112 J. P. UILL. 



The latter alone constitutes the unilaminai- wall of the 

 blastocyst and has the embryonal knot adherent at one spot 

 to its inner surface. The formative cells which compose 

 the knot thns take at first no part in the constitution of 

 the outer wall of the blastocyst, and may or may not 

 do so in later stages according as the covering layer of the 

 trophoblast (the Deckschicht or Raub^r's layer) is tran- 

 sitory or permanent. This peculiar developmental con- 

 dition, characterised by the internal position of tiie formative 

 or embryonal cells within the blastocyst cavity, has been 

 termed by Selenka ('00) " entypy " (Eutypie des Keim- 

 feldes).^ It is a phenomenon exclusively found in the 

 Eutheria and characteristic of them alone, amongst the 

 mammals. In the Marsupial, as in the Monotreme, the 

 formative cells are freely exposed, ;ind constitute from the first 

 part of the blastocyst wall just as those of the Sauropsida form 

 a part of the general blastoderm. Limited as entypy thus 

 appears to be to the higher mammals, the probability is that 

 we have to do here with a purely secondary, adaptive feature. 

 If we proceed to inquire what is the significance of this 

 remarkable difference in the early developmental phenomena 

 of the lower and higher mammals, it seems to me that we have 

 to take account, in the first place, of the differences in the 

 structure of their respective eggs, and especially we have to 

 bear in mind that the Eutherian ovum is considerably more 

 specialised than even the Metatherian, It is on the average 

 smaller than the latter, i.e. it has suffered in the course of 

 phylogeny still further reduction in size, and has lost, to an 

 even greater extent than the Marsupial ovum, the store of food- 

 yolk ancestrally present in it. Moreover, it has suffered a still 

 further reduction in respect of its secondary egg-membranes. 

 The Metatherian ovum still retains in its shell-membrane a 



1 " Unter Eiitypie des Keimfekles mitclite ich dalier verstanden 

 wissen : Die nicht durch Bildung typischer Auuiionfalten gescheliende, 

 sondera durch eine sclion wiihrend der Gastvulation erfolgende Abscli- 

 niirung des Keimfeldes ins Imieve der Eil>l;isenhi'ille (Cliorioii) " ('00, 

 p. 203). 



