326 THE RECAPITULATION THEORY 



yet been seen, and probably does not occur, as the 

 larva still possesses sufficient food yolk to carry it 

 on in its development. It is therefore permissible 

 to hold that the neurenteric canal may be a mere 

 embryological device, and devoid of any deep mor- 

 phological significance. 



The question of variation in development is one 

 of very great importance, and has perhaps not yet 

 received the attention it deserves. We are in some 

 danger of assuming tacitly that the mode of develop- 

 ment of allied animals will necessarily agree in all 

 important respects or even in details, and that if 

 the development of one member of a group be known, 

 that of the others may be assumed to be similar. 

 The more recent progress of embryology is showing 

 us that such inferences are not safe, and that in 

 allied genera or species, or even in different indi- 

 viduals of the same species, variations of develop- 

 ment may occur affecting important organs and at 

 almost any stage in their formation. 



Great individual variations in the earliest pro- 

 cesses of development i.e. t the segmentation of the 

 egg have been described by different writers. In 

 Renilla, Wilson found an extraordinary range of 

 variation in the segmentation of eggs from which 

 apparently identical embryos were produced. In 

 some cases the egg divided into two in the normal 

 manner ; in other cases it divided at once into eight, 

 sixteen, or thirty-two segments, which in different 

 specimens were approximately equal or markedly 

 unequal in size. Sometimes a preliminary change 

 of form occurred without any further result, the egg 



