104 ON THE PLACENTAL STRUCTURES OF THE TENREC 



development of any one of these vascular connexions does not 

 destroy the distinctive and specific character of that peculiar system. 

 The perirrhachidian or dorsispinal anastomosis is chiefly developed 

 in the right side in the pig 1 , which has a left azygos also ; its 

 endorhachidian or intervertebral factors have absorbed its lower 

 intercostal tributaries in the porpoise ; and in this animal there is 

 no vena azygos on the left. These points of difference are, how- 

 ever, but slight as compared with the difference which the state- 

 ments ordinarily made on this point would lead us to expect. 

 The Cetacea, we may add, have been observed to resemble the sheep 

 and pig and horse, in the deficiency of the rudimentary structure 

 known as the Eustachian valve, which, however, is by no means 

 invariably present in deciduate mammals. A few points of re- 

 semblance between the placentae of the Ungulata and the Mutica 

 have escaped notice. First, in Cetacea and in certain Ungulata we 

 find the membranes of what is often a solitary embryo prolonged 

 from one cornu round into the other, and projecting by a caecal 

 extremity into the short corpus uteri. Such a condition of the 

 structures is figured from the mare by Colin (1. c), and has been 

 seen by myself in the membrane of a small cetacean, sp.? ; and 

 in the cow and other ruminants a similar extension of the mem- 

 branes of a foetus lodged in one cornu round into the other is not 

 rarely seen, but without any caecal diverticulum markedly deve- 

 loped in their short corpora uteri. The multiparous sow does not 

 of course resemble its less fertile congeners in this particular ; but 

 the membranes of the deciduate and ordinarily uniparous seal (Phoca 

 vitulina) have been observed to be confined to the uterine cornu 

 which contained its single foetus 1 . 



Secondly, we find on the umbilical cord of the foetal cetacean, 

 filiform outgrowths of the amnios which are undoubtedly homo- 

 logous with the similarly placed growths in the early ruminant, 

 and in the soliped embryo, as well as with those on the amnios 

 of the tenrec, as already described. In the amnios of the pig no 

 such growths are observable, but certain dilated microscopic 

 vesicles 2 have been supposed to take their place and function. 



1 Barkow, ' Zootomische Bemerkungen,' p. 7. [The Editor observed this to be the 

 case also in the grey seal (Halichoerus gryphus). ' Trans. Boy. Soc. Edinb.' 1875, 

 vol. 27.] 



2 Birnbaum, ' Untersuchungen iiber den Bau der Eihaute,' Berlin, 1863, pp. 18 and 

 67. 



