DEPARTMENT OF GENETICS. 115 



half-century great advances have been made in our knowledge of the 

 details of sexual reproduction, we have not advanced far in our knowl- 

 edge of the essential nature of that protoplasmic dimorphism which is 

 sex. 



About the nature of that essence of sex one can only frame hypo- 

 theses. One thinks of the double nature of matter — positively charged 

 and negatively charged. One thinks of the dimorphism of certain 

 organic materials, such as dextrose and levulose. Other hypotheses 

 could be framed and doubtless have been. But material adequate for 

 testing these hypotheses has hitherto been lacking. The ordinary 

 animal or plant is a nutritive, growing, perhaps moving and sensory 

 organism, besides being a sexual organism, and the sexes differ greatly 

 in their other functions and in their form. The Carnegie Institution 

 is in a peculiarly favorable position, since it possesses an organism 

 supposed by some to be sexless until Dr. Blakeslee discovered that 

 it was practically all sexual. Since this discovery Dr. Blakeslee has 

 preserved and carried on these sex-strains for 20 years and has brought 

 them with him to this department. This organism is the bread mold — 

 Rhizopus. It consists of a mass of threads which under certain condi- 

 tions may unite to form reproductive bodies. There is no difference 

 in form between the male masses and the female masses. Apparently 

 one mass of threads, which may be increased to any desired amount, 

 is all male throughout its whole extent; another mass of threads is 

 all female throughout its whole extent. The male and the female 

 masses are alike; the only way of telling which is which is by bringing 

 two masses into contact. If they react by forming sexual bodies they 

 are of different sex; if they fail to react they are of the same sex. 



Now, this organism presents in its simplest form material upon 

 which to study the fundamental nature of sex. This Institution has 

 the unique opportunity to utilize this material and if successful in 

 its search, it would have to its credit one of the greatest discoveries 

 in biology. Dr. Blakeslee is still maintaining his strain of mucors. 

 He has during the past year accumulated additional data, especially 

 from the "imperfect hybridization" reaction, which increases the evi- 

 dence of a strict sexual dimorphism in this group. 



The Nature and Metabolic Basis of Sex in Pigeons. 



During the year Dr. Riddle has completed his study of the differen- 

 tial survival of male and female dove embryos, subjected to increased 

 and decreased pressures of oxygen. A summary of the results is given 

 in tables 4 and 5, described by Dr. Riddle as follows : 



"Table 2 shows that the treatment with high concentrations of oxygen 

 killed a lower percentage of males, a higher percentage of males survived, 

 and the total number of males obtained is disproportionately high. Treat- 

 ment with reduced pressures of oxygen killed a higher percentage of males, a 

 lower percentage of males survived, and fewer males than females were ob- 



