464 MR HAROLD AXEL HATG ON 



arteries and vein. The amnion then spreads out on either side of the line of closure, 

 the resulting folds passing anteriorly, posteriorly, and laterally to the margins of the 

 placenta, over which it then spreads, covering the fcetal surface of that structure. 

 The branches of the umbilical arteries and vein are conducted by the above-mentioned 

 folds to the margins of the placenta, and finally divide up into numerous twigs which 

 enter the substance of the chorion. 



The placenta itself is of the zonary type, occupying a median zone of the uterus : 

 its histology corresponds very closely with the descriptions of GROSSER * and DUVAL 

 for the zonary placentae of the cat and bitch, with the exception of certain minor 



FIG. 2. A portion of a transverse section of the uterine wall and placenta of the 

 Weddell Seal. (Semi-diagrammatic. ) 



m. Muscle layers of uterine wall. 

 b. Maternal blood-vessels. 

 . Epithelium lining uterine glands. 



I. Lumina of glands containing secretion (em- 



bryotrophe of Grosser). 

 tr. Interglandular fastening septum. 



v. Villi of placental labyrinth. 



The foetal blood-vessels exist in considerable numbers in the villi, but are not represented in the figure ; 

 the foetus contained in the above uterus had undergone about one-third of its development. 



details. Thus we find that a vertical section taken through the placenta and uterine 

 wall presents the following main features (fig. 2) : 



(a) A sheet of amnion (not shown in the figure) covering the fcetal surface of the 



numerous "cotyledons" of the placenta. 



(b) The fcetal portion of the placenta, composed of the rather wide laminae of the 



so-called " placental labyrinth " t between which mesodermic tissue lies ; 

 the sheets of this labyrinth are made up of a syncytium (derived from 

 the trophoblast of earlier stages) which encloses and surrounds large 



* Vergleichende Anat. und Entvrickelungsgesch. der Eihaute u. Placenta, 1909. 

 t See GROSSER, op. cit. 



(ROY. soc. EDIN. TRANS., VOL. L., 242.) 



