I»LACENTA IX PERAMELES 159 



plete, the allanto-cliorion Ijeiiig united with the maternal 

 epithehiim over an area whose diameter, following the well- 

 marked uterine folds, is some 1B-I4mm. 



The area of attachment is not co-extensive with the whole of 

 the chorion since there is a marginal zone of the latter quite 

 free, as in the 6-6 mm. stage. 



Structural Details of the Allantoic Placenta. 



As would have been expected from the description of the last 

 stage there is b}^ this time a most intimate fusion of the maternal 

 and foetal tissues in the diploplasm;!. the proliferation of the 

 troplioblastic ectoderm and its invasion of the maternal 

 syncytium having advanced considerably beyond the condi- 

 tion found in the 6-6 nun. entbryo of P. gunni. 



Here again, therefore, in the trophol)lastic proliferation there 

 can be recognized two portions, a basal cellular layer corre- 

 sponding to the cytoblast or cytotrophoblast of higher mam- 

 mals, and externally to this a proliferating plasmodial portion, 

 the plasmodiblast or plasmoditrophoblast. 



The cytoblast is throughout its extent fairly distinct. Over 

 a fair area towards the centre, however, it has already begun 

 to lose its integrity — having disappeared in many places as 

 a cellular layer and become converted here into plasmodiblast 

 (tig. 18). It was upon this condition of the cytoblast in its 

 central portion that Hill based his suggestion of the degenera- 

 tion of the chorionic ectoderm. 



In the marginal portion (fig. 21) the cytoblast still consists of 

 high columnar cells separated from one another by distinct 

 cell walls. Externally, however, the boundaries of these on 

 their plasmodial aspect are indistinct. Where intact, they 

 have somewhat elongated nuclei arranged at right angles to 

 the surface of the uterus. These nuclei stain well and are rich 

 in chromatin. They do not, however, stain so darkly as the 

 more granular plasmodial nuclei to which they give origin. 



At the edge of the placental area the development of plas- 

 modiblast from cytoblast is in its minimum condition of activity, 

 as is shown in the figure (fig. •21). 



