104 



THE VERTEBRATE OVARY AND ITS RELATIONSHIP TO REPRODUCTION 



NON -PREGNANT CYCLE 



PREGNANT CYCLE 



COPULATION NOT COPULATION PERMITTED 

 J PERMITTED ^ 



ESTRUS ESTRUS 



UTERINE 

 CYCLE 



HEMOR RHAGE 



HEMORRHAGE 



GLANDS HIGHLY DEVELOPED 



PARTURITIOM 



DAYS 10 21 10 20 210 TO 335 



Fig. 57. Reproductive and pregnancy cycles in the cow. (Parturition period based upon 

 data supplied by Kenneth ('43), also by Asdell ('46). Other data for chart derived from 

 Asdell ('46). 



Three main characteristics of heat or estrous period are evident: (1) A duration of 

 heat of only about 10 to 18 hours; (2) abundant secretion (;luring heat of a "stringy 

 mucus," derived from mucoid epithelium of vagina and from sealing plug of cervix when 

 cow not in estrus (Asdell); and (3) ovulation occurs from 13'/2 to \SVi hours after ter- 

 mination of estrus (Asdell). Variation in time of ovulation may be considerable, from 

 2 hours before end of estrus to 26 hours after (Asdell). 



mature pattern established during earlier development (fig. 58A, A', B). The 

 male mammary gland may remain similar to the condition shown in fig. 58A. 

 The maturing of the egg follicles within the ovary and the concomitant for- 

 mation of estrogen which accompanies sexual maturity is linked with the 

 mere complex state of the mammary-gland system shown in fig. 58B. 



The next step of mammary-gland development is carried out under the 

 influence of progesterone. Progesterone is necessary for the development 

 of the terminal glandular tissue or alveoli associated with these ducts (fig. 

 58C, D). Finally, the pituitary lactogenic hormone (luteotrophin [LTH];, 

 prolactin) stimulates the actual secretion of milk (fig. 58E). Recent research 

 also has shown that the lactogenic hormone collaborates in some way with 

 estrogen and progesterone in the development of the mammary-gland tissue. 



