126 T. THOMSON FLYNN 



Finally, I must not omit to pay a tribute to the friendly 

 interest which has been shown in my work by my former 

 teacher, Professor W. A. Haswell, who has been always ready 

 to help with kindly criticism and advice. To him, also, I am 

 indebted for the loan of literature otherwise inaccessible to me. 



2. Eeview and Criticism of Previous Work. 



The discover}' by Hill of a true allantoic placenta in the 

 two forms, Perameles obesula and nasuta, was of 

 the greatest importance in regard to the phylogeny of the 

 marsupial group and its relationship to the remainder of the 

 mammalia. The actual placental material, upon which Hill 

 was able to base his conclusions, consisted of three stages. 

 The first of these. Stage C, concerned an embryo 7 mm. long. 

 This was followed by a Stage D describing the placental con- 

 nexion in embryos 8 to 8-75 mm. long. In a subsequent 

 paper Hill described the similar phenomenon in a 12-5 mm. 

 embryo which was also designated D. For the sake of clearness, 

 when speaking of these stages, I will designate them by their 

 length in each case. 



In his earliest (7 mm.) stage the fixation of the embryo was 

 already completed and the allantoic placenta was well on the 

 vv-ay towards being fully established. 



From an examination of these three stages Professor Hill 

 showed that the epithelium of the uterus becomes converted 

 into a vascular syncytium, the nuclei of which arrange them- 

 selves in groups in the lower portion of this layer. 'At the same 

 time maternal capillaries pass up between the syncytial lobules, 

 penetrate the syncytial protoplasm, and form a network on 

 and just beneath the surface " (1897, p. 387). 



The fixation of the embryo is brought about in the usual 

 way by means of the chorion. The ectodermal portion of this 

 membrane, by which actual approximation of the foetal 

 membranes to the uterine wall is first achieved, consists of 

 a single layer of large cells by means of which the attachment 

 is l)rought about. 



With the somatic mesothelial layer of the chorion, the 



