CLE A FACE AND DIFFERENTIATION 



nucleus the number of chromosomes equal to that of the 

 fertilized Qgg. The germ cells of the adult in their terminal 

 stages after the process of reduction again possess half the 

 somatic number. Thus the cycle from germ cell to germ 

 cell is complete. 



During the cleavage stages, the chromosomes are halved 

 in quantity at each cell-division. Since, as we know, they 

 never reach the zero-point, they must increase in mass 

 during the period of cleavage. Says Morgan^: "During 

 development, especially during early cleavages, the amount 

 of chromatin steadily increases in amount, giving an expo- 

 nential curve resembling the first half of a curve of a mono- 

 catalytic reaction." What evidence we possess definitely 

 warrants the statement that the chromatin material 

 increases in amount during cleavage. 



Suppose we consider the egg of a large fish, a carp or 

 salmon. Such an egg after fertilization has two groups of 

 chromosomes, one from the sperm-nucleus and one from 

 the egg-nucleus, which united give the total number of 

 chromosomes characteristic for this particular species of 

 carp or salmon. Eventually from this single fertilized egg 

 the adult fish develops, which is composed of millions and 

 millions of cells constituting all of the organs of the fish; 

 if it is a female, it produces thousands or even millions of 

 eggs, if it be a male, billions of spermatozoa. Eggs or 

 spermatozoa of an adult fish have chromosomes equal in 

 number, size, form and, presumably, in weight to those 

 found In the egg- or sperm-nucleus of the fertilized egg from 

 which this adult fish was developed. 



If the chromosomes grow — and It is Inconceivable that 

 they do not — they grow at the expense of something; they 

 can not grow directly out of themselves without obtaining 

 the new materials for themselves. The synthesis of chro- 



^ Morgan, ig2j. 



311 



