiser region. 

 Luther (1936) divided the pre-gastrulation blastoderm of the trout^'into 

 four equal quadrants, each of which was then isolated by grafting it into 

 the yolk-sac epithehum of an older embryo. All four quarters were 

 found capable of differentiating into all the main embryonic organs, 

 although these were usually somewhat chaotically arranged owing to the 



Figure 11.5 



In a an early gastrula of the trout is opened, the tongue of invaginating 



material cut away and grafted into the opposite side of the blastoderm; h 



surface view, showing the graft (dotted) ; c and d, stages in the developm^ni 



ot the host and mduced embryos. (From Luther 1935.) 



abnormal mechanical conditions. Soon after the beginning of gastrulation 

 this equality between the four sectors disappeared. The sector diametri- 

 caUy opposite the position in which the embryo will form soon loses its 

 power to differentiate axial organs. There develops, in fact, a well-marked 

 gradient, according to which the capacity for differentiation falls off from 

 each side of the mam embryonic region. In Fundulus, Oppenheimer (1053) 

 finds rather better differentiation of embryomc regions (three successful 

 out of six) than of extra-embryonic germ-rmgs (five out of seventeen) 

 but the figures are hardly sufficient to estaUish the existence of a gradient 

 In the Amphibia at similar stages (i.e. blastula and early gastrula) the 

 capacity of isolated ventral portions to regulate and form a complete 

 embryo is, of course, very much less than that just described for the teleost 



