DEVELOPMENT OF THE ALBINO RAT 103 



have before him the stages showing the anlage of the mesoderm 

 seems clear. Sobotta's ('11) observations, mouse material, 

 deserve fuller consideration. In interpreting his results, I am 

 mindful of the fact that he was unable to locate the line of union 

 between primary embryonic and extraembryonic ectoderm, as 

 can readily be done in suitable rat material, as has previously 

 been shown by Robinson and Widakowich, and to which atten- 

 tion has constantly been drawn in this communication. I am 

 unable to state from personal observation whether in the white 

 mouse these two types of ectoderm which form the lining of the 

 proamniotic cavity, can be differentiated on ascertaining the 

 right technical method. Sobotta's material seems well fixed. 

 If not, it would seem to me difficult to determine definitely 

 the exact place of origin of the mesodermal cells, whether extra- 

 embryonic or embryonic. Sobotta recognized the anlage of 

 the mesoderm in the mouse during the last hours of the seventh 

 day or first hours of the eighth day. This is said to appear at 

 the caudal end of the future embryo as a group of loosely ar- 

 ranged cells lying between the inner and outer layers of the 

 egg-cylinder. At the place where the mesodermal cells arise 

 from the inner layer of the egg-cylinder, there is developed a 

 fold, recognized as the caudal amniotic fold ("Schwanzfalte 

 des Amnios"). After discussing these observations at length, 

 Sobotta concludes as follows: 



Was die Deutung dieser friihen Stadien der Mesodermbildung in 

 der Keimblase der Maus anlangt, so handelt es sich hier nicht um die 

 Bildung des embryonalen Mesoderms, die erst mit der eigentlichen 

 Gastrulation spater einsetzt, sondern um Entstehung ausserembryo- 

 nalen Mesoderms, besonder des Teils des mittleren Keimblattes, dass 

 bei der Bildung der primaren Eihaute, Amnios und Chorion in Betracht 

 kommt und des den ausserembryonalen Teil der Leibeshohle, das 

 Exocoelom auskliedet, der Hohle, die eben Amnios und Chorion von- 

 einander trennt. Es erfolgt also, um einen kurzen Ausdruck zu gebrau- 

 chen, die Bildung des Amniosmesoderms. 



An embryonic anlage is said not to exist at this stage; this is 

 recognized only after the development of the primitive streak. 

 It is not my purpose to enter fully into a discussion of this im- 

 portant question in this communication. This would involve 



