THE VITAL UNITS CALLED CELLS 201 



spireme. At the same time the diplosome divides giving 

 rise to two centrosomes which separate and migrate in 

 opposite directions. 



c. The spireme sphts longitudinally throughout its length. 

 This is important because, if the hereditary determinants 

 occupy a definite Hnear arrangement in the substance of 

 the spireme, as they appear to do, it provides for their 

 quahtatively equal separation into two parts. 



D. The double spireme now becomes segmented into a 

 series of pairs of rod-Hke bodies called chromosomes. The 

 nuclear membrane disappears; the centrosomes move further 

 apart. The chromosomes become disposed in such a way 

 that those formed from each^half of the spireme are placed 

 on opposite sides of a plane known as the equator of the 

 cell and represented by a dotted hne. 



E. The groups of chromosomes separate and then fuse 

 together losing their discrete outhnes. A circular groove 

 appears around the equator of the cell and gradually deepens. 



F. Finally the masses formed from the chromosomes 

 become enclosed in nuclear membranes and the groove 

 pinches the originally single cell into two cells which are 

 qualitatively similar as far as their nuclear components 

 are concerned. 



DETERMINATION OF SEX 



A fundamental difference has been discovered in the 

 chromosomes of male and female sex cells. The former 

 originally contain one x, or sex, chromosome which is often 

 larger than the others and easily identified; while the latter 

 possess two of them. As the sperms and eggs mature the 

 number of chromosomes in each is reduced by one half, 

 because they are later to combine to produce tissue cells 

 with the whole number. This means that half of the males 

 contain an x chromosome and that the other half do not, 

 further, that each and every female has now one, in place of 

 the original two. On fertilization there are two possibilities 

 expressed by the following equations. 



1 . Egg X + Sperm x = individual 2X, a female 



2. Egg X + Sperm = individual ix, a male 



