Haddon — Note on the Blastodermic Vesicle of Mammals. 545 



and Halniaturus ruficollis the cells of the suhzonal membrane over- 

 lying the yolk-sac outside the sinus terminalis become amoeboid, 

 and throw out pseudopodia-like processes which fit in between the 

 cells of the uterine epithelium and serve to attach the blastodermic 

 vesicle to the uterus (fig. 7). This attachment is entirely non-vas- 

 cular, 'and is the sole means by which the vesicle is attached to the 

 uterus, no villi being formed. 



From the nature of the case no adhesion occurs between the 

 .embryo and the parent in the Prototheria. In the Metatheria a 

 very slight connexion does occur, but in this the yolk-sac alone 

 takes part. As the latter was the sole nutritive organ of the 

 earlier mammals, it would but slowly part with this function. 

 Hyder (19) has suggested that the degeneracy of the yolk of the 

 mammalian ovum may be due to the development of the so-called 

 uterine milk from the uterine glands, and to have atrophied in 

 consequence of the perfectly parastie connexion subsisting tempo- 

 rarily between the maternal organism and the embryo, as was sup- 

 posed by Balfour. At this stage of evolution the allantois was 

 purely respiratory, as it is in the Sauropsida, and the yolk-sac 

 (blastodermic vesicle) was becoming less nutritive in function. 



In the Eutheria the placenta is formed by fusion of the allan- 

 tois alone with the suhzonal membrane, the yolk-sac becoming in- 

 creasingly reduced. In the rabbit the yolk-sac spreads over a large 

 portion of the subzonal membrane, leaving but a small area for the 

 necessarily discoidal placenta ; and, save for the non-adherence of 

 the yolk-sac, and for the production of an allantoic placenta, a 

 diagram of a rabbit's ovum in an advanced stage of pregnancy 

 ((/. Balfour, loc. cit. ii., fig. 148, p. 200), corresponds closely with 

 that of Caldwell's Marsupials (fig. 7). In higher mammals the 

 yolk-sac has a diminished and the allantois an increase.^ extension. 



Bearing the above in mind, it is very interesting to find that 

 the first attachment of the embryo to the parent in some rodents 

 is by the covering cells (Deckenzellen) which are later replaced by 

 the villi of the allantoic placenta. It may be that this has relation 

 merely to special oases ; but it is probable that when our knowledge 

 is more complete in such details it will be found that in many 

 Eutheria there is a temporary adhesion of the walls of the blas- 

 todermic vesicle to the uterus which is subsequently supplanted by 

 the allantoic villi. 



