6 J. P. HILL. 



As concerns my own observations, I venture to think it is 

 possible to bring them into line with what we know of the 

 early ontogeny in the other two mammalian sub-classes, and 

 I have attempted to do so in the concluding chapter of this 

 paper, with what success the reader can judge, whilst as regards 

 the divergence between Selenka's results and my own, I am 

 perfectly convinced that the explanation thereof is to be 

 found in the fact that the whole of Selenka^s early material 

 was derived from but two pregnant females, and that much 

 of it consequently consisted of eggs which had failed to 

 develop normally. From the one female, killed 5 days 

 after coition, he obtained one egg in the 2-celled stage, 

 one with about twenty cells and nine unfertilised ova. From 

 the second, killed 5 days 8 hours after coition, he obtained 

 " ausser zwei tauben, 14 befruchtete Eier namlich je ein Ei mit 

 4, 8, 42, 68 Zellen, eine junge und eine altere Gastrula mit 

 noch dicker Eiweisschicht und endlich aclit audi gleichen 

 Entwickelungsstufe stehende weit grossere Keimblasen, deren 

 Wand noch grosstentheils einschichtig war" {'86, p. 112). 

 Selenka recognised that the last-mentioned blastocyst "die 

 normale Entwickelungsphase repriisentiren," since he found 

 as a rule that all the embryos from one uterus were in the same 

 developmental stage. Nevertheless he proceeded to describe 

 the segmenting eggs and the two " gastrula " which lagged so 

 far behind the blastocysts, as if they were perfectly normal 

 developmental stages. He does, indeed, question whether or 

 not the 42-celled stage is normal, but decides in the aflfirma- 

 tive, " denn wenn ich von zwei Zweifelhaften Fallen absehe, 

 so habe ich niemals Eier ans den ersten Tag aufgefunden, 

 welche auf irgend welche Anomalie der Entwickelung 

 hinwiesen." This, however, can hardly be accepted as a 

 satisfactory reason for his conclusion, since apart from the 

 other eggs of the same batch, he had but the two eggs from 

 the other female for comparison, viz. the 2-celled egg (and 

 even that is, in my view, not quite normal), and the 20-celled 

 egg, which is stated to have suffered in preparation. With 

 the exception of the two eggs just mentioned, all the crucial 



