304 
PROFESSOR ARTHUR ROBINSON ON 
and with polecats ; moreover, the structure of the ovaries is the same in all, there- 
fore, for the purpose in hand, they may be looked upon as one group ; but for 
convenience of description, and to indicate clearly in the following account whether 
a ferret in the ordinary sense or a so-called “ polecat,” which I believe to be a 
hybrid, is being described I have termed the so-called “polecat”' in all cases a 
ferret-polecat hybrid. 
In the course of the investigation 266 animals were used, but in many cases 
the ovaries of the animals were in functional stages,' which have only indirect 
bearing on the points to be considered now, or they offered evidence only on 
subsidiary points, therefore I have selected 118 "animals from which 232 ovaries, 
obtained in satisfactory condition, were cut into continuous sections. Details 
with regard to these animals are given in the accompanying tables ; at the same 
time it is to be understood that one or both the ovaries of all the 266 animals 
have been cut into continuous sections and have been examined, and that the 
statements to be made with regard to conditions which are found at different 
periods are based upon the whole group. 
Some of the animals were obtained from a dealer in Norfolk, others from a 
dealer in Kent. Some of the animals were reserved for me at the end of one 
breeding season, were kept separate from the male, and were sent to me either 
before signs of oestrus appeared or immediately thereafter ; these animals were 
inseminated by healthy, vigorous males in my own laboratory, and under my own 
observation. Other animals were inseminated under the dealers’ observation, and 
were sent to me immediately thereafter or some days later, care being taken to 
prevent access to the male after the insemination. Both dealers thoroughly under- 
stood what was required of them, and as the results obtained from the animals 
inseminated under their observation correspond with the results obtained from 
the animals inseminated in my laboratory, there can be no doubt that they carried 
out the instructions given to them effectively. 
The ferret possesses certain advantages for the purposes of investigations deal- 
ing with the phenomena of early development in mammals, for it is one of the 
few highly specialised carnivora which are obtainable in considerable numbers, 
and which are small and easily dealt with. It is also one of the group of animals 
which possess an almost completely closed ovarian capsule of the type present 
in the racoon and the badger, to which I have previously directed attention (32), 
(figs. 1, 2, PI. I), and in which, therefore, it might be- hoped that all the ova shed 
at an ovulation would almost certainly be found in the genital passages ; this 
hope has, for the main part, been fulfilled ; though, as will be seen later, it is 
possible for an ovum to escape through the opening in the peritoneal capsule into 
the peritoneum. The presence of the peritoneal capsule is also an advantage, 
inasmuch as it protects the surface of the ovary from injury during the manipula- 
tions which are necessary in association with the fixation, hardening, and embedding 
