530 A. A. W. HUBRECHT. 



pressure, the outer layer of cells of the mammalian embryo — 

 the trophoblast — has no doubt been specially entrusted. For 

 this purpose it is undeniably the most favorably situated. 



It is certainly significant that in all ^lammalia it forms a 

 closed sac at the very earliest period after segmentation of the 

 ovum has commenced. More significant yet is the fact which 

 I have noticed in Sorex that a considerable increase in size of 

 the early blastocyst is brought about (cf. figs, 5 and 6 with 

 figs. 8 — 11) without any adequate increase of the number of 

 cells composing it. This is the actual demonstration of the 

 fact that the increase in size is due to an increased tension, 

 which, in the case of the spherical blastocyst, can only be 

 brought about by the accumulation of liquid contents that 

 are under a higher pressure inside the blastocyst than is the 

 surrounding medium. This in its turn has to be ascribed to 

 inherent properties of the protoplasm of the trophoblast-cells — 

 properties which may either be of a more secretive or of a more 

 osmotic nature, as will some day have to be more carefully 

 determined. The actual high elasticity of a mammalian blasto- 

 cyst has often been observed and been commented upon. 



The utility of this arrangement has probably contributed 

 more towards the retention of what I would call the pseudo- 

 meroblastic condition of the blastocysts of the higher Mammalia 

 than has the hereditary tendency towards the production of 

 this condition. Moreover, other factors came into play to 

 increase the significance of this elastic and spacious blastocyst. 

 It offers a very safe lodging for the developing head of the 

 embryo, which already in Reptilia is seen to be enclosed in a 

 proamnion that bends downwards into the yolk. Such a 

 protection is all the more effective for the mammalian embryos 

 that are no longer protected by a hard shell, but enclosed in 

 moveable and contractile maternal tissue.^ 



^ Another reason which might apparently be given for the elasticity and 

 the increase in bulk of the mammalian blastocyst has here been intentionally 

 left in the background, viz. the reason that thereby the walls of the uterus are 

 bulged out, in consequence of which nutritory facilities are obtained. I am 

 not inclined to attach any value to this argument, which appears to me to be 



