372 ARTHUR ROBINSON. 
Bonnet has described it as originating partly in the region of 
the primitive streak and partly in the marginal zone of the 
germinal area from the hypoblast (5). More recently three 
chief seats of mesoblastic formation have been described,—the 
anterior part of the germinal area, the marginal zone of the 
germinal area, and the primitive streak (28). 
Under these circumstances no apology is needed for the 
publication of the results of a series of observations, dealing 
with the questions in dispute, which have been carried on 
during a period of several years, upon the developing ova of 
Mus musculus and Mus decumanus, which were selected 
on account of the comparative ease with which they can be 
obtained in large numbers. Although, at first sight, the 
peculiar “inversion of the layers” which occurs in the early 
stages might be held as an objection to their use for the study 
of ordinary developmental phenomena, experience proves that 
this is not the case. On the contrary, the “inversion” is an 
aid to the investigator, for it facilitates observations on the 
extension of the middle germinal layer and upon the compara- 
tive rate of growth of the primitive streak. 
Some of the observations to be referred to in the subsequent 
pages have already been recorded in a thesis presented to the 
University of Edinburgh for the degree of Doctor of Medicine, 
in 1890 (41). Since that period, however, amongst a number of 
fresh specimens which have served to confirm the most 
important conclusions drawn from my previous work, I have 
found some intermediate stages which have thrown further 
light upon some of the phenomena, and which have enabled 
me to form certain definite conclusions concerning the pro- 
cesses of coelom formation and extension, and the relation of 
amnion formation to the “ inversion of the layers.” 
As the results of my observations are not in harmony with 
those obtained by previous observers (9, 18, 44, 45), it is 
advisable to precede the record of results by a short account 
of the methods used in the preparation of the specimens, so 
that it may be shown at the outset that the differences to be 
noted are not due either to the manipulation of the specimens 
