358 PRINCIPLES OF EMBRYOLOGY 



mass (a so-called 'Balbiani ring') ; in the other two lobes the same region 

 has a perfectly normal structure. The swelling in the chromosomes of the 

 forelobe begins to retract at the end of larval life and during the prepupal 

 stage, exactly at the time when this lobe produces a brownish secretion. 

 In Pavan's case (Pavan I955) there is also a transitory swelling of particular 

 bands at particular times in certain tissues but not in others. This seems as 

 direct evidence as one could hope for of the activity of individual genes at 

 characteristic times and places. 



The fact that a given gene produces a different intensity of effect in 

 different types of cytoplasm is, of course, obvious enough from the mere 

 occurrence of the differentiation of gene-controlled processes. It is also 

 demonstrated very clearly in certain particular cases. Baltzer (1940, 1952&, 

 c) and his students have carried out many experiments in which an egg of 

 one species of urodele has been fertilised by sperm of some other species 

 e.g. Triton taeniatus or palmatus egg fertilised by T. cristatus sperm. These 

 hybrids are often viable, but some combinations eventually die, usually 

 at fairly late stages of development. A more interesting situation is pro- 

 duced if, after the fertilisation but before the conjugation of the nuclei, 

 the egg nucleus is sucked out with a pipette. This leaves only the foreign 

 sperm nucleus in the cytoplasm of the other species; such animals are 

 known as hybrid merogons. In all the combinations tested, they die 

 before completing development and at an earher stage than do the corre- 

 sponding hybrids in which the female nucleus has been left in situ. Pre- 

 sumably the origin of the nucleus and cytoplasm in the hybrid merogons 

 from different species makes it impossible for them to interact in a satis- 

 factory manner. In some combinations, e.g. the Triton ? + Salamander <^ 

 hybrid (with female nucleus intact), the lethahty affects all cells of the 

 embryo more or less equally. In others, however, some tissues may be 

 much more strongly affected than others. In the hybrid merogon in which 

 there is only a T. cristatus nucleus in taeniatus cytoplasm, it is the head 

 mesoderm which suffers most severely and which dies earliest. This must 

 mean that there is some reaction between the nucleus and cytoplasm of 

 this particular tissue which cannot be properly performed when they 

 belong to different species (Fig. 16.6). 



It is noteworthy that in several such cases the merogonic tissue which 

 would die or fail to develop if left in situ can be kept aUve for a con- 

 siderable time, and will often continue its differentiation, if it is grafted 

 into a normal host embryo (e.g. Hadom 1937). Apparently the substances 

 which the merogonic tissues cannot make are diffusible and can reach it 

 from healthy tissue in the neighbourhood. Their nature is quite unknown, 

 and would seem likely to repay investigation. 



