ON THE ARRANGEMENT OF THE FCETAL MEMBRANES IN THE CETACEA. 487 



In the pluriparous sow, again, though the poles of the ovum are smooth and 

 almost non-vascular, the third bare spot is not present, for the membranes 

 surrounding each embryo are confined to a single horn, and do not pass across 

 the corpus uteri."" In the sow also the folds of the chorion and uterine mucous 

 membrane do not, as in the whale and mare, lie in the direction of the long axis 

 of the uterine cornu, but at right angles to it. 



The bare spots at the poles of the greatly elongated chorion of the cetacean 

 are undoubtedly homologous with the smooth ends of the shorter ellipsoidal 

 chorion of the carnivora, but the non- villous portion is absolutely and relatively 

 much smaller in the former than in the latter. As the carnivora are, as a rule, 

 pluriparous, the conditions necessary for the formation of a third bare spot are 

 non-existent in them, just as is the case in the pig. 



But the closer affinity of the mare than of the sow to the cetacean, in the 

 characters of the chorion, is shown also in the arrangement of the villi, and in 

 the distribution of the capillary blood-vessels. The villi of the chorion of the 

 mare are thickly distributed over its surface. They are for the most part com- 

 pound and arranged in tufts ; though isolated simple villi are scattered in the 

 intervals between the tufts. The tufts are, as a rule, somewhat larger than 

 those in the cetacean, and they have more of a brush-like form than of that of 

 the head of a cauliflower, a difference which I find to be due to the secondary 

 villi being elongated and filamentous rather than club-shaped. Not only is a 

 rich capillary network situated within the villi, but this plexus, as I have 

 satisfied myself, from the examination of an injected chorion, freely communi- 

 cates, as in the whale, with an abundant extra- villous sub-chorionic plexus, from 

 which the rootlets of the umbilical vein arise, so that in both animals a diffused 

 capillary area is produced by similar modes of distribution. 



Von Baer and Eschricht long ago pointed out that in the pig the trans- 

 verse folds of the chorion are notched at the free margin, and the teeth, by grow- 

 ing, become converted into villi. These villi are smaller than in the mare and 

 whale, and at an early period of development can scarcely be said to be present. 

 In a gravid uterus which I examined, where the embryos were 1*3 inch long, 



opposite horn was considerably larger, being 2^ inches long by from \ to f- inch broad, and with radiated 

 bare processes passing off from its two ends. The bare spot opposite the os internum was twice as large 

 as in the cetacean, and had several strongly-marked, radiating, non-villous processes. The presence of 

 these bare spaces, or at least their exact position and signification in the chorion of the mare, seems to 

 have escaped the notice of veterinary anatomists. Neither Chauveau nor Gurlt make any mention of 

 them in their well-known treatises, and Franck (" Handbuch der Anatomie der Hausthiere," Stutt- 

 gart, 1871), whose work is the most recent and fullest in detail of any which I have been able to con- 

 sult, merely says, " in one or other horn roundish spaces are found, where the villi are sparse and 

 feeble, and here the chorion has a semi-transparent appearance." 



* In one pig's uterus, I found that the membranes belonging to the embryo, situated lowest down 

 in the left horn, actually did pass across the corpus uteri into the right horn, but this of course was 

 not the case with the other ova. 



VOL. XXVI. PART. II. 6 L 



