55 



closely enveloping the little embryo, the intermediate space between the surface of the embryo and the amniotic bag being very 

 small. Moreover the amnion, which consists of an inner stratum of flattened epithelium, and an external thin layer of 



connective tissue, seems in a great measure to be separated from the surrounding subchorionic mesenchyme by a fissure. 



The vesicle-like dilatations of which the further connections and meaning could not with certainty be explained, seem to me 

 to be abnormal formations, the supposed umbilical vesicle being also, in all probability, abnormally developed, and by evident 

 thickenings of the walls, proves itself to be in a progressive condition. 



It seems not improbable that this instance represents the commencement of a condition which in man is known 

 by the name of vesicular mola. 



Fætus i. On considering the fætal membranes of the 30 millim. long fætus, we are met by a more advanced 

 developement. The fætus is figured lying in its amniotic bag (PI. VI, fig. 7). 



The amnion still envelopes the fætus somewhat closely, although its cavity is relatively much larger here than 

 in the 8 miUim. long fætus. In the region surrounding the point of attachment of the umbilical cord, several fine vessels 

 are observable, forming a large-meshed network on the amniotic bag, especially on that part which answers to the embryo's 

 ventral surface. On examining it microscopically, there is found on the inside towards the lumen of the bag, the usual 

 epithelium, which is flattened and simple; outside this lies a thin stratum of connective tissue (parietal mesoblast), in which 

 fine vessels spread, as previously stated, from the insertion of the amnion upon the umbilical cord, forming a large-meshed 

 scanty vascular net-work. Inside the fine vessels, there were plainly seen nucleated coloured blood corpuscles, and in a few places 

 beautifiil illustrations of the formation of new blood-vessels. 



The umbilical cord is 8 or 10 millim. in length and about 4 millim. in diameter immediately below the place 

 of insertion. It runs ventrally towards the head for a little way, and is resolved into its elements after a short course, the 

 relatively large vascular stems being divided in the chorion. 



The very capacious chorion presents an elongated shape corresponding to the uterine cavity, but of which the 

 details were not distinguishable, on account of the imperfecrion of the preparation. — The surface of the chorion in the 

 specimen is very wrinkled, the larger folds following the long axis. By dilating the chorion, the folds do not disappear, but, 

 on the contrary, the "nodular" appearance becomes very distinct; for there are a number of small, scattered, wart-like 

 prominences on the surface, which are the chorionic villi, but they are very short and widely separated. 



Even with only a comparatively good magnifying power, on viewing the chorion from the surface, a large-meshed 

 net-work of rather thicker vessels is observable, among wliich is spread a fine-meshed capillary network. On a microscopical 

 ex.imination of the chorion's free surface, the epithelium presents a beautiful mosaic appearance. On investigation of the 

 serial sections, one notices on the extreme outside, a layer of cubical epithelium, which has, in many places, a cylinder form, 

 and in others approaches stratified epithelium. The substratum of connective tissue consists, beneath the epithelium, of 

 spindleshaped (rarely star-shaped) cells in one or two dense strata, and then of mesenchyme with sparse cells. It is also 

 observable here, that the widely-extended sub-chorionic cælom, which has. moreover, become much more restricted bv the 

 amnion and allantois, is partly filled with a serous, albuminous liquid, from which, by hardening, various probably albuminoid 

 substances have been precipitated. This, therefore, strongly resembles that in the previously described stage. As for the 

 vessels, the serial sections show that the large vascular stems lie imbedded more or less deeply in mucous membrane, while 

 the capillaries lie close to the subepithelial stratum of connective tissue, and often just below the epithelium, so that only 

 the simple epithelium and the vascular wall itself separates the fætal blood from the uterine wall. In those places where the 

 surface of the chorion forms large or small swellings, a very rich capillary net-work may often be observed. 



The allantois forms a large, wide bag, which, in the well-preserved portions of the membrane, lies close to the 

 chorion, from which it is separated by a layer of embryonic tissue, as before mentioned. Its relation to the amnion is 

 peculiar. At the place whence the vessels of the umbilical cord begin to spread themselves out into the subchorionic 

 tissue between the chorion and the allantoic bag, the allantois lies close to the amnion, and adheres firmly to it, its thin 

 wall seeming almost to insert itself on the amnion over an extent corresponding to the ventral surface of the embryo (cf. 

 PI. YI. fig. 7, dotted line). — The allantoic bag itself, as a thin membrane, can easily be separated from the chorion, and 

 from the subchorionic connective tissue. 



Microscopically it proves to consist of a simple, very flat, stratified epithelium on the side turned towards the 

 allantoic cavity (entoderm epithelium), and outside this, of connective tissue with scattered, spindle-shaped and star-like cells, 

 with smaller vessels imbedded (visceral mesoblast). 



It also appears in this specimen, that the allantois is of larger dimensions than the amnion. The histological 

 conditions agree with those previously described. 



