192 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



the four unmodified chromosomes. This latter cell (the thick arrow at 

 the top of Fig. 87 points to it) is the primordial germ-eell, the ancestor 

 of all the cells that constitute the gonad of the individual. 



Fig. 87. 



Diagram to illustrate the nuclear differentiation of sonaa from gonad in Ascaris megalocephala. 

 Z represents the zygote-nucleus with its four chromosomes. I, II, III, IV represent successive 

 generations of nuclei descended from the original zygote-nucleus. I — two nuclei, those of the first 

 two blastomeres. That on the left (somatic) shows the commencement of the process of chromatin 

 diminution. II — 4-cell stage. The two nuclei on the left have undergone diminution : the cast- 

 off ends of the chromosomes are seen outside the nuclei. Of the two nuclei on the right the lower 

 shows commencing diminution. Ill — 8-cell stage. Of the eight nuclei, six (somatic) have undergone 

 diminution, one (to the right) is conmiencing diminution, and the remaining one retains the four 

 chromosomes unaltered. IV — 16-cell stage. Of the 16 nuclei, 14 (somatic) have undergone diminu- 

 tion, and of the remaining two (above and to the right) one (somatic) is commencing diminution while 

 the other retains the normal four unmodified chromosomes. The last mentioned is the primordial 

 germ-cell — the ancestor of all the cells of the gonad. 



During subsequent development the cells go on dividing with ordinary- 

 mitosis — vast numbers of cells poor in chromatin forming the compli- 

 cated soma of the adult ; vast numbers of others with the full amount 



