EAELY ONTOGENETIC PHENOMENA IN MAMMALS. 233 
and hedgehog, and I have always found the pairs cross- 
ways. 
On the other hand, the fact that a three-segment stage is 
often met with suggests that one segment as a rule divides 
before the other segment, and so brings about the cross-ways 
position. 
Discussing the arching of the epiblast plate while bursting 
its way through the trophoblast as it does in Tupaja, Talpa, 
Ovis, Sus, etc., Hubrecht sa,ys : “ The causes of the folded 
condition of the embi’yonic shield can hardly be so simply 
mechanical as Selenka supposed. They remain obscui’e for 
the present, and will come anew under consideration when the 
origin of the amnion will be discussed ” (p. 10). It is diffi- 
cult to believe that the arching does not aid in the rupture ot‘ 
the trophoblast, though the fact that it brings about the 
rupture — if it is a fact — is not necessarily the reason why the 
arching occurs, because the rupture is brouglit about by other 
means in cases like Lepus or Sorex. 
The causes are less obscure on the hypothesis that Eutheria 
derive their peculiar couditions from Sauropsidan ancestry. 
With reference to the Monotremes Hubrecht speculates as 
follows. He admits that “ our knowledge is as yet very 
scanty,” but brings what is known about the segmentation 
into line with the higher mammals by homologising the 
outer layer (which Wilson and Hill regard as epiblast) with 
the trophoblast of the Eutherian, and Wilson and Hill’s (also 
Semon’s and Caldwell’s) entoderm Avith the embi-yonic knob 
of Eutheria, and regards the yolk as an accumulation on an 
ancestral type peculiar to Prototheria and not derived from 
Sauropsida. 
For this view I can see no reason derivable from actual 
specimens described and figured by those four authors. 
Hubrecht’s fig. 07, p. 23, might be that of a Sauropsidan, e.g. 
sparroAv ; nor do 08, 07, GO, really differ from the Saurop- 
sidan blastoderm. Possibly the segments in 60 look rather 
less part of the yolk than is often the case in Sauropsidan 
eggs, but it is certainly remarkable that there should be the 
