TIME RELATIONS OF PARTURITION, MATURATION, ETC. 21 



haustively the reasons for failure in so many (54) cases, but some of the 

 apparent causes will be given for the benefit of those who may wish to 

 use the method for breeding purposes, or for a continuation of the study 

 of the phenomena of fertilization in mammals. 



The number of hours after parturition when mice were inseminated 

 varied from g% to 28^; and the time between insemination and killing 

 varied from 3^ to 17^ hours, as many time combinations as possible 

 being made. Before considering the two classes of eggs from the insem- 

 inated individuals — the fertilized and the unfertilized — 14 cases can at 

 once be deducted from the latter, because the eggs in those 14 mice were 

 found in the ovaries, where semination is of course not to be expected. 



In the first class (cases resulting in fertilized eggs) the times of 

 insemination were pretty evenly distributed between i6£ and 28^ hours 

 p.p. ; two, however, lay outside these limits, being at g% and 14^ hours p.p., 

 respectively. All these mice were killed at from 4 to 13^ hours after 

 insemination, the one inseminated at q4 being killed 13^ hours later 

 (23 hours p.p.). an( i the one at 14^, 6 hours later (20^ hours p.p.). In 

 the second class (resulting in unfertilized eggs) most of the insemina- 

 tions were made between 11 and 18^ hours p.p.; a few, however, were 

 evenly distributed between 20^ and 28^ hours p.p. The animals were 

 killed at from 3^ to 17^ hours after insemination. 



A comparison of the two classes brings out the fact that the insemi- 

 nations in both extend over almost exactly the same period of time, but 

 with a somewhat different distribution; and a more detailed examina- 

 tion of the data (not recorded here) shows that as the inseminations 

 were delayed more and more after parturition the proportion of suc- 

 cessful ones increased. Accordingly, the optimum time for insemination 

 lies between 18 and 30 hours p.p. 



The most obvious causes of failure are (1) too early insemination, 

 in which case possibly the conditions of the uterus are sometimes un- 

 favorable for the continued existence of the spermatozoa, (2) killing too 

 soon after insemination to allow the spermatozoa time to reach the eggs, 

 (3) late ovulation, and (4) combinations of two or all of these factors. 



The time required for the spermatozoa, after introduction into the 

 uterus, to reach the eggs in the first part of the oviduct nearest the 

 ovary varies from 4 to 7 hours in mice inseminated about the same 

 number of hours p.p. Of these eggs some contained the heads of sper- 

 matozoa, some both pronuclei. Assuming, as is reasonable, that all the 

 eggs, because they lie very near one another, are seminated at nearly 

 the same time, one must conclude that the time required for a sperma- 

 tozoon to develop into a pronucleus is very short indeed. According 

 to the same reasoning pronuclei must grow very rapidly. Since the first 

 spindle never persists until the egg reaches the oviduct, semination occurs 

 only during the stage of the second maturation spindle. An account of 

 the effect of semination on maturation is given on p. 35. 



