DEVELOPMENT OF THE ALBINO RAT 85 



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 6f 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. X 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; ex.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 1st das primare 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: 



