62 J. W. JENKINSON. 



— on Mustela ferox^ to substantiate in some measure, as he 

 believed, his previous assertions. His work, however, was 

 received, if we may judge from an article of Born's (8) and 

 from Duval's (12) comments on "les precieuses observations 

 publiees recemment par Robinson sur la cavite de segmenta- 

 tion des Rongeurs,'' with less criticism and suspicion than it 

 would seem to have deserved ; and I hope, therefore, that I 

 need not apologise for publishing the results of some investi- 

 gations which I have undertaken with the view of finally 

 deciding, if possible, whether Robinson's account or that of 

 the earlier observers was correct. 



I have been fortunate enough to obtain between thirty and 

 forty embryos of the critical age (about the fifth day, when 

 the blastocyst is still free in the uterus). These were pre- 

 served in rather weak Flemming's solution, which was injected 

 into the uterus, and the latter, with the contained embryos, 

 was cut into series and stained in various ways. In order to 

 ensure a most careful examination of the sections, I have 

 taken the precaution of making camera drawings of all of 

 them, and I append a list of those embryos of which I have 

 counted the nuclei, which will show, I think, that my series 

 is tolerably complete : 



22.— 195 

 23.— 200 

 24.— 220 

 25.-256 

 26.— 261 

 27.-285 

 28.-295 



Table showing in ordsr the number of nuclei in twenty-eight mouse 

 embryos. In the youngest of these the inner mass is quite undifferentiated, 

 and the blastocystic cavity is small. In the oldest the epiblastic mass is well 

 formed, and the hypoblast is partially extended round the inside of the much 

 enlarged blastocyst. 



According to Robinson, the blastocyst of the mouse in its 

 earliest stage consists of a hollow oval vesicle, one end of 

 which — the " floor " — is formed of a mass of cells two or 

 three deep, " which is not separable into an inner mass of 



