101 



However this may be, something of the same sort has taken 

 place without affecting the ovulation-unit in Ungulata and man. lu 

 horse, sheep, pig, and man the critical unit is no longer the equivalent 

 of one ovulation-unit, but almost of two such. This is interpreted as 

 meaning, that the gestation-length has been doubled without altering 

 the grade of development at birth, and it is put into association with 

 a probable increase of size in the course of the ancestral history, without 

 excluding the possibility, that other factors may have come into play. 



In all mammals ovulation during gestation is either abortive or 

 suppressed; and this is necessary, for a normal ovulation during 

 gestation would lead to abortion. 



The corpus luteum is probably a contrivance for the suppres- 

 sion or rendering abortive of ovulation during gestation. The com- 

 mencing degeneration of this structure some little time before the 

 end of the gestation (like its rapid atrophy, where fertilisation has not 

 taken place) allows of preparation being made for a new ovulation. 

 As already indicated, the approach of this ovulation is, in a reflex 

 manner, the direct cause of birth. 



In the complete work consideration is given to the causes of 

 many abortions in various mammals and to the nature of menstruation. 

 It is insisted that the critical period (with the consequences follow- 

 ing in its train), multiples of this, and the ovulation-periods nor- 

 mally-suppressed during gestation must be very frequently times of 

 abortion in various mammals, particularly in those, like horse and 

 man. where the gestation is a prolonged one of several units. 



Menstruation is comparable to an abortion prior to a new ovul- 

 ation, and it is an abortion of a decidua prepared for an egg, which 

 was given off subsequent to the preceding menstrual period, and which 

 has escaped fertilisation. It is an abortion corresponding in its 

 periodicity with a former critical unit, which obtained before the present 

 one arose by a duplication. Thus, it is comparable to an abortive 

 birth at a former critical period. 



So far it has appeared, that, under the conditions governing it, 

 gestation in Metatherian and Eutherian mammals is, like their ovul- 

 ation , dominated by a rhythm , and a similar and connected one 

 really underlies lactation. 



This latter originally, and even now in many cases, had no effect 

 on gestation, and a female, suckling young, could, and in some species 

 can, become pregnant again immediately after the birth of a litter. 

 This, as is well known, holds true of the mouse, rabbit, and cavy. 

 But in others the effect of lactation has been either to destroy the 



