DEVELOPMENT OF THE ALBINO RAT 87 



cavity. A lumen is obtained in the antimesometrial portion later 

 than in the mesometrial portion. As development proceeds, this 

 sharp demarkation of antimesometrial and mesometrial portion 

 is gradually lost. This, as stated in his own words, reads: 



Sehen wir von dem die (der Keimhohle zugekehrte) Oberflache 

 des Cylinders tiberziehenden Dotterentoderm zunachst ab, so sieht 

 man, dass die Furche, welche die oben erwahnten mesometralen und 

 antimesometralen Abschnitte in Stadium der Fig. 11 u. 12 trennte, 

 jetzt wieder wenig deutlich ist. Es bahnt sich eine Verschmelzung 

 beider Abschnitte wiederum an, was man am leichtesten daraus er- 

 sieht, dass bald (Fig. 14) beide Abschnitte ein gemeinsames Lumen 

 erhalten. 



With the formation of a continuous proamniotic cavity, 

 this is bordered by a single layer of 'ectodermal cells/ with al- 

 ternately placed nuclei. The cells are described as being the 

 same throughout; neither in text nor figure does Sobotta differ- 

 entiate between ectodermal cells derived from the antimesome- 

 trial portion of the egg-cylinder and those derived from the 

 mesometrial portion. Melissinos also recognizes antimesome- 

 trial and mesometrial portions in the development of the egg- 

 cylinder of the mouse, in his figure 34. According to this ob- 

 server, the antimesometrial portion of the proamniotic cavity 

 is the first to appear; later it appears in the mesometrial por- 

 tion, the two cavities joining as development proceeds. The 

 parts of the ectoderm derived from these two portions may be 

 recognized, however, after a single proamniotic cavity has 

 developed. This Melissinos states in the following words: "Trotz 

 aller Vereinigung der beiden Hohlungen bleibt die Unterscheidung 

 des normals abgesonderten antimesomtralen Abschnittes von 

 dem mesometralen immer leicht zu machen, sei es durch eine 

 klare Grenzlinie oder durch eine an der Peripherie des visceralen 

 Dotterblattes befindliche Furche." The account of Melissinos 

 is more in agreement with the presentations as observed in the 

 albino rat than is that of Sobotta. 



Selenka, Sobotta, and Melissinos recognize three different 

 regions of constriction to which significance is given, in the 

 egg-cylinder of the mouse. As stated by Sobotta, the first con- 



