DEVELOPMENT OF THE ALBINO RAT 345 
ectoderm. This portion in the section figured, passes a little 
to one side of the mid-sagittal plane. The two sections preced- 
ing the one figured enclose the mid-sagittal plane, and in them, 
the group of cells found between primary embryonic ectoderm 
and entoderm are in closer relation to the ectodermal layer and 
at all points distinctly separated from the entoderm. They 
are regarded as having wandered from the primary embryonic 
ectoderm to the place they oceupy, a fact which is more easily 
ascertained in cross sections of a similar stage, as will appear 
from further discussion. From a study of very slightly older 
stages it can be determined that this region constitutes the 
primitive streak region of the future embryonic area. It is not 
my purpose at this time and in this communication to give es- 
pecial consideration to the much discussed question of the 
origin of the mesoderm in Mammalia. In the rat, this question 
is complicated by the question of the anlage of the amniotic 
fold, which separates the proamniotic cavity into amniotic 
cavity proper and the ectoplacental cavity, the development 
of which will be considered in a projected contribution. In 
anticipation of this second publication, however, the following 
facts may here receive consideration. Widakowich presents 
in his figure 4, giving only the antimesometrial end of an egg- 
cylinder obtained the latter part of the 7th day, the anlage of the 
mesoderm as observed by him. This figure and my own figure 
31 present almost identical relations, his figure showing only 
three mesodermal cells between primary embryonic ectoderm 
and entoderm. His own words concerning the anlage of the 
mesoderm in the albino rat, with which I find myself in full 
accord, except as to the age of the egg-cylinder, read as follows: 
Das erste auftreten des Mesoderms beobachtete ich an Keimen 
vom Ende des 7 Tages. Die ersten Mesodermzellen liegen im Bereiche 
der vom mesometralen Ende des stirker fairbbaren primiren embryo- 
nalen Ectoderm gebildeten Falte. Es kommt hier eine ganz bestimmte 
Stelle in Betracht, die dort liegt, wo sich spiter das hintere Ende des 
Primitivstreifens befindet. 
There is, however, wide divergence of the views of authors 
as concerns the anlage of the mesoderm in the rat and mouse. 
