Nature of Heredity. 335 



the germ-cells usually become specialised at length for special func- 

 tions concerned with their union, and with this specialisation they 

 may be supposed to lose the power which they have hitherto pos- 

 sessed of receiving and storing up stimuli, of which they have already 

 received the full complement requisite for controlling the develop- 

 ment of the new organism to which thev will give rise. 



5. THE INTERPRETATION OF ONTOGENY AS A PROCESS 

 OF PROGRESSIVE EQUILIBRATION AND THE BIO- 

 GENETIC LAW. 



Leaving out of the question for the present the complications 

 introduced by the sexual process, we must now enquire how the pro- 

 cesses of ontogeny or individual development may be explained in 

 accordance with the above enunciated views as to the nature of the 

 germ-cells and their relations tO' the somatic or body-cells. 



The fertilized ovum, or egg-cell, at the time of commencing its 

 development, consists of a certain amount of protoplasm, often 

 charged with food material, and containing a nucleus in which we 

 suppose to be stored up ancestral stimuli which will control the 

 development. Let us imagine these stimuli to be given out again in 

 the order in which they were received. 



The first stage in the development of a typical organism is the 

 division of the ovum intO' two cells, preceded, of course, as always, by 

 division of the nucleus. This two-celled body must make, and must 

 always have made since it was first developed, its quotum of impres- 

 sion upon the nuclei, must produce, that is to say, probably within 

 the nuclei, a system of forces which requires a two-celled body to 

 ■equilibrate it. 



This system of forces constitutes a stimulus which is stored up 

 in the nuclei, and handed on by nuclear division tOi the future germ- 

 cells, and this will be the first stimulus given out by the nucleus of 

 the developing ovum. Therefore, the developing ovum will divide 

 into two cells exactly in the same way as its ancestors did, this being 

 the only way in Avhich equilibrium can be attained. 



But although the two-celled body may have been at one time 

 a more or less permanent condition, it is now only temporary, and is 

 rapidly succeeded by a four-celled stage, and this also must always 

 have made and must still make its impres.sion upon the arrangement 

 of forces in the nuclei, and so on, through all the different stages of 

 ■development with their increasing complexity of structure. At every 

 successive stage of development the stimuli stored up in the nuclei 

 must give rise, by internal equilibration, to a form of body similar 

 to that which originally transmitted these stimuli to> the nuclei, and 

 which is the only form which can balance these particular forces. 



Wh\ . it mav be asked, should we suppose that the stored up 

 stimuli in the nuclei of the developing organism must always be 



