ON THE ARRANGEMENT OF THE FCETAL MEMBRANES IN THE CETACEA. 497 



two forms of placentation, which at the first glance seem, and are indeed 

 usually regarded as, so widely separated, blend with each other through 

 gradational forms which occur in the other mammalia. 



Hence, whilst it may be convenient to employ the terms caducous or non- 

 caducous, deciduate or non-deciduate, in zoological classification, as expressing 

 the shedding or non-shedding of distinctly recognisable portions of the uterine 

 surface, along with the foetal envelopes, they ought not to be regarded as signi- 

 fying absolute differences in anatomical structure in the two groups of placental 

 mammals.* 



Turning now to a comparison of the arrangement of the membranes which 

 lie within the chorion, we find that, although the allantois in the cetacea is 

 elongated towards the two poles of the chorion, yet that it does not reach their 

 extremities as in the mare and ruminants; still less does it pass through and 

 beyond to form those pouch-like sacs, which Von Baer many years ago 

 described in the pig as the diverticula attantoidis. Neither do we find that it 

 lines either the whole, or even the larger part of the inner surface of the 

 chorion, as in one or other of these animals, or as in the carnivora ; but its 

 chorionic attachments are limited to that aspect of the latter membrane which 

 is opposite the abdominal aspect of the foetus. In the persistent condition of 

 its allantois it differs moreover from the human subject and the other mammalia 

 in which that membrane disappears at a comparatively early period of gestation. 



The amnion in Orca, though it does not reach the poles of the chorion, yet 

 preponderates over the allantois, which is just the opposite condition to the 

 arrangement met with in the solipeds, ruminants, and pachyderms. Projecting 

 into the sac of the amnion, though invested by that membrane, are the small 

 corpuscles, " filiform outgrowths, which are undoubtedly homologous with the 

 similarly placed growths in the early ruminant, and in the soliped embryo, as 

 well as with those on the amnion of the Tenrec," as Rolleston has already 

 pointed out. Dr Sharpey also informs me that he has met with a similar set 

 of bodies in connection with the amnion in Manis. It is to be observed that 

 these structures are not limited to the part of the amnion which invests the 

 cord, but are distributed irregularly in connection with its entire surface. 



The umbilical vesicle, again, disappears in Orca some time before birth, as 

 in the mare, pig, and ruminants, and does not persist in the form of a consider- 

 able sac, as in carnivora, rodents, bats, and insectivora; or as a rudiment, as is 

 sometimes seen in the human subject. 



In its placental affinities the Orca, as will be recognised from the statements 

 made in the comparison I have just instituted, approaches more closely to the 



* It is right to state that Professor Huxley, by whom the terms deciduate and non-deciduate 

 were introduced, " by no means intended to suggest that the homologue of the decidua does not exist 

 in the non-deciduate mammals." — Elements of Comparative Anatomy, p. 103. 



