FCEXAL MEMBRANES OP CHIROMYS MADAGASCARIENSIS. 1157 



individuals of the same age. From Milne Edwards's observations 

 we knovv^ that in younger specimens it is simpler than in older. 



It should be noted that the main sac and its lobes do not com- 

 pletely clothe the inner surface of the chorion (text-figs. 2 & 3), 

 there being left between the lobes considerable areas where the 

 amnion is fused directly with the inner surface of that membrane. 



The entodermal lining of the allantois is formed by a well-marked 

 cubical epithelium, quite different from that lining the allantoic 

 canal. Its inner surface is not everyv/here smooth, but is pro- 

 duced into low folds, more numerous in the main sac than in the 

 accessory lobes. Its mesodermal wall is relatively thin, and 

 consists of fibrillar connective tissue, dense just outside the ento- 

 dermal lining, but delicate and loose elsewhere both in the inner 

 and outer walls. In parts it is reticular in character. It is 

 rich in cells both fixed and migratory. 



3. Yolk-sac. 



We have not found any definite traces of the yolk-stalk and 

 yolk-sac in our specimens, unless an irregular epithelial strand in 

 process of cornification, which is situated below the amnion of the 

 umbilical cord in the region of junction and bounded on its inner 

 side by a mass of gelatinous connective tissue, represents the 

 degenerate remains of the stalk. Milne Edwards (3) figures 

 the yolk-sac in relatively late foetuses of Fropiihecus diadema 

 and Indris hrevicaudatus [v. especially his pi. 120, where in 

 a late but not yet haired foetus of the latter species it is 

 shown as a quite small, flattened, and stalked vesicle, situated 

 towards the distal extremity of the allantoic stalk — i. e., approxi- 

 mately in the position of our cornifying strand). Strahl (14) 

 simply mentions that in a 36-mm. foetus of Galago the yolk-sac 

 is " gut nachweisbar." Jenkinson (16) states that in Leinlemur 

 " no trace of the yolk-sac was found." 



4. Amnion, 



The amnion forms an oval sac fitting loosely around the foetus. 

 It invests the umbilical cord throughout its length, and at the 

 distal extremity of that spreads out to become loosely fused with 

 the inner wall of the allantoic sac and its lobes. In the intervals 

 between the latter it is fused directly with the connective tissue 

 of the chorion. The most extensive areas of the amnion that are 

 in this way in direct apposition and fusion with the chorion lie 

 towards the head-end of the foetus (text-figs. 2 & 3 and PL II. 

 figs. 2 & 3). There is an extensive area covering the whole of the 

 right side of the head and shoulders (text-fig. 3), and narrower 

 patches over the left side of the head and shoulders (text-fig. 2) 

 and along the back between the apices of the allantoic lobes. As 

 in the higher Primates, the expansion of the amniotic sac and its 

 adherence to the inner wall of the allantois have brought about 

 the complete obliteration of the extra- embryonal coelom. 



PROC. ZooL. Sec— 1922, No. LXXVII. 77 



