118 CARNEGIE INSTITUTION OF WASHINGTON. 



duction in Cladocera must miscarry. Yet the same influence may in one 

 case call forth males and in another case sexual eggs. Since males and not 

 ephippial eggs appear in most of our cases and ephippial eggs and not males 

 in the other case, evidently somewhat different conditions must be obtained 

 to produce ephippial eggs where males are now obtained and males where 

 ephippial eggs have been obtained." 



Differential viability of "male^^ and "female'' sperm. — As noted in 

 previous Year Books (1919, p. 135, 136; 1920, p. 124-127), Dr. Little 

 has been carrying out researches in this field. 



"In last year's report the suggestion was made that a differential viability 

 of the male-forming and female-forming sperms might underlie the various 

 departures from an equality sex-ratio which are commonly met with and 

 which are especially marked in first litters or first births and in first-genera- 

 tion hybrids between various strains and races of laboratory mammals and 

 different nationalities in man. During the summer Mr. Fehx Saunders, 

 Miss Isabelle Graves, and Miss Vera Goddard have been working under 

 Dr. Little's direction on methods and technique of obtaining the hydrogen- 

 ion concentration of the seminal fluid, vaginal secretions, and uterine secre- 

 tions in rats and dogs. For this purpose female rats of known stage in the 

 cestrous cycle are obtainable by a daily analysis of the laboratory colony, 

 by the methods used and described by Long and Evans. The Bovie self- 

 recording potentiometer is being used for the determination of the hydrogen- 

 ion concentration and special methods have been devised by which it is 

 hoped that readings may be obtained from secretions while they are still 

 within the vagina or the uterus of the female." 



Differences in size between "male'' and "female" sperm. — It is known 

 that in mammals and some insects the sex of offspring is determined by 

 the one of two unlike types of sperms that fertilizes the eggs. The 

 two types differ in the number of chromosomes. Some years ago 

 C. Zeleny, Faust, and Senay (1915) showed that certain spermatozoa 

 differ also in size and fall into two overlapping groups of head- 

 length. It was important to know if the same is true of mammalian 

 sperm. Under Dr. Little's direction, Miss Graves and Miss Goddard 

 have measured and tabulated the head-length of over 1,300 unstained 

 dog sperm, all obtained from the same animal. A magnification of 

 2,000 times was used. To secure accuracy, camera-lucida projections 

 were drawn and then measured. 



When tabulation of the results was made by division into classes 

 of 0.25ju, the bimodal curve is not obtained which one expects if there 

 are two size-types. If two modes are actually present they are masked 

 by being so close together that the methods available for measuring 

 sperm fresh from the animal are not accurate enough to establish their 

 existence beyond doubt. The personal error introduced by the ob- 

 server would probably be fully as great as the distance between the 

 modes. The difference between the two types of sperm in dogs is not 

 sufficiently large to measure in material as nearly as possible in the 

 actual condition of its entrance into the female reproductive tract. 



