TYPES OF (ESTROUS CYCLE 69 



synchronized, and some extraordinary distortion must 

 have taken place in the human cycle if fourteen days 

 (i.e. half the total length of cycle) elapse between 

 prooestrus and ovulation. 



(c) More recently, Marshall has suggested that menstrua- 

 tion represents both pseudo-pregnant and prooestrous 

 degeneration telescoped into one (448). In animals 

 such as the guinea-pig with a dioestrous cycle con- 

 taining a luteal phase, the end of the latter (pseudo- 

 pregnancy) is rapidly followed by prooestrus, and a 

 further contraction of the cycle at this point might 

 result in these stages becoming merged. This view of 

 Primate menstruation is satisfactory in explaining 

 many difficulties. It admits the essential pseudo- 

 pregnant nature of the premenstrual growth of the 

 uterus, and explains menstruation in non-ovulating 

 animals by the supposition that the part of menstrua- 

 tion occurring under such conditions is the prooestrous 

 stage only. Corner's (126) observation that menstrua- 

 tion without ovulation is not preceded by the typical 

 premenstrual growth of the endometrium is therefore 

 of great interest. Allen's (12) results can also be 

 explained on the grounds that the rupture of large 

 follicles and the cessation of injection of the oestrus- 

 producing hormone precipitate the destruction of 

 prooestrous growth. 



Even this view, however, has the difficulty that 

 the destruction of prooestrous growth will have ended 

 some nine days before ovulation, and it does not 

 explain such abnormal phenomena as menorrhagia 

 associated with cystic follicles. 



