THE PHYLOGENETIC SIGNIFICANCE OF THE MARSUPIAL 

 ALLANTOPLACENTA. 



By Professor T. Thomson Fltnn, D.Sc, University of Tasmania. 



[Read 29th, November, 1922.] 



In a paper -which is now in the press (Quart. Journ. Micr. Se.) I have 

 been able, with the aid of certain early stages in the development of the marsupial 

 Perameles, to extend in a fruitful direction our knowledge .of the marsupial al- 

 lantoplaeenta. In this communication, following out Assheton's suggestion (1909) 

 — against that of Hubrecht (1908) — I have used the term "placenta" for any 

 mutual development of the foetal membranes and the uterine wall — whether of 

 intergrowth or of mere apposition — which has for its purposive function the 

 nutrition of the embryo in utero. 



In this way, in a typical mammal, in the ontogeny of which the placental 

 cycle is fully developed, there are three successive stages of this cycle, (a) 

 metrioplacental, (b) omphaloplacental, (c) allantoplacental. 



The following scheme will indicate the scope and nature of these stages : — 



II 

 1-1 



(a). This is the stage in which the nutrient material is ab- 

 sorbed, without the aid of bloodvessels and, in fact, long before 

 bloodvessels are developed. During this period cleavage and , ■g^S' 

 early development of the blastocyst take place. The work ^ - - 

 of this period is carried on in later growth by the lower non- 

 vascular portion of the yolk-sac Vi'all (= m.etrioplacenta) . 



(b) . The bloodvessels of the vascular area of the yolk-sac wall have _g 



now appeared and are functional. This area becomes closely _g 



applied to or actually fused with the uterine wall (^ omphalo- o. 



placenta) . The chorion prepares a way for the oncoming allantois. S 



(c). The allantois becomes attached to the placental area of the uterine wall 

 and an intimate fusion of embryonic and maternal tissues takes place, with 

 a corresponding close apposition of the two sets of bloodvessels, the whole 

 forming the allantoplacenta. 



It will be evident that in the first stage the metrioplaeenta is the only one 

 present. In the next stage the metrioplaeenta and the omphaloplaeenta function 

 together, the latter being the more important, while, in the final period, the 

 allantoplacenta which is the dominant one functions with the other two. 



Suppression of one or other of these stages of the placental cycle may take 

 place. In general, in marsupials it is the third stage which is lacking. In 

 Perameles alone, as far as is known, does it assume an impoi-tance 

 with that of the Monodelphia. 



