DEVELOPMENT OF THE ALBINO RAT aol 
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) Oberfliche 
des Cylinders iiberziehenden Dotterentoderm zuniichst ab, so sieht 
man, dass die Furche, welche die oben erwihnten 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, sel 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- 
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