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mammals ^). However this may be, and it is a matter of no apparent 

 import, the initiation of birth at the critical period put restrictions 

 upon ovulation, if none previously existed. 



Ovulation was rendered impossible during gestation, and under 

 the most favourable circumstances it could only recur shortly after 

 every birth. 



Under the conditions imposed by uterine gestation the number of 

 offspring is at best a restricted one, no case being known, in which 

 the uterus can normally harbour more than 27 developing embryos ^), 

 the average number being much smaller, even in the most prolific 

 forms. 



It, therefore, became necessary to repeat the ovulation as often 

 as possible under the limitations of frequency and number of eggs 

 imposed. 



Thus it arose, that the ovulation-period became almost equal to, 

 but actually a little longer than, the interval of time necessary for 

 development to the critical period, or, in other words, to the birth. 



The latter interval may be termed "the critical unit" in any given 

 case, and in the same way the period of time between any two ovul- 

 ations may be described as "the ovulation- unit". 



It is probable, that the critical unit first fixed the ovulation-unit ; 

 or, at any rate, that the interaction of the two led to a correspondence, 

 such that tht latter always slightly exceeded the former. Owing to 

 this it resulted, that a coming ovulation, i. e. a reflex message from 

 the ovary, was the direct cause of birth. 



Tyler Smith in 1849 almost foresaw this; but, though he as- 

 signed the cause of birth to the ovary, he failed to recognise that a 

 coming ovulation was at its basis. 



When an allantoic placenta was initiated, with consequent pro- 

 longation of the gestation, the correspondence and relationships of the 

 critical and ovulation-units determined the conditions, under which 

 this prolongation might take place. It could only be extended in the 

 first instance by being made to embrace two critical units instead of 

 one, so as to maintain the correspondence of the critical and the 

 ovulation-unit, i. e. so that birth might still be induced and succeeded 

 by a new ovulation. 



The carrying-out of this extension of gestation to two critical units 



1) The conditions in Monotremes might be interesting in this 

 connection. 



2) Didelphys according to Selenka. 



