PLACENTA IN P BRAMBLES 147 



on to the cy to blast, the cells of which exercise a secretory 

 and selective role. 



'Hie cytoblast appears to have as one of its duties the ampli- 

 tication of the area of attachment. Cell outlines are always 

 distinct between the cytoblastic cells due to radial divisions. 

 These result in the formation of new cells and consequent 

 increase of the area of attachment. As a result, new marginal 

 zones of plasmodial formation are brought into being, and these 

 attack fresh sources of nutrient material in the maternal 

 trophospongia. This areal increase is subject, of course, to 

 certain limitations, and in Perameles, as will be seen 

 later, the growth in thickness of the plasmodial formation is 

 determined also by the thickness of the proliferated maternal 

 epithelium. In many mammals, however, as is well known, the 

 plasmodiblast extends much further, even in some cases as far 

 as the muscularis. causing the degeneration and disappearance 

 of practically all the structures in its track. 



A second function of the cytoblast cells is, as indicated 

 above, the elal)oration of a secretion which is passed into the 

 extra-embryonal coelome. They therefore exercise a certain 

 selective capacity on the material passed to them by the 

 Plasmodium. 



Abundant evidence of this secretory activity is to be seen in 

 sections, the actual secretion being easily observed. From the 

 above it will be seen that structurally and physiologically the 

 proliferating trophoblast of Perameles over the allantoic 

 placental area is quite comparable with the corresponding layer 

 in the Eutheria. 



Stage 2 . Perameles g u n n i , 6-6 mm. 



The specimen on which the following description is based 

 was brought to me by a trapper on September 17, 1919, about 

 two hours after being trapped. Both uteri were swollen and 

 found to be pregnant. The left uterus after being opened was 

 fixed in Hill's fluid, and the whole uterus with embryo in 

 situ was later sectioned. 



The right uterus was examined in salt solution. It was 



L 2 



