796 



SCIENCE 



[N. S. Vol. XLIV. No. 1144 



a considerable period of time. This has been 

 done by Leo Loeb for the guinea-pig. For the 

 rat there are no published observations except 

 those by Kirkham and Burr (1913), from which 

 it is to be inferred that the ovarian cycle has 

 a length of twenty-one days. 



Although further studies on the rat are 

 being carried on by the senior author, it seems 

 worth while at this time to present in outline 

 the chief conclusions arrived at, reserving for 

 a later paper a more complete presentation 

 and discussion of evidence. 



The most obvious and certain evidence of 

 the occurrence of ovulation is the presence of 

 eggs in the oviduct. It is chiefly upon this 

 kind of evidence that the conclusions are 

 based. There is also a further source of infor- 

 mation concerning the ovarian cycle in the 

 corpora lutea, formed in most cases from the 

 ripe follicles which have discharged their eggs. 

 The corpora lutea grow and undergo such 

 changes before degenerating that there may be 

 as many as 40 in one ovary, of which only the 

 youngest and oldest can sometimes be identified 

 with certainty. However, the newest corpora 

 up to an age of about 2J, days can be distin- 

 guished from older ones. Such young corpora 

 are always present when eggs are in the ovi- 

 duct, and their absence when no eggs exist in 

 the tubes is additional proof that ovulation 

 either has not occurred (especially if the ovary 

 contains large follicles), or took place several 

 days before. 



All of the 80 females used were isolated from 

 males before their last litters were born, and 

 thereafter were kept alone or with other fe- 

 males. Also their young were at once removed, 

 usually before being suckled. 



The ovaries and oviducts were sectioned, the 

 position of the eggs (when present) in the 

 oviduct was determined, and the condition of 

 the corpora lutea noted. The animals were 

 killed at intervals during 101 days after par- 

 turition, 67 of the 80 rats being taken during 

 the first four 10-day periods as follows : 



1 to 9 days, 18 rats 



10 " 19 " 15 " 



20 " 29 " 17 " 



30 " 39 " 12 " 



40 " 42 " 5 " 



making an almost complete series at one-day 

 intervals. They are grouped at still closer in- 

 tervals about the tenth, twentieth, thirtieth 

 and fortieth days. The rest of the animals 

 were killed only at about ten-day intervals from 

 50 to 101 days. 



Unfertilized eggs pass through the oviduct 

 in about three days, usually having degenerated 

 by the end of that time, as determined by a 

 study of 15 animals killed during the first four 

 days post partum. Accordingly the distance 

 traveled by the eggs in the oviduct is of impor- 

 tance and was taken into account in estimating 

 the time of ovulation. 



Of the 80 animals examined 49 revealed eggs 

 in the oviduct. To these may be added 14 

 more in which it is permissible to estimate the 

 time of ovulation. Summarized they are as 

 follows : 



Ovulating atter Parturition 

 Rats Days 



15 S- 1 



11 9A- 131 



1 15J / 



13 19 - 23J\ 



1 24* 



5 27|- 34$ 



5 38 - 41* 



2 49*- 50 



2 57$- 58J 



2 671- 701 



2 78-82 



2 87-89$ 



2 97J-101 



301 

 39i 

 50 

 58 



99 



Of the other 17 rats none had eggs in the ovi- 

 duct, and the ovaries presented no evidence of 

 recent ovulations. They were killed between 

 the periods enumerated above. 



The foregoing indicates that female rats 

 when kept isolated from males ovulate on the 

 average every 10 days. 



J. A. Long, 

 Jessie E. Quisno 

 Zoological Laboratory, 

 University or California 



OVULATION IN MICE 



It has been known since the time of Tafani 

 (1889) that mice normally ovulate soon after 

 giving birth to litters. According to Sobotta 

 (1895) a second ovulation takes place in 



