HEREDITY 317 



nated here as M and M'. When the primary oocyte divides, reduc- 



S 



ing the number in the secondary oocyte to—, one of the two daugh- 

 ter cells is not a functional secondary oocyte, but an abortive small 

 cell called the first polar body, or first polocyte. It contains either 

 all M or all M' chromosomes, which are thereby lost, for, although 

 the first polocyte usually divides again, it never becomes functional 

 nor seems to serve any useful purpose. It must be remembered that 

 the polocyte also contains an X chromosome. The loss of chromatin 

 represented by the nucleus of the first polocyte means that some of 

 the inheritable characters may be dropped at this point in the 

 chromosome cycle, for only those in the functional secondary oocyte 

 ever have an opportunity to realize the characters of an adult. 



The secondary oocyte, with — chromosomes, then divides again 



by ordinary mitosis to form ootids. But again one of these is an 

 abortive, small second polar body, or second polocyte. The remain- 

 ing ootid is now the only functional cell derived from the original 

 oogonium, a marked contrast in number with the four spermatids 

 derived from a single spermatogonium in spermatogenesis. The 

 chromosomes of the ootid are one-half the somatic number, either 

 all M or all M' elements, one of which is an X chromosome, des- 

 tined to become the synaptic mate of the PX or PY chromosomes 

 in the sperm. The ootid now becomes transformed into a mature 

 tgg, the transformation including an increase in size and usually 

 the secretion of considerable quantities of food reserves, commonly 

 known as yolk. 



Composition of the Nucleus o£ the Zygote (Fig. 203). 

 When the tgg is fertilized by the spermatozoon the somatic num- 

 ber of chromosomes is restored and P and M chromxosomes become 

 the synaptic mates in the soma cells of the resulting new organism. 

 If the sperm that fertilizes the egg happens to be one of those which 

 contain the PX chromosome^ the PX of the sperm and the MX 



