THE BIOLOGY OF THE CELL SURFACE 



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ated from the somatic, are in their turn differentiated from 

 each other. 



The primordial germ-cells pass through a period of mul- 

 tiplication which ends with the production of many cells 

 called ovogonia (in the female) and spermatogonia (in the 

 male) whose nuclei still possess the somatic number of 

 chromosomes, i.e., the number characteristic for the species. 

 Ovogonia and spermatogonia without increase in their 

 numbers are transformed into primary ovocytes and pri- 

 mary spermatocytes respectively by the pairing of the 

 chromosomes in each nucleus; thus the somatic number of 

 chromosomes is "reduced" to one-half, the gametic or 

 haploid number. Then follows the period of growth which 

 is especially expressed in the egg. The maturation 

 (meiotic) divisions succeed the period of growth and differ 

 somewhat in the male and female sex-cells. 



In maturation usually each primary spermatocyte divides 

 into two secondary spermatocytes and each of these again 

 into two cells which are called spermatids. Hence a pri- 

 mary spermatocyte gives rise to four spermatids. By a 

 series of cytoplasmic changes together with nuclear con- 

 densation the spermatids become spermatozoa. Only 

 spermatozoa are capable of fertilizing eggs, whilst many 

 eggs, as we shall see soon, can be fertilized before, after or 

 in various stages of their maturation. 



The maturation (meiosis) of the egg occurs as follows: 

 at the end of the growth-period two divisions of the primary 

 ovocyte follow each other paralleling those in the primary 

 spermatocyte but the cells so arising are markedly unequal 

 in size. The small cells, the polar bodies, contain nuclei 

 enclosed by a minimum of cytoplasm. Of the three ovo- 

 tids (or four, if, as sometimes happens, the first polar body 

 divides) from one primary ovocyte there is only one, the 

 mature e.^%, capable of fertilization; the two (or three) polar 

 bodies are abortive eggs. 



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