DEVELOPMENT OF THE ALBINO RAT 329 
ment very similar to that of the egg-cylinders shown in longitudi- 
nal section in figure 26. 
Widakowich, after discussing very briefly the mode of develop- 
ment of the egg-cylinder, discusses and figures an egg-cylinder 
of the albino rat, obtained 6? days after the last coitus. His 
figure 3 corresponds in stage of development very closely to 
that shown by me in A of figure 27. In his figures, there is pre- 
sented an egg-cylinder showing the anlage of the mesometrial 
Fig. 28 A series of cross sections at different levels of an egg-cylinder of the 
albino rat after the anlage of the antimesometrial portion of the proamniotic 
cavity. XX 200. Rat No. 27, 7 days, 17 hours, after insemination. The sec- 
tions selected for the several levels drawn, A to D, are as follows: A, middle 
of ectoplacental cone; B and C, through extraembryonic ectodermal portion of 
egg-cylinder, just below junction with ectoplacental cone (B), and just above 
ectodermal vesicle (C); D, through middle of ectodermal vesicle. Compare 
with B, figure 26, a longitudinal section of an egg-cylinder of the same stage of 
development; p.ect., parietal or transitory ectoderm; ez.ect., extraembryonic 
ectoderm; pr.emb.ect., primary embryonic ectoderm of the ectodermal vesicle; 
v.ent., visceral entoderm; pr.emb.ent., primary embryonic entoderm; a.met.pr., 
antimesometrial portion of proamniotic cavity. 
portion of the proamniotic cavity. Emphasis is given to the 
fact that in the antimesometrial portion of the egg-cylinder, 
there may be recognized the primary embryonic ectoderm. 
His own words with reference to this point read as follows: 
Der Schnitt zeigt nun sehr deutlich, dass sich die Zellen, die die 
antimesometrale Hohle so begrenzen, dass die alte Kugel-oder Eiform 
dieses Teiles noch zu erkennen ist—das primire embryonale Ectoderm 
—intensiver farben wie die Zellen des mesometralen Abschnittes oder die 
des Ectoplacentarconus—das extraembryonale Ectoderm. Die Kerne 
zeigen keinerlei Unterschied in der Farbung, wohl aber das Plasma, 
dass im antimesometralen Teile von dichterer Structur zu sein scheint. 
This description corresponds very closely to that given by 
me for a similar stage. The differentiation of these two kinds 
of ectoderm was also recognized by Robinson, who states: 
