AND AFFINITIES OF TARSIUS. 



477 



vesicle and subsequently miites with and vascularises the entire 

 chorion), we see features all of which we are familiar with in the 

 development of the lower members of the Mammalian series, and 

 all of which are, in my opinion, primitive. 



Moreover, the primary relations of their fcetal membranes 

 are precisely those Avhieh we are justified in postulating for 

 the primitive ancestral stock from which presumably both the 

 Didelphia and Monodelphia diverged ; whilst their simple, 

 diftiise, non-deciduate placenta (involving the loose interlocking 



Text-figure 1. 



nlt.ck. 



amn. 



]Sfycticel)us tardigradns. Diagram (after Hubveolit) to show the arrangement of 

 the foetal membranes of the early emhrj-o. Note especiall.y the allantois [all.) 

 already fused with the chorion {ch.) to form a discoidal area of allanto-chorion 

 (all.cJi.). The arrows indicate thedirection of extension of the extra-emhryonal 

 ccelom (ex.coe.) into the mesoderm of the omphalopleure {omph.) so as to 

 separate that into yolk-sac wall and chorion, amn. amnion, y-s.c. yolk-sac 

 cavitj'. 



Ectoderm (including- trophoblast) is represented by a thin line, entoderm by 

 a thick line, and mesoderm b}' a dotted line. 



of short villous outgrowths of the allanto-chorion with corre- 

 sponding cr3^pts of the uterine mucosa, the persistence of the 

 iitei'ine epitheliiun and of the uterine glands) is, in my view, 

 essentially primitive, presenting us with a simple little spe- 

 cialised type of placenta from which the much more advanced, 

 and presumably more efficient, ari^angements in the other 



