106 University of California Publications in Zoology \Vou.9 
INTRODUCTION 
The solution of the complex problems of heredity, of trans- 
mission and of sex determination requires experimental studies 
as well as investigations of the normal events and conditions of 
reproduction. It was with this idea in mind that the writers 
undertook the present series of studies on mammalian sex cells, 
the first published results of which have already appeared (Long 
and Mark, 1911; see also Mark and Long, 1911). At present an 
attempt is being made to cross rats and mice, as well as to study 
the course of early development in each species, both under 
normal conditions and also under artificial conditions which 
simulate natural ones as closely as possible. It is believed that 
a comparison of the results thus obtained, both by hybridization 
and independently of it, will be instructive. 
The following preliminary communication (No. 3 of these 
studies) is the outcome of an attempt to devise methods of 
manipulation which will enable the experimenter more success- 
fully to control the material under investigation, and especially 
to enable him to follow in the living organism the changes which 
are inferred from the conditions in preserved material. Further 
communications on the results of artificial insemination and on 
other problems growing out of the whole undertaking are 
reserved for future papers. 
The present paper is the result of work done in the Zoological 
Laboratory of the University of California, and is a continuation 
of investigations carried on in the Zoological Laboratory of Har- 
vard University, some of the results of which have been published 
under the title ‘‘The Maturation of the Egg of the Mouse.’’ It 
is published as a contribution from the Zoological Laboratory of 
the University of California and also as no. 225 of the Contribu- 
tions from the Zoological Laboratory of the Museum of Compar- 
ative Zoology at Harvard College. 
The running expenses of the work have been defrayed in part 
from grants made by the Carnegie Institution of Washington, 
and in part by funds furnished through the Zoological Labora- 
tory of Harvard University; but the special equipment required 
