1158 PROF. J. p. HILL AND MR. R, H. BURNE ON THE 



Minute Structure of the Allantoic Chorion and its Villi. 



The general appearance of the chorion and its villi in section 

 is illustrated in Pis. Y., YI. figs. 13, 14, and 15. 



The allanto-chorion consists essentially of the outer wall of the 

 allantois fused with the chorion pi^oper. It thus comprises the 

 following layers : — (1) the allantoic entoderm ; (2) the composite 

 allanto-chorionic connective tissue which is continued out to form 

 the cores of the complexly folded laminar villi, and in which are 

 situated the branches of the umbilical vessels; (3) the superficial 

 covering of trophoblast (tropho-ectoderm, chorionic ectoderm). 



The trophoblast i fig. 16, tr.) throughout its extent is singularly 

 uniform in character, varying only in thickness, and that with 

 no apparent regularity, though frequently it is thinned in places 

 over the apices of the villous branches. It attains a maximum 

 thickness of "024 mm. and a minimum of "005 mm. or even slightly 

 less. Where it is thickest, it consists of a well-defined single 

 layer of cubical cells, with their outer surfaces projecting con- 

 vexly. The nuclei are large and deeply staining. The cytoplasm 

 of the outer zone of the cell-body is homogeneous or very finely 

 granular and strongly eosinophil ; that of the deeper zone below 

 the nucleus is reticular in character, and stains lightly. There is 

 no basement membrane, and the basal ends of the cells rest, just 

 like those of the uterine epithelium, directly on the endothelial 

 walls of the underlying capillaries (fig. 16, cap.). These are 

 relatively large and very abundant, forming a richly developed 

 subepithelial plexus, first figured by Milne Edwards from injected 

 specimens of Propithecus cliadema and Indris hrevicaudatus (3, 

 pi. 121, figs. 1, 2, & 4). In Galago also, according to Strahl (14), 

 the capillary network of the villi lies in direct contact with the 

 epithelial covei^ing. 



The mesodermal core of the villi (PI. Yl. fig. 16) is formed by a 

 variety of fibrillar connective tissue, differentiated into two zones : 

 (a) a looser less deeply staining central zone {c.t.), containing 

 numerous leucocytes and lymphocytes and in which are situated 

 the smaller branches of the umbilical vessels ; and (h) a highly 

 characteristic peripheral zone in the form of a perfectly continuous 

 and more deeply staining layer of uniform thickness which lies 

 immediately below, and evidently forms a support for the subtro- 

 phoblastic capillary plexus. This peripheral zone (PI. YI. fig. 16, 

 c.t' .) consists of a coarse fibrous reticulum, the stronger fibres of 

 which run out at right angles to the surface, so that under a, low 

 power the zone presents a striate appearance, suggestive at first 

 sight of a columnar epithelium, more especially as the nuclei of 

 its cells of origin are mainly arra,nged in an irregular row, defining 

 the deep surface of the zone. Round the nuclei there is some- 

 times visible a small amount of cytoplasm, which appears to be 

 directly continuous with the fibrous network. This zone would 

 seem to be distinctive of the villi of Chiromys. It is not present 

 eitlier in Lejnlemur or Nycticehus. In both these forms the 



