ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



192 



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

 *7 points to it) is the primordial germ-cell, the ancestor 

 that constitute the- ^onacl of the individual. 



Diagram t<> illustrate tin- inn 1< -.ir dillercntiation of soma from gonad in Ascaris megalocephala. 

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

 M 1. 1 descended from the original zygote-nucleus. I two nuclei, those of the first 

 'lli.it in tlir 1' it i ---im.-itic) shows the commencement of the process of chromatin 

 II i (ill -t.i,:;i'. 'lln'twn nuclei on the left have undergone diminution: the cast- 

 [] nui-.ide the nuclei. Of the two nuclei on the right the lower 

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

 U commencing diminution, and the remaining one retains the four 

 IV id-iell stage. Of the i<> nuclei, i.\ (somatic) have undergone diminu- 

 tion, and of tin- ti-iii.iiiiiiig two (above and to the right) one (somatic) is commencing diminution while 

 the other r.-t.mis tin- n-ii-ni.il four unmodified chromosomes. The last mentioned is the primordial 

 germ-- [ aU the cells d the K onad. 



During suUsi-quriit drvc-lopment the cells go on dividing with ordinary 

 t nninhcrs of cells poor in chromatin forming the compli- 

 >t tlic adult : vast numbers of others with the full amount 



