152 



THE LATER STAGES OP DEVELOPMENT 



Fig. 53. Extirpation of the tail primor- 



dium of Triton at an early stage (a) 



results in an entirely tail-less larva (h). 



After Sehaxel. 



therefore, is a develop- 

 mental phase of limited 

 duration during which 

 regulation exists no 

 longer, and regenera- 

 tion not yet. 



Results similar to 

 those described for the 

 tail primordium have 

 been obtained with 

 other organs as well at 

 this stage. Complete 

 extirpation of a limb 

 primordium, for in- 

 stance, results in last- 

 ing absence of the limb 

 concerned, both in am- 

 phibians, and in the 

 chick embryo (Fig. 54). The same applies to eyes, gills, etc., 

 and even to the blood of amphibians : removal of the rudiment of 

 the blood cells leads to the development of larvae without blood 

 corpuscles (Frederici, 1926). 



The high degree of independence of the organ primordia 

 is also demonstrated by their behaviour on transplantation. 

 In the previous chapter we have seen many examples of the fact 

 that, once they have been determined, organ primordia grafted 

 into a foreign environment can continue their development in 

 complete independence, and that they can in this way reach 

 a high degree of differentiation. This, moreover, is true not 

 only of amphibians, but of other animals at the same stage 

 as well. Organ primordia of the chick embryo, grafted into the 

 egg membranes of another, much older embryo, will often 

 continue their development, and differentiate into the organ 

 they were destined to form. The same applies to fishes. Here 

 organ primordia have been grafted onto the yolk sac of other 

 embryos. Organ rudiments of mammals, e.g. rabbits and rats, 

 can be grafted onto the omentum in the abdominal cavity of a 

 rabbit. In that case, and, what is more, even after transplantation 



