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General Biology 



gathers in a thick mass towards one side of the nucleus. This is known 

 as the synapsis stage. From this thick mass of chromatin there will 

 emerge just one-half the number of chromosomes usually found in cells 

 of the particular species we are studying. Such cells are said to have 

 the haploid number of chromosomes. 



Each of these chromosomes is double, the two parts either lying 

 side by side, or end to end. This stage of half the number of chromo- 

 somes (but where each is a double chromosome), is called pseudo-reduc- 

 tion. Real reduction then follows. The two parallel portions of each 

 chromosome divide longitudinally, while the entire chromosome con- 

 tracts into small four-portioned chromosomes, each of which is called a 

 tetrad (Figs. 31 and 33). A mitotic figure now forms and moves toward 



chr. 



Fig. 32. Fertilization of the Amphibian Ovum. 



A, outline drawing of a section parallel to the axis of the egg; the superficial 

 pigment of the animal hemispheres of the egg is indicated, but the yolk granules 

 are omitted, co., entrance cone; spz., spermatozoon lying at the bottom of the 

 entrance funnel; s.sp., spermsphere. 



B., a meridional section through the egg at a later stage; cf, sperm nucleus, 

 also called the male pro-nucleus; $, egg-nucleus, also called the female pro-nucleus; 

 as., sperm-aster; ph., polar body. The size sperm-and egg-nuclei has been exag- 

 gerated. 



C, portion of a section through an egg showing an early stage in the forma- 

 tion of the fertilization spindle, highly magnified; d" sperm-nucleus; $, egg- 

 nucleus; cs., centrosomes. 



D, portion of a section of an egg showing the early stage of the metaphase of 

 the fertilization spindle; chr., the chromosomes derived from the sperm-and egg- 

 nuclei lying unevenly, but still in two distinct groups, in the equatorial plane. 

 (After Jenkinson.) 



the outer rim of the egg. The nucleus divides equally, so that one-half 

 of each tetrad passes to a daughter nucleus. 



Although the nucleus divides equally, the cytoplasm does not. This 

 produces one large egg cell and one small particle, this latter with one- 

 half the chromatin, but with little or no cytoplasm. The smaller portion 

 is the first polar body. This is pinched off from the egg cell proper. 



Both egg cell and polar body now begin to divide again. It is in 

 this second division that each remaining half-tetrad (now called a dyad), 





