PLACENTA IN PERAMELES 167 



(/) The outgrowth of foetal plasmodium does not extend 

 any farther than to involve the proliferated maternal epithelium. 



(g) The attachment of the allantois is effected when the 

 embryo has attained a length of approximately 6-5 mm. 



(h) The outward migration of the basal cytoblast cells (when 

 converted into plasmodiblast) gives opportunity for the 

 maternal and foetal vessels to come into intimate apposition. 



(i) In the final stage the foetal nuclei in the placenta are 

 found to be in a state of degeneration. 



(j) Kemains of the maternal epithelium still probably exist 

 in the full-term placenta. 



(/.:) The uterine glands persist throughout gestation, but the 

 portion of their epithelium within the diploplasma disappears. 



(l) All maternal vessels have definite endothelial walls : 

 hypertrophy of the endothelial cells does not occur and lacunae 

 are not formed. 



(m) An allantoic placenta is recorded for P . g u n n i . 



2. Yolk-sac Placenta. — A virtual yolk-sac placenta is 

 present in P . g u n n i as in other species of P e r a m e 1 e s , 

 brought about by the intimate apposition of the complex 

 system of vessels in the vascular omphalopleure with the highly 

 vascular portion of the uterine syncytium just beyond the 

 placental area. 



ferated to form a basal cytoblast and an external plasmodiblast. The 

 ]3roliferations of the latter are related to glands and nuclear nests. The 

 ca])illaries ramify just below the cytoblast layer. 



(c) Allantoic attachment is now in its first stages. Further proliferation of 

 the foetal tropho blast has taken place and the plasmodiblast now occupies 

 the greater part of each nest. Within the diploplasma the gland 

 epithelium has disappeared and the gland is closed in above by cytoblast 

 plus plasmodiblast. Capillaries still ramify below the cytoblast. 



(d) The foetal plasmodium now occupies the whole of each nest with the 

 exception of (possibly) a few remains of maternal tissue. The cytoblast 

 layer has broken down, having mostly been transformed into plasmo- 

 diblast. A few islands of trophoblastic tissue are, however, still left 

 in the cytoblast position. The very much branched and interlocked 

 foetal and maternal vessels have now come into intimate apposition. 

 The gland epithelium in the diploplasma has entirely disappeared, and 

 the glands are closed in above by all antoic vessels alone. 



all., allantois; all. cap., allantoic capillary; cyt., cytoblast; gl., gland; 

 mat.cap., maternal capillary ; pb., plasmodiblast ; troph., trophospongia. 



