148 TiiANSACTIOxNf.S AND PROGEKUINGS OF THK [Se.sh. Lxvi. 



little or nothing in common with previous ones. Under- 

 lying it is something more than a mere morphological 

 continuity of germ-cells. From its nature it might be 

 termed " the understudy-theory of heredity." Given in a 

 certain life-history the period of formation of the primary 

 germ-cells. Of these let there be for simplicity but two, 

 A and A\ On one of these falls the lot of developing 

 into an embryo. To which of the two this happens is 

 not of consequence for the argument. In all its es.sential 

 characters the remaining primary germ-cell (whose im- 

 mediate destiny it is to become the founder of the " sexual 

 products " of the said embryo) is the exact counterpart 

 of the developing one. So much so is this the case, that 

 if both form embryos, these are like-twins. 



In the ancestry neither of the primary germ-cells A 

 and A^ had ever been a Metazoan ; neither they nor their 

 ancestors had ever formed parts of a Metazoan body. 

 But their ancestry is continuous with a long line of germ- 

 cells, and at regular intervals these were exactly like . 

 certain sister-cells, which did develop and form embryos. 

 Although the cell A^ does not itself give rise to an 

 embryo, it retains for itself and for all its immediate 

 progeny the properties of A, those characters which, were 

 it or its progeny to develop, would make it or them like- 

 twins with A. 



In the drama of heredity there are always understudies, 

 which for a certain essential period are endowed with all 

 the identical properties of that germ-cell from which the 

 player arises. These understudies, the primary germ-cells, 

 are never employed upon the stage as such — except in 

 instances of like-twins — but some of them, in new guises 

 and after new conjugations, are the immediate ancestors of 

 those which become the acting characters in new scenes of 

 the cyclical drama of life. 



We now pass to the consideration of the primary germ- 

 cells as the equivalents of the spore-mother-cells of plants. 

 The theory of an antithetic alternation of generations as 

 the basis of Metazoan development postulates something 

 resembling the formation of spore-mother-cells in plants. 

 It is clear that the final reduction of chromosomes has 

 been deferred to a later portion of the life-cycle in Metazoa 



