240 PRINCIPLES OF EMBRYOLOGY 



of the rabbit epiblast to react to neuralising stimuli was demonstrated by 

 cases in which such an effect was produced by grafts of chick primitive 

 streak (Fig. 11.9); and rabbit primitive streak was also able to induce 

 when grafted into the chick. It is highly probable, therefore, that the 

 mammalian primitive streak is an organisation centre with functions 

 similar to those of the same structure in the bird embryo. It was also 

 showai that development of the posterior region is possible in the abseace 

 of the anterior end of the streak (Hensen's node), which some authors had 

 considered to be an essential focus of embryo formation ; again a result 

 which parallels that in the bird. Toro (1939) has reported the induction 

 of neural tissue in the rat by grafts placed in the amniotic cavity, but his 



Figure 11.9 



A small piece of chick primitive streak was grafted under the ectoderm of a 



rabbit embryo of the streak stage, which was then cultured in vitro. The 



graft has formed some neural tissue, and has induced a neural plate in the 



rabbit host. (After Waddington 1934.) 



evidence, as published, is not very convincing, and it would be unexpected 

 to find that tissues lying freely in such a cavity, in contact with the outer 

 side of the ectoderm, could successfully induce; it seems more likely that 

 he was dealing with embryos which were distorted as a result of the 

 operations. 



The results of direct operative experiments on mammal embryos have 

 been supplemented by evidence of quite a different kind, which comes 

 from an analysis by methods which have as yet been little used in other 

 groups of animals, except the insects. A fairly considerable number of 

 hereditary factors or genes are known which cause abnormahties in early 

 developmental stages of mammals ; perhaps their comparative frequency 

 in this group is connected with the extremely rapid and radical changes 

 which have led to the evolution of the group. The effects of these genes 

 are often somewhat variable, and by a study of the variations it is some- 

 times possible to decide that one particular aspect of the abnormality is 

 primary, and the remainder secondary consequences; from such argu- 

 ments some insight into the causal sequences of epigenesis can be attained. 



