DEVELOPMENT OF THE ALBINO RAT 351 
respective figures, these forming the region of the future anterior 
ends of the respective embryos. In C of figure 32 is shown a 
drawing of one of the sections of a series of cross-sections of an 
egeg-cylinder taken from rat No. 41, 8 days, 16 hours, after in- 
semination, presenting a stage in which the primitive groove 
may be definitely made out. This figure is not unlike figure 6 
of the article of Widakowich, obtained from an egg-cylinder 
secured on the eighth day. Concerning this figure he states: 
“Das Ectoderm steht in direktem Zusammenhange mit zwei 
Mesodermzungen die gegen die der Primitivrinne gegeniiberlie- 
gende Seite zu auswachsen.”’ The section drawn in C of this 
figure is taken from the region very near the junction of the 
primary embryonic and the extraembryonic ectoderm, as may 
be observed from the character of the entodermal cells, in the 
lower part of the figure. The increase in the thickness of the 
mesodermal wings, the result, in part at least, of proliferation 
of mesodermal cells, as evidenced by the presence of mitotic 
figures, is clearly shown in this figure. The mesoderm is dis- 
tinctly separable from the entoderm as also from the primary 
embryonic ectoderm except in the region of the primitive streak 
and groove. The growth of the mesoderm after its anlage has 
been correctly shown for the albino rat by Selenka, Robinson, 
and Widakowich; the latter especially giving excellent figures. 
His figure 5 is especially instructive. In this, he represents the 
appearances shown by two views of an isolated egg-cylinder, with 
the primitive groove in anlage, showing the lateral extensions 
of the mesoderm. Sobotta (’11) has given the best and most 
comprehensive account of the anlage and growth of the meso- 
derm in the mouse. An excellent cross-section of a mouse egg- 
cylinder in the primitive streak stage is presented in his figure 
6, which presents very similar appearances to my C of figure 32. 
None of the figures of cross-sections of egg-cylinders included 
by me show the very beginning of the anlage of the mesoderm, 
though A of figure 32 approaches this very closely, as does also 
figure 31, presenting a sagittal section. The evidence at hand 
warrants the conclusion that in the albino rat, the mesoderm 
has its anlage in the caudal region of the primary embryonic 
