264 THE GERM-PLASM 



the daughter-cells. TJiat portion of the ids of the inothej'-cell^ 

 tJierefore, luhich is for the time latent^ exerts a defnite influence 

 on the determination of tJie subsequent developjuent ; on it alone, 

 indeed, depends the number and order of succession of the 

 determinants which will subsequently become active, and all 

 those characters which result from the number and grouping of 

 the cells, as well as from the histological nature of the individual 

 cell, are also determined in the first instance by their ids. 



We can thus explain why cross-breeding is only successful in 

 the case of nearly-related species, and not in that of members of 

 entirely different families. If it were possible, for instance, for 

 the ovum of a sea-urchin to be fertilised by the spermatozoon of 

 a worm, such as Rhabditis nigrovenosa^ the disintegration of 

 the ids of germ-plasm derived from the father and mother 

 respectively would take place in an entirely different manner 

 even in the first stage of segmentation ; the maternal ids might 

 become disintegrated into the groups of determinants for the 

 right and left halves of the body^ while the paternal ids became 

 separated into the groups for the external and internal germinal 

 layers. But dissimilar groups of determinants of this kind could 

 not co-operate and give rise to intermediate structures ; and 

 even if the ontogeny advanced a few more stages, embryonic 

 structures could never be produced which would work together 

 harmoniously. 



The term homologous determinants and ids may be applied to 

 those which control homologous cells and groups of cells. It 

 may therefore be stated that structures which are intermediate 

 between those of the two parents may arise wJienever homologous 

 detern?inants and ids come together. If a small spot is present 

 on a certain region of the wing in two allied species of butter- 

 flies, for instance, and this is represented in the germ-plasm by 

 a single determinant, the homologous determinants of the two 

 cells in which this spot was first contained will be brought to- 

 gether when these two species are crossed, and will eventually 

 be able to share in the control of the cell in question. They 

 need not, however, necessarily be exactly alike ; the spot may 

 be brown in species A, and red in species B. In this case the 

 determinants would be homologous but not homodynajnous, and 

 might possibly combine to form a reddish-brown spot. The 

 essential point in amphigonic heredity is, that the idioplasm in 

 each cell in the entire ontogeny contains ids which are individually 



