IV. i INITIAL STRUCTURE OF THE GERM 227 



fertilized, segments as a whole with the polar lobe in correct 

 proportion and gives rise to a dwarf larva with all its parts 

 complete (Fig. 137 a, c}. Many, however, die. 



A meridional half-egg is able to develop normally, or nearly so. 



The behaviour of the enucleate vegetative fragment of the 

 fertilized egg is interesting. Though deprived of nucleus and 

 centrosome it forms its polar lobe synchronously with the 

 divisions of the animal portion ; three times is the lobe protruded 

 and three times withdrawn: the fourth time it is not taken 

 back ; this is the moment when the first somatoblast is given off. 

 In this enucleate fragment the polar lobe is not in proportion 

 but of the same size as in the whole egg. 



Even the isolated polar lobe, or pieces of it, exhibits periodic 

 movements, amoeboid, or of elongation, or protruding a smaller lobe. 



FIG. 137. Dentalium. Development of egg- fragments, the plane of 

 section being indicated in the small figures. , 6, Twins, after horizontal 

 or oblique section near animal pole ; c, trochophore developed from 

 a fragment resembling that shown in a. (After Wilson.) 



The existence in the egg of preformed substances specified for 

 the production of particular organs is thus incontrovertibly 

 established, and as long as an egg-fragment or blastomere 

 contains all these it will give rise to an embryo which is, in form, 

 perfect, though lack of enough material (or the shock of the 

 experiment) may bring its life to an untimely end. 



At the same time these substances, though preformed, are not 

 necessarily prelocalized ; their original is not the same as their 

 ultimate situation, and in their redistribution we have to deal 

 with a process which is, as Wilson remarks, truly epigenetic. 



Before bringing this section to a conclusion reference must 

 be made to another case in which it seems probable that definite 



