4 BLT.T.F.Tiy 905 3 U. S. DEPARTMENT OF AGRICULTURE. 
egg cell, a change takes place in the latter which prevents the entrance 
of others. 
SEXUAL MATURITY. 
Ani in a Is reproduce only during the part of then lifetime following 
sexual maturity and preceding senescence. During this period most 
of them reproduce only in a certain season of the year, the breeding 
season, and with hi this season a given female will breed during; only 
/¥ M 
\ 
•A 
F". 1 —a — : uy :■: zL~. :iia; :t:::v;::-t :ells :r:aa i aaehe arasshrrrer. The :ari:ai 57:- ile-sh. :t: 
figure, which is characteristic of dividing cells, is clearly brought out. In the formation of the repro- 
ductive cells, the chromosomes, which appear solid black in the illustration, come together in pairs 
around the equator of the spindle, separate and move to opposite poles, iormiDg two nuclei as a pre- 
'- —---■--, ~ t t: ihasha :;hae cell as 3 — h:le Zich :: chereralcia;: :■ ell 5 his :nl~ hiliche rririail aaahc-er 
::' ehromosam es. In ordinary cell divisions, each chromosome splits, the halves moving to opposite 
roles, each ::' —hi :h : as rhraias the saaae aaaiher :■: :: ::a;:s raies is :ae rriaiaal cell. Only ; ''.-■" :•:' 
:he car :aacs races ars ia : a- ia :he aicrare. — hich ~^,s thaea thTcagh a hirh-r-;— erraicr: scare. C-cur- 
a very brief period, the period of heat or oestrus. In many cases the 
cestrous peri" 1 : recurs at definite intervals during the breeding season. 
The age of maturity, while in a general way characteristic for a 
given kind of a nim al, depends on a great number of factors in indi- 
vidual cases. A warm climate, liberal feeding, and good care in 
era! are conducive to early maturitv. There are also hereditarv 
differences. Earlv sexual maturitv is doubtless correlated with the 
