THE PLAOENTyVTION OP PERAMlilLES. 891 



undergone active proliferation. They now form an irregular 

 band occupying the mid region of the syncytium, and arc so 

 numerous as to frequently overlap even in very thin sections 

 (fig. 2). They varj^ considerably in shape, mostly ovalish or 

 elongated, and are evidently in a most active phase. Though 

 I have not been able to make out undoubted mitotic figures in 

 my preparations, there can be no doubt that marked pro- 

 liferation of the syncytial nuclei has taken place. 



Minot's description (4) of the early changes taking place in 

 the uterine epithelium of the rabbit prior to its complete 

 degeneration is equally applicable to Perameles. He says 

 ''the thickening [of the uterine epithelium] is due to the en- 

 largement and fusion of the epithelial cells, and this enlarge- 

 ment of the cells is due to the proliferation of the nuclei, and 

 to the growth of the protoplasm which begins later, and con- 

 tinues longer (as later stages show) than the multiplication of 

 the nuclei." It may be pointed out, however, that the agree- 

 ment in the two cases goes no further than the earliest stages. 

 As we know from the researches of Minot (4), Duval (5), and 

 others in the rabbit, this nucleated protoplasmic layer formed 

 from the uterine epithelium soon degenerates and disappears ; 

 in Perameles, on the other hand, as will be abundantly evident 

 further on in this paper, the syncytial layer derived from the 

 uterine epithelium not only does not degenerate, but, increas- 

 ing in size and becoming vascuiarised by maternal vessels, 

 persists throughout the whole period of pregnancy, and takes 

 a most essential part in placental formation. 



Stage B. — P. obesula. 



The left uterus was somewhat larger than the right, measur- 

 ing 17 ram. in length by 11 mm. in breadth. It contained 

 two blastodermic vesicles, with the "granulosa membran " of 

 Selenka still in greater part persistent round them. The 

 embryo measures about 7 mm. in length, and possesses at least 

 fifteen mesodermal somites. It is characterised as follows : — 

 Anterior end strongly flexed and enclosed in the large pro- 

 amnion ; medullary plate in anterior cerebral region still 



