PLACENTAL STRUCTURES OF THE TENREC. 307 



merit, now ordinarily' made, to the effect that the Cetacea have no azygos veins at all, 

 as this peculiarity would, if it really did exist, differentiate them from all other mam- 

 mals whatever. But on looking at the words of Von Baer^, upon whose authority this 

 statement is made, and at the facts with which Von Baer's words are usually in accor- 

 dance, it will be found that the vena-azygos system not only does exist in Cetacea, but 

 actually furnishes us with an additional point of affinity between them and the large 

 ungulate Mammalia. Von Baer's words, in the ' Bulletin de I'Academie Imperiale des 

 Sciences de St. Petersbourg,' I. c, are, " la veine dite impaire qui manque excepte le bout 

 anterieur, et dont les sinus de la colonne vertebrale tiennent lieu." This anterior end, 

 however, is a very considerable vessel ; and in the ' Nova Acta' {I. c.) I find that Von Baer 

 describes it as having a trunk as larg-e as that of the vena cava superior, which it joins ; 

 and he goes on to state that its large size is not to be accounted for by its intercostal 

 factors, but by the junction to it of a trunk from within the vertebral canal, of large size 

 and connected by its constituent factors, as the vena azygos is, with those of the lower 

 cava as also with the lower intercostal veins. If we examine a Pig, we shall find it to 

 possess a large azygos vein on the left side ; and this the Cetacean, it is true, does not ; 

 but it will be seen to have on the right side a short vena azygos, just as the Porpoise 

 has, with four or five affluents from the upper intercostal spaces, as well as very con- 

 siderable tributaries from the muscles of the back and scapulae. The vena azygos 

 system is well known to be intimately connected with venous ramifications situated 

 about, around, between, and within the spinal column ; and the greater development of 

 any one of these vascular connexions does not destroy the specific and distinctive 

 character of that peculiar system. Its perirrhachidian or dorsispinal anastomosis is 

 chiefly developed in the right side in the Pig, which has a left azygos also ; its endo- 

 rhachidian or intravertebral 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, however, but slight as compared with the difference which the statements 

 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 rudi- 

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

 invariably present in deciduate Mammals. 



A few points of resemblance between the placentae of the Ungulata and the Mutica 

 have escaped notice. First, in Cetacea and in certain Ungulata we find the mem- 

 branes of what is often a solitary embryo prolonged from one cornu round into the 

 other, and projecting by a csecal extremity into the short corpus uteri. Such a condi- 

 tion of the structures is figured from the Mare by Cohn {I. c), and has been seen by 

 myself in the membranes of a small Cetacean, sp. ? ; and in the Cow and other Ruminants 



' 'PhU. Trans.' for 1849, p. 152. Milne-Edwards, 'Physiologic Comp.' vol. iii. pi. 2. p. 593. 

 " Nova Acta, vol. xvii. pt. 1. p. 408, 1834 ; Bull. Acad. St. Petersbourg, torn. i. 1835 ; Froriep, ' Notizen,' 

 50, p. 38. 1836. 



VOL. V. PART IV. 2 S 



