
PLACENTATION OF THE SEALS. 299 
in the degree in which the shedding takes place. The Felide have a higher 
grade of deciduation than the Canide, and with the latter the Phocide corre- 
spond. Hence the Dogs and Seals, in their placental affinities, are less removed 
from the Cetacea, the Suide, and the Solipedia than are the Cats. The pits and 
trenches of the mucosa, which one sees on the uterine zone, after the separation 
of the placenta in a seal, a fox, or a dog, are obviously similar in their morpholo- 
gical characters to the crypts of the mucosa of a mare, a cetacean, or other 
animals with a diffused placenta. In the seal, the pits and trenches possess a 
precision of form more than is seen in the dog and fox, a circumstance which 
is undoubtedly due to the subdivision of the placenta of the seal into definite 
minute lobules. The higher grade of deciduation in a cat may perhaps be 
accounted for by the broadly laminated villi, their very sinuous form, and the 
depth in the mucosa to which their terminal bud-like offshoots penetrate, giving 
to the foetal part of the placenta a “grip,” if I may so term it, over the 
maternal part, so as to interlock the latter more firmly with the villi, and thus 
to cause the mucosa to be more completely shed in the process of separation. 
For, as I have already pointed out in a previous memoir,* the shedding or 
non-shedding of maternal tissue, along with the feetal, during the act of 
parturition is determined by the degree of interlocking of the foetal and 
maternal portions of the organ with each other, and not from the presence 
in the deciduata of a structure or structures which do not exist in the 
non-deciduata. 
In the fox and seal the intra-placental prolongations of the mucosa are 
subdivided into a reticulated arrangement of slender trabeculae, each bar of 
which contains only a single dilated capillary; but in the seal this sub- 
division is carried out to a greater extent than in the fox. In the seal occurs 
that very remarkable anastomosis of the distal ends of the primary branches 
of the chorionic villi, which gives to the placenta its. precise lobular subdivision, 
and walls in each lobule at its uterine periphery with the greyish membrane. 
From a somewhat cursory examination of the placenta of a Phoca vitulina in 
the Museum of the Royal College of Surgeons of England, it appeared to me 
that a similar membrane existed also in that animal, so that I am disposed to 
consider the arrangement as one which is of more than generic, indeed of 
ordinal value. 
From the general correspondence in shape and structure between the 
placenta of the Pinnepedia and that of the true Carnivora, there can be no 
doubt that in both orders the early stage of formation is marked by the pro- 
duction of crypts in the placental area of the uterine mucosa, and that these 
crypts are formed, quite independently of the utricular glands, by a great growth 
and folding of the interglandular tissue. In the grey seal, the villi of the 
* Trans. Roy. Soc. Edin. 1871, p. 426. 
VOL. XXVII, PART III. at 
