208 Arthur Willey, 



The placenta of the beaver adheres to the Uterus over a dis- 

 coidal area, but the placenta itself is not disc-shaped. It forms a 

 massive kidney-sliaped organ, projecting bodily into the exocoelom, 

 the lower or foetal part of it embraced by the amnion, as already 

 described and again represented in Fig. J. 



With a foetus having a crown-rump measurement of 130 mm. 

 the placenta is 60 mm long, 34 mm wide, and about the same 

 (34 mm) in height. The hilus of the kidney-shaped organ is situated 

 on its superior or uterine aspect, and is marked by the insertion of 

 the placental stalk or root (radix placentae). 



The surface of the placenta presents a somewhat lobulated 

 structure, the shallow grooves between the lobules giving passage 

 to the superficial blood-vessels, maternal veins and foetal arteries. 

 of the placenta. Internally there is no marked differentiation into 

 lobules, the entire mass of the spongy placenta consisting of a 

 uniform dark vascular reticulum, the allanto-chorionic ingrowths 

 being diffuse and not sufficing to produce a definitely septate divi- 

 sion of its substance. Median vertical sections show a definite con- 

 tact-zone where the dark spongy ectoplacenta impinges upon the 

 whitish serotina or trophospongia at the root of the placenta. The 

 central mass of the serotina appears, at this stage. to be in a con- 

 dition of pulpy histolysis. 



The vascular labyrinth of the spongy ectoplacenta consists of 

 sanguimaternal canalicules with plasmodial walls, separated by 

 allanto-chorionic (foetal) capillaries. I propose to postpone the histo- 

 logical examination of the placenta and associated structures to a 

 future occasion when it may be possible to have procured earlier 

 stages. The macroscopic condition of the parts, at this period of 

 gestation, appeared to me to present a sufficient number of inter- 

 esting features to justify their presentation in this form and on this 

 memorable occasion. 



The inferior or foetal aspect of the placenta is occupied by the 

 umbilical cord and the allantoic vesicle. The latter appears as a 

 bladder füll of clear fluid, firmly adhering to the placenta. Upon 

 pressing the bladder between the fingers, it proved to be a closed 

 vesicle, the fluid not finding any means of exit. 



The spirally twisted umbilical cord, containing both the allantoic 

 and the omphalo-mesenteric vessels, passes forwards to the cephalad 

 end of the placenta, where the omphalo-mesenteric cord leaves it and 

 bends upwards (i. e. towards the back of the embryo; it should be 



