6 JAS. p. HILL. 



considerably less than that of a Macropod embryo at a 

 somewhat earlier stage of development. What, however, 

 especially characterises the vascular area of this stage, as 

 well as that of my preceding Stage D, is its relatively 

 extremely poorly developed capillary system. Branches 

 are, indeed, given off from the sinus into the vascular area, 

 but these are small and by no means numerous, and ap- 

 parently undergo very little secondary branching. 



I describe below the foetal membranes of an embryo of 

 Macropus parma, and from that account and the accom- 

 panying figure (PI. 2, fig. 9) it will readily be seen that 

 the branches given off from the sinus terminalis into the 

 vascular area are not only numerous but richly branched ; 

 they form a highly developed capillary system, almost entirely 

 unrepresented in Perameles, in correspondence with the fact 

 that in Macropus the embryonal nutrition is carried on 

 solely by means of the yolk-sac vessels of the vascular area, 

 while in the Perameles embryo of this stage the allantoic 

 circulation is now playing the dominant part in the nutrition 

 of the embryo, only an insignificant proportion of the nourish- 

 ment necessary for the growth of the latter being conveyed 

 through the yolk-sac vessels. In my previous paper I put 

 forward the suggestion that probably the entire yolk-sac wall 

 disappeared before the end of intra-uterine life. The present 

 stage, however, shows that, at all events up to this period 

 of development, the entire yolk-sac wall remains intact, 

 although greatly reduced in functional importance, as shown 

 by the poor development of its vessels. 



The yolk-sac splanchnopleure is supplied with blood by 

 numerous fine wavy vessels (fig. 1, a. v.), which arise directly 

 from the vitelline artery — especially from its first part — as it 

 courses over that membrane. In fig. 2 one main branch is 

 seen passing from under the free margin of the umbilical 

 stalk. It divides up into numerous small wavy branches 

 which pass up parallel with the vitelline veins. Behind this 

 numerous branches arise separately from both sides of the 

 artery (fig. 2, vit. a.). In the terminal part of the course of 



