December 1, 1916] 



SCIENCE 



797 



nursing mothers on an average 21 days after 

 parturition, a discovery he made use of in his 

 study of maturation. There are no other 

 printed records of spontaneous ovulations in 

 addition to that coming immediately after 

 parturition. 



The following is a summary of the results, 

 with respect to the occurrence of ovulation, of 

 an investigation, still under way, of the ova- 

 rian cycle in mice. The study is being carried 

 on in the same way as for rats outlined in 

 the preceding article. 



Sixty-two female mice of various coat colors 

 were bred, allowed to have their litters when 

 isolated from males, kept alone or with other 

 females, and killed at intervals during a pe- 

 riod of 91 days. Most (52) were killed during 

 the first 56 days at. intervals of about 2 days, 

 except between 18 and 21 days, 34 and 38, and 

 50 and 56 days when the interval was a day or 

 less. The rest of the animals were taken be- 

 tween 70 and 74£, and 87j and 91 days. 



The sections of the ovaries and oviducts 

 were examined for eggs in the oviduct and for 

 the youngest corpora lutea. In determining 

 the time of ovulation the position of the eggs 

 in the oviduct was considered; and the pres- 

 ence or absence of the youngest corpora lutea 

 was used as a check. 



The examination of these mice indicated 

 that the second ovulation occurred at from 15 

 to 19 days following parturition, the third at 

 about 35, the fifth at 69 to 72, and the sixth 

 at 87 to 90. No ovulation was found at the ex- 

 pected fourth, perhaps because too few ani- 

 mals were killed at that time. But it is sig- 

 nificant that of those animals killed at 70 to 74, 

 and 87 to 91 days which fall within the ex- 

 pected later ovulation periods, 3 and 2 animals 

 were found to have ovulated at the sixth and 

 seventh periods respectively; also that none 

 of the mice killed between the ovulation periods 

 was found to have ovulated. 



It thus appears that the normal ovulation 

 period in mice recurs at about 17! to 18 days. 

 J. A. Long, 

 H. P. Smith 



Zoological Laboratory, 

 University op California 



AGAR AGAR FOR BACTERIOLOGICAL USE 



Agak agae is used by so many, as a basis of 

 nutrient media, that any suggestion as to how 

 to select the most suitable grade is worthy of 

 consideration. 



Fellers 1 has recently published some bac- 

 teriological studies on agar agar. The same 

 author 2 has also prepared a paper on the com- 

 position of agar agar and given methods for 

 purifying commercial agar. No matter how 

 easy the method proposed for the purification 

 of a substance is, we have to select that which 

 we intend to purify. One of the best ways of 

 determining the stability of an organic sub- 

 stance is to find out how much it will be 

 hydrolyzed under the conditions it is to be 

 used. Hydrolysis is, generally, increased with 

 temperature, and thus increased acidity at high 

 temperatures is often due to greater hydrolysis 

 at the high temperatures. If substances show 

 an increased acidity at high temperatures, but 

 when cooled back to normal temperatures re- 

 turn to the acidity they had before the heating, 

 the high temperatures have not materially 

 changed their composition. Some samples of 

 agar agar have been known to develop a large 

 increased permanent acidity due to autoclav- 

 ing. It is evident that such samples should 

 not be used for accurate work. The increased 

 acidity due to autoclaving and due to titra- 

 tion made in hot solutions can be made use of 

 in selecting agar agar for laboratory use. 



The following described test has been found 

 to designate the superiority of some samples of 

 agar agar over others. Samples chosen by 

 means of this test are always those which go 

 completely in solution when heated with car- 

 bon dioxide free distilled water. Further 

 media made with them have a lower acidity 

 than media made with agars not so good by 

 the test. 



THE TEST 



The test depends on the increase in acidity 

 of water solutions of the agar due to auto- 

 claving and to titrations made near 100° C. 



i Fellers, Soil Science, Vol. II., No. 3, p. 255. 

 2 Fellers, Jour. Ind. and Eng. Chem. (Article 

 to appear soon.) 



