SEAL-EMBRYOS. 19 



In most of the recorded cases the uterus contained but a single fetus, as is the 

 case in the uterine specimens in this collection. Mayer, quoted by Turner, records an 

 instance in P/ioca vitulina in which the left horn of the uterus contained five embryos 

 and the right horn four. Turner also had in his possession twin foetuses from the 

 uterus of a Phoca greenlandica. There can, however, be no doubt that the presence 

 of more than a single fetus is quite exceptional, and that the fetus is situated in 

 one or other of the cornua, the non-gravid horn being very slightly, if at all enlarged. 



There is little or nothing to add to the description already given by Turner of 

 the macroscopic characters of the placenta. As is the case in the Carnivora generally 

 the placenta of the seal is zonary. The fetal surface (fig. 16) shows a series of 

 elongated cord-like elevations with intervening depressions. These elevations lie more 

 or less parallel with one another and run in the long axis of the placenta itself (fig. 16). 



The histological characters were examined by means of longitudinal and transverse 

 sections from the margin and central portion of placenta of different ages. In order 

 that no point of importance should be overlooked I submitted the sections for 

 examination to my friend Mr. Richard Assheton, who has done so much to elucidate 

 the structure and comparative anatomy of that organ, and to his kindness I am 

 indebted for the description here given. 



Two distinct stages are represented in the specimens examined. The earlier one 

 is of an age equivalent to the 24th to 26th day of pregnancy of the dog, the older of 

 an age equivalent to perhaps the 40th to 45th day of pregnancy of the same animal. 

 There is, as one would expect from Turner's description (10), a very close resemblance 

 to the placenta of Carnivora such as the dog or ferret. 



In the earlier stage the angioplasmode formation of Duval has become well 

 established but forms as yet only a thin layer. The mouths of the uterine glands are 

 blocked by the trophoblast and by degenerated uterine epithelium, but the preservation 

 of the material is not sufficiently good to determine the boundary between the two. 

 The distal parts of the glands are expanded, and by this expansion and consequent 

 thinning out of the intervening tissue the " lamelles mesenteriques " are formed (fig. 17). 



In the younger stage the embryonic blood corpuscles are still nucleated. Even 

 in the early stages lacunae containing extravasated maternal blood, lying between the 

 maternal tissue and the trophoblast or bounded on nearly all sides by the trophoblast 

 have commenced to appear. (Compare Assheton 1, pi. 13, Canis.) 



In the older specimen (fig. 18) the angioplasmode layer has increased enormously. 

 In the earlier stage it is only about one-sixth to one-quarter of the thickness of the 

 sub-mucous layer, whereas in the later stage it is about twice the thickness of that 

 layer. The dilated glands are now still more dilated, and Mr. Assheton thinks that in 

 many cases the angioplasmode projects into their cavities. 



The lacunae of extravasated maternal blood are large and the trophoblast cells 

 forming their walls are gorged with red maternal blood corpuscles. 



There appear to be no important differences between the placenta of the seal and 



D 2 



