658 SIR WM. TURNER ON THE PLACENTATION OF HALICORE DUGONG. 



In the Cetacea the chorion occupies both uterine horns ; in the Dugong it is situated 

 in only one horn, at least in the later stage of gestation. In the Cetacea the villi are 

 diffused over "both horns of the chorion, except at the poles and opposite the os uteri 

 and the uterine crypts are also diffused; in the Dugong in the fully-developed placenta, 

 the villi are aggregated into a zone on one side of the equator of the chorion, and there 

 is a corresponding limitation of the uterine crypts. In the Cetacea the allantois though 

 extensive, and lying in both horns, does not reach the poles of the chorion, as it does in 

 the Dugong; and whilst the latter possesses allantoic bodies, there are none in the 

 former. In the Cetacea the amnion extends into both horns, reaches beyond the allantois, 

 and possesses numerous amniotic corpuscles. In the Dugong the allantois extends 

 beyond the amnion, and the latter has no corpuscles. Neither in the Cetacea nor the 

 Dugong has an umbilical vesicle been recognised. The Cetacea are undoubtedly non- 

 deciduate. The Dugong is also without doubt in the main non-deciduate, though there 

 is the possibility of an exception in the case of the longer villi and the walls of the 

 deeper crypts. But in this matter I would again point out that in the separation of the 

 foetal from the maternal placenta, which had taken place in my specimen before I opened 

 the uterus, the longer villi had detached themselves from the chorion and remained 

 imbedded in the uterine wall ; whilst on the hypothesis of this part of the placenta being 

 deciduate, one would have expected rather that the longer villi would have continued to 

 be attached to the chorion, and the walls of the deeper crypts to have torn away from 

 the uterus and have enclosed them. There is nothing in the structure of their respective 

 placentas to justify the retention of the Cetacea and Sirenia in the same order. 



By some zoologists the Sirenia have been regarded as allied to the Ungulata. 

 Amongst ungulate mammals the placenta is not uniform in its arrangement. The Pigs, the 

 Equidae, the Camelidee, Tragulidse, the Tapir, Hippopotamus, and probably the Rhinoceros, 

 furnish typical illustrations of the diffused variety of placenta ; the Ruminants generally 

 exemplify the cotyledonary form ; whilst the Giraffe * and Cervus mexicanus,i in addition 

 to a number of large and small cotyledons, had patches of villi diffused over portions of 

 the chorion intermediate to the cotyledons, so that they combined the characters of both 

 types. All these animals agree in possessing a large and persistent allantoic sac. The 

 umbilical vesicle, however, either disappears at an early period of utero-gestation, or is 

 so small as to be recognised with difficulty. All ungulate mammals, which possess a 

 diffused or cotyledonary placenta, are non-deciduate in the sense in which that term is 

 usually employed. 



Hyrax, which has certain ungulate affinities, also possesses a zonary placenta, which, 

 as I showed some years ago,| corresponds in structure with that of the domestic Cat. 

 The sac of the allantois is so large as to reach the opposite poles of the chorion, but the 

 umbilical vesicle apparently atrophies at a comparatively early period of utero-gestation. 



* See my Lectures on the Placenta, p. 67. 



+ See my description of the Placenta of the Mexican Deer, in Journal of Anatomy and Physiology, vol. xiii. p. 195, 

 1879. 



t Proc. Roy. Soc. Lond., Dec. 16, 1875. 



