THE GERM-PLASM THEORY 299 



favor of this view are the facts that during mitosis 

 the number and shape of the chromosomes are con- 

 stant in every species (variations sometimes occur) 

 and the complex series of processes in indirect nuclear 

 division seems to be for the sole purpose of dividing 

 the chromosomes equally between the daughter 

 cells; even during the intervals (interkinesis) be- 

 tween successive mitoses the chromosomes may be 

 recognized in certain species as prochromosomes 

 (see Digby, 1914, for review of literature). During 

 the maturation of the germ cells chromosomes 

 seem to play the most important role, uniting in 

 synapsis, and separating in the reducing division. 

 The chromosomes of the minute, motile sperma- 

 tozoa equal in number those of the comparatively 

 enormous, passive egg; the spermatozoon consists 

 almost entirely of chromatin, and this is the only 

 substance present in the zygote that is equally 

 contributed by both egg and spermatozoon. The 

 processes following the penetration of the spermato- 

 zoon into the egg bring about a combination of the 

 chromosomes of the two gametes into a single 

 nucleus ; in certain animals at least some characters 

 depend upon the presence of a certain chromosome, 

 the X-chromosome ; in certain cases of polyspermy 

 the addition of extra male chromosomes seems to 

 be the cause of the abnormal development of the egg. 

 These and many other facts of chromosome be- 

 havior that have been discovered by observations 

 and experiments have convinced most biologists 

 that the chromatin is the germ-plasm. 



