SIR WM, TURNER ON THE PLACENTATION OF HALICORE DUGONG. 653 



General Observations on the Placentation of the Dugong. 



I have already stated that a description of the foetal membranes of the Dugong was 

 given a few years ago by Dr Harting of Utrecht. His specimen was at a much earlier 

 stage of development than mine, for his foetus was only 27*8 cm. long, whereas mine 

 measured 163 cm., i.e., 5 feet 4 inches. The chorion presented a very different appear- 

 ance in the two specimens. Harting describes it in his example as being, with the 

 exception of its poles, covered on its entire surface with villi ; the length of which varied 

 from 0'7 to 1"5 mm. They were most dense at the middle of the chorion, where the 

 branches of the umbilical vessels reached the chorion. The villi extended to about 

 3 cm. from the posterior pole, but had amidst them spaces bare of villi, so that the 

 border of the chorion adjoining the naked pole was very sinuous. The villi approached 

 the anterior pole to from 4 to 5 cm. The villi were formed of connective tissue covered 

 by a layer of round or oval cells, and contained capillary blood-vessels. The posterior 

 non- villous pole had probably been in relation to the tubal end of the uterine cornu and 

 the anterior non- villous pole to the ostium uteri, but in the absence of the uterus their 

 relations could only be surmised. Close to the anterior pole of the chorion was an 

 appendage covered by villi, which Harting thought might have been lodged in the 

 non-fecundated horn of the uterus. Owing to the distribution of the villi over so large 

 a proportion of the surface of the chorion, Harting came to the conclusion that in the 

 Dugong the placenta was diffused as in the Cetacea, the Pig, and the Mare. 



My dissection of the gravid uterus in a much more advanced stage of development 

 puts the placentation of this animal in quite a new and different light. As has been 

 stated in the description, the placenta is zonary, the zone not being situated around the 

 equator of the chorion, but somewhat nearer to the caudal end of the embryo ; and the 

 zone is as perfect in form as in the Cat or other carnivorous animal. 



It will be necessary, therefore, to consider how the change from the more diffused 

 arrangement seen by Harting to the zonary form observed by me was brought about. 

 For this purpose I may refer to observations which I made some years ago on the 

 placenta of the common Cat at different stages of development.* In an early stage the 

 whole chorion, except an area yxjth inch in diameter at each pole, was covered with villi. 

 In a more advanced stage, the non- villous area was somewhat larger at each of the poles, 

 though much the greater part of the chorion was still villous, and the placenta had 

 assumed the form of a broad zone (fig. 4). In the latter half of gestation the villi 

 were limited to a comparatively narrow zone surrounding the equator of the chorion, 

 and a much larger area was smooth and free from villi than that which was occupied by 

 the placenta. 



Corresponding changes in the proportion of the villous to the non- villous chorion 

 take place also, I believe, in the development of the placenta in the Dugong. At first 

 the chorion is in all probability entirely covered with villi as is the case indeed at an 



* See for further details my Lectures on the Comparative Anatomy of the Placenta, p. 72, Edinburgh, 1876. 



