Nettie Maria Stevens f. 
By 
Th. H. Morgan. 
Miss N. M. Stevens graduated from Lelaiid Stanford University in 
1899 and received her Ph. D. at Bryn Mawr College in 1903. Her first 
published work “Studies on Ciliate Infiisoria” appeared in 1901. She 
was forty years of age at tliis time. It is rare for one who begins bis 
scientific work so late in life to attain in a few years such high rank as 
an investigator. In Miss Stevens’s case this was made possible by her 
natural ability and devotion to her work, as well as by the liberality of 
Bryn Mawr College, which created for her a research professorship. 
Her investigations lay almost entrrely in the field of cytology, and in- 
cluded not only extensive studies of the germ cells, but several papers 
on the histology of regenerative processes in planarians and hydroids. 
Miss Stevens wiU be best remembered through her discovery in 
1906 relating to sex determination. She found that the male of a beetle 
( Tenebrio molitor) produces two kinds of sperm, differing in that one half 
the sperms have a large chromosonie and the other half a smaller chromo- 
some. Two such classes of sperm were already known in certain other 
insects, and McClung had earlier suggested their connection with sex 
production. Miss Stevens discovered that the small chromosome is 
eonfined to the male line, while in the feniale its place is taken by the 
larger one. She drew the correct inference that since all unfertilized 
eggs are alike in their chromosomal content, therefore a female results 
from the fertilization of an egg by the sperm containing the larger chromo- 
some, and the male by the sperm containing the smaller chromosonie. 
A similar relation was discovered at the sanie time by Prof. E. B. Wilson. 
Düring the foUowing six years Miss Stevens extended her studies 
in this subject over a wide field. In 50 species of beetle she found a 
unpaired chromosonie in twelve cases, and an X Y pair in thirty-eight 
