42 2 HEREDITY AND DEVELOPMENT 



essential mosaic or organisation within the chromosomes of the 

 nucleus may induce a secondary mosaic or localisation in the 

 building material which the general substance of the egg-cell 

 affords. It need hardly be pointed out that the organisation 

 or architecture which is thought of in such cases is something 

 infinitely finer than the microscopically visible (reticular or 

 alveolar) structure which all living matter exhibits. 



What is Distinctive in DeYelopment ? — Unicellular organ- 

 isms divide and redivide rapidly ; it is their normal mode 

 of multiplication. The germ-cells of multicellular animals do 

 the same in the early chapters of their history. The fertilised 

 egg-cell does the same ; but the daughter-cells or blastomeres 

 cohere to form an embryo^ just as the daughter-cells into which 

 some Protozoa divide also cohere to form a " colony." For a 

 time there is no growth among the cleavage-cells into which 

 the fertilised ovum divides, so that an embryo of sixty-four cells or 

 more is no larger than the undivided Q,^g. This, again, is paral- 

 leled by cases of spore-formation in Protozoa, where many 

 divisions occur in a short time and within the limited space of 

 the mother-cell. In some cases the young embryo shows a large 

 number of nuclei derived from the division of the fertilised 

 nucleus of the egg-cell, while the cell-substance is slow in being 

 segregated around the nuclei into unit-areas or cells. This, 

 again, is paralleled by some multinucleate Protozoa. 



Thus the really distinctive fact in development is the 

 progressive differentiation. The daughter- cells do not remain 

 homogeneous ; some start a lineage of nerve-cells, others 

 a lineage of digestive cells, and so on. In a gradual, orderly 

 fashion the apparently simple gives rise to the obviously com- 

 plex, and throughout the process there are striking phenomena 

 of adjustment to temporary conditions, of " self- differentiation " 

 on the one hand and mutual influence on the other, and of 

 integrated " regulation " throughout. We are so familiar with 

 the orderly succession of events that we hardly realise the 



