220 Transactions. — Zoology. 



Each foetus lay coiled up in its compartment (fig. 1), some part of its 

 body, in many cases, pushing one of the partition walls and causing it to 

 bulge out into the adjacent compartment. In the specimen figured, for 

 instance, the head of foetus h encroaches upon compartment g ; the 

 trunk of g encroaches upon compartment h, and its head upon f. The 

 foetuses are thus packed as closely as if they were not enclosed in separate 

 chambers. 



In the specimen figured there were eight foetuses in the uterus, but the 

 usual number seems to be five. In some cases one foetus was considerably 

 less developed than the rest : this is the case with a in fig. 1 : in one 

 instance there was in the anterior end of the oviduct a mass of yolk, evi- 

 dently an egg which had undergone no development. 



The precise anatomical relations are as follows : — The wall of the 

 uterus, as mentioned above, is very thin : it consists of an outer peritoneal 

 investment (fig. 2, p), then of a remarkably thin muscular layer (m), and 

 finally of the mucous membrane (m.m.). The latter is produced into a 

 series of reduplications which extend across the cavity to the opposite wall, 

 and in this way the foetal compartments are formed. 



From this it is evident that the outer walls of the foetal compartments 

 are simply portions of the uterine walls, and are lined with epithelium, but 

 that the party walls (ps. ch.) consist of mucous membrane only, covered 

 with epithelium on both sides. The mucous membrane has a yellowish 

 colour, is raised on its free surface into numerous folds, and is abundantly 

 supplied with blood-vessels, so that each foetus is surrounded with a 

 vascular membrane. 



From the inner surface of the mucous membrane, a thin colourless 

 transparent non-vascular layer (fig. 2, ^s. am.) can be readily dissected off. 

 From the relations of the mucous membrane, as just described, it follows 

 that this non-vascular membrane must occur in the form of a series of 

 closed sacs, forming the actual lining of the several compartments. 



As a consequence of this arrangement, when the peritoneal and muscular 

 layers of the uterus are stripped off — -which can be done with great ease — 

 and the Fallopian tube and cloacal end of the uterus removed, the mucous 

 membrane of the uterus proper is obtained in the form of a single perfectly 

 closed sac, but on removing the mucous membrane itself, a number of 

 closed sacs are obtained, each enclosing a foetus with the surrounding fluid, 

 and consisting of the non-vascular membrane just described. 



It will be seen at once that the transparent non-vascular sac in which 

 each foetus is directly enclosed, has the same general relation to the foetus 

 as the amnion of Sauropsida and Mammalia, from which it differs in being 

 a product, not of the foetal but of the maternal tissues. I propose, there- 



