58 HISTORY OF THE HUMAN BODY 



are equally represented. Neither the preponderating bulk of 

 the ovum (macro-gamete) nor the flagellum and other loco- 

 motor apparatus of the spermatozoon (micro-gamete) are 

 of any significance in hereditary transmission, but are mere 

 adaptive characters, of provisional functional importance, and 

 without influence in directing the development of the new or- 

 ganism ; while the chromosomes, to effect the union and equal 

 division of which the other parts have been developed, form 

 the true germ-plasm, transmitted in direct continuity from both 

 parents and entering every cell as it develops, directing both 

 the architectural plan which these cells assume and also their 

 gradual differentiation into the tissues which form the adult 

 soma of the succeeding generation. 



In this " Continuity of the germ-plasm " is found the ma- 

 terial basis also for the recapitulation theory, the law of 

 biogenesis explained in the first chapter; for the continuously 

 living chroniatin, which pervades each cell of an organism, 

 has in its ozvn existence actually experienced all the somatic 

 modifications of its entire past history, traces of which it 

 must retain in some form of structural expression, enabling it 

 to control the development of the soma during every stage of 

 its existence. How this is effected is far beyond our present 

 means of observation, and perhaps of experiment, but the re- 

 sults presuppose an inconceivably complex structure in the 

 chromatin in order to render such results possible. 



The first stage in the development of all Metazoa, that of 

 the fertilized ovum or zygote, is followed, in most cases imme- 

 diately after fertilization, by a succession of cell-divisions, or 

 cleavages, as they are here termed, which, in typical cases, fol- 

 low a general geometrical plan and result in the formation of a 

 mass of cells that shape themselves into a definite embryological 

 stage, that of the blastula. As the various geometrical forms 

 assumed by the cells during the cleavage stages are all rep- 

 resented among colonial one-celled organisms, so there are 

 also a few such that, in the arrangement of their cells, closely 

 resemble the blastula. In this stage the cells form a hollow 

 sphere, one cell in thickness, and in cases in which the blastula 



