WITHIN THE EGG 183 



unspecialised cell-lineage, a direct descendant of the fertilised 

 egg from which the parent arose. The idea of germinal con- 

 tinuity has brought much light into biology. 



We know in the case of the chick, for instance, exactly 

 how stage succeeds stage, often in a circuitous fashion as if 

 not going straight to the end; but we do not know the 

 mechanics of development, how each stage conditions the 

 next, except in a vague sort of way. The Archiv fur 

 Entwicklungsmechanik contains, however, many papers 

 which show that some analysis is quite feasible. In some 

 cases we know quite precisely that e will follow a, b, c, d, but 

 we do not know how it comes about. The descriptive 

 account is sometimes nearly perfect ; the causal analysis lags 

 far behind, especially because (i) development is growth, and 

 growth is of the very essence of life ; and (2) development is 

 the expression of a mosaic inheritance which is gradually 

 realised. Is it not very difficult often, in spite of our theories 

 of association, selective memory, controlled attention, infer- 

 ence, and so on, to give any feasible account of how one 

 thought grows into a bigger and better one, though we feel 

 sure that the idea had a development, is a product of con- 

 scious or subconscious antecedent stages ? The same sort of 

 puzzle faces us in the study of development. Out of the 

 apparently simple there emerges the obviously complex ; we 

 see the stages, but we do not yet fathom the how of them. 

 We often think of a rich inheritor with a thick cheque- 

 book ; we can give some consecutive account of him, but 

 ever and anon he surprises us by a new departure, which 

 means that he has cashed part of his legacy. So the develop- 

 ing bird is progressively cashing, i.e. expressing or realising, 

 part of its inheritance. But its ways are very orderly. 



In a few hours the apparently simple drop of formative 

 protoplasm has become a disc of cells, in a few days it has 

 passed through the great events in vertebrate development 



