770 The American Naturalist. [August, 
formed when the egg was fixed upside down and one of the first two 
cells killed; in some cases, however, half embryos were formed even 
under these conditions. 
The advance made lies in recognizing that results obtained are not 
final till all the conditions of the experiment are considered, and that 
the state of the egg determines the development of half or whole forms 
irrespective of theories of post-generation or qualitative-division. 
The Mouse’s Egg.—Dr. J. Sobotta, of Berlin, contributes to the 
May number of the Archiv fiir Mikroskopische Anatomie a fully illus- 
trated account of his researches on the fertilization and cleavage of the 
mouse’s egg. 
His work has been extended over five years and has involved the 
death of 750 mice yielding 1459 eggs, only 57 of which were degener- 
ate or not fertilized. 
While still warm the ovaries, oviducts and part of the uterus were 
killed in mixtures of corrosive sublimate and picrosulphuric acid or, to 
even better advantage, in osmic acid mixtures. The entire organs 
were cut into serial sections about 10 microns thick, and fixed and 
stained by special methods given in detail in the paper, to which the 
reader is referred for a full account of the technique employed. 
The author discovered that in the mouse there is besides the period 
of heat occurring just after parturition, as in many mammals, a second 
period twenty-one days later. At this time the young are weaned, and 
by permitting fertilization at this second period only the young aresaved 
for future experiments, whereas they perish if the mother becomes 
again pregnant at the first period. The ages of the embryos obtained 
were most accurately determined by reckoning from this second period 
of heat, at which time the male was admitted. | 
Ovulation takes place at the first period whether copulation is effected 
or not. Between the periods of heat copulation is prevented by the 
fact that the walls of the vagina are grown together. 
The process of copulation lasts but one minute and is difficult to 
observe even in the most tame of the white mice that the author had, 
as it takes place in the night towards morning, and the animals are 
then shy. In this process the uterus becomes very greatly distended 
with sperm containing clusters of sperms and also some isolated sperms, 
all moving in the liquid. The vagina is distended by a large mass of 
a homogeneous secretion of the seminal vesicle of the male. | 
Twenty to thirty hours after copulation the vaginal plug softens and 
falls out; before this the uterus has become small again and the sperms 
are dead, as they live but a few hours. 
