54 EIGHTEENTH REPORT. 



some pass to the opposite poles. The result is that every new 

 gamete has one of each kind of chromosome present while the 

 zygote nucleus and its descendants possess two of each kind. 



The complexity' of the process for exactly halving the chromo- 

 somes in ordinary mitosis and for properly distributing the whole 

 chromosomes in reduction division makes it seem certain that the 

 chromosomes play a very important role in the cell. The fact that 

 the mode of distribution of the chromosomes at meiosis takes place 

 exactly in the manner that the Mendelian theory requires for the 

 distribution of the structures responsible for the main ^'mendeliz- 

 ing" characters has led further to the assumption tliat it is tlie 

 hereditary characters that are borne by the chromosomes. 



We talk verj'^ glibly about the "bearing of heredity characters." 

 Just what is meant by this phrase? I teal it is a term that we all 

 use verj' frequently without really considering what is included in 

 the expression. We know, in general, that "like begets like." 

 What does this mean in terms of cells and their activities? 



Every plant and every animal develops from a single cell, the 

 fertilized egg, leaving out of consideration the rather numerous 

 host of lower plants in which sexual reproduction is lacking 

 ( Myxophj'ceae, Oauleii)a, and scattered forms in other groups). 

 The latter, although not producing eggs, mostly possess at one 

 stage of the life cycle but a single cell. Even in those forms that are 

 always multicellular we find that all parts of the individual arise 

 by cell divisions or in case of coenocytes like Caulerpa by cell en- 

 largement accompanied by nuclear divisions. Thus all the complex 

 features of the structure of the most highly developed plants or 

 animals must be bound up in the limits of the single cell which 

 bridges the gap from one generation to the next. We may acce])t 

 the theory of those who would greatly limit the complexity of these 

 details by arguing, in manj' cases doubtless with right, tlmt the 

 development that any jjarticular cell of a multicellular plant or 

 animal undergoes is mainly merelj' a direct response to the im- 

 mediate environmental conditions. But even then we must ad- 

 mit that there must be vast differences in the factors present with- 

 in the nucleus of this cell to account for the fact that the eggs of 

 different animals e. g., fishes, frogs, toads, not to mention the 

 host of the acquatic invertebrates, and of many algae all develop 

 into their own jiroper species although the external environment 

 is identical. 



The structure of the mature individual is a result of cell divisions 

 and the modification of the resultant cells in various disections. 



