56 



No vesicula umbilicalis was visible to the naked eye. There is every reason to believe that tliis formation 

 becomes atrophied very early, as previous authors have supposed. 



Foetus 3. In the 53 millim. long fætus, which is figured on PI. VI, fig. 8, lying in the amniotic bag. and with 

 a portion of the fætal membranes attached to it, we find in these last only slight changes from the stadium immediately preceding. 



The amnion forms a much wider bag. Its surface is especially remarkable for the large-meshed net-work formed 

 by numerous vessels of coarse and fine calibre, which are found on that portion of the bag answering to the embryo's ventral 

 side. The conical, tapering umbilical cord describes a curve, concave to the embryo's ventral side, and extends backwards 

 towards the tail, passing on the right side of this, in order to attach itself, and send out vessels. The cavity of the amnion 

 here extends a little way down, and forms an enlargement, its wall attaching itself to the place of insertion of the umbilical 

 cord. On a microscopical examination, it appears that the entire wall of the amnion consists of a flattened cubical epithelium, 

 with an extremely thin layer of vascular tissue outside it. 



The allantois, in this as in the preceding stage, adheres for some way to the amnion, in such a manner that 

 the starting point is where the umbilical cord begins to dilate. When the allantoic bag is opened and the amniotic bag 

 filled, the latter pushes a little invagination in the former. The thin transparent allantoic membrane lies very close to 

 the chorion, but is easily and without tearing it, loosened from it. A microscopical examination of the surface shows a 

 polygonal epithelium with an equally thick connective tissue outside. In transverse sections, the allantoic wall is seen to 

 consist of a simple stratum of epithelium resting upon a mucous tissue of sparse, partially spindle-shaped cells (PI. VII, 

 fig. 3. All). The intercellular substance of the mucous tissue takes a darker colour from carmine than the thin mesenchyme 

 lying between the chorion and the allantois. 



In the specimen examined, the greater part of the chorion belonging to the corpus uteri, and one of the horns 

 is in good condition. The surface of the chorion is exceedingly wrinkled and folded, and, on being magnified, proves to be 

 beset with low, round, wart-like excrescences, which become especially distinct on stretching the chorion out over a white 

 ground or in front of a light. These wart-like villi are of greater extent, and therefore appear denser here than in the 

 stadium previously described. The larger folds are generally longitudinal. The outermost third of the chorion's cornua 

 is free from villi, and shows fewer folds, being in many places almost smooth. The verrucose villi also appear more 

 scattered farther out over the end of the horn, and more numerous nearer in towards the part corresponding to 

 the corpus uteri. In section, the usual epithelium of the chorion (PI. VII, fig. 3, E) is observable; it is simple and cubical 

 as in the preceding stages. In the depressions between the villi, it is cylindrical. The subepithelial tissue is much more 

 rich in cells in the villi than elsewhere, while the space between the chorion and the comjjaratively adjacent allantoic bag is 

 filled with a serum-impregnated mesenchyme, with few cells (PI. VII, fig. 3, M). 



The vessels have become far more numerous in this stage, and fine capillaries lie close in under the epithelium. 



The villi are larger and have a narrower collum, with expanded peripheral Iree ends. There are numerous sul)- 

 epithelial vessels. 



The fætal iiieinl)ranes in the more advanced embryos show principally a further growth of villi. The chorion 

 in the 92 millini. long embryo had thus a mass of folds, both large and small, running in dirterent directions. The wart-like 

 villi were distinctly seen at this stage of developement as prominences from 0.5 to 1.5 millim. broad, in some places very 

 dense, in others more scattered. As previously stated in the less-developed stages, the wart-like villi decrease in number 

 towards the narrower peripheral portions of the chorijnic bag. 



Recapitulation. It must be noted as a peculiarity, that only when the ovum has attained to a considerable 

 size and the embryo's length may be measured by centimetres, does the surface of the chorion begin to assume a villous 

 character. The amnion and the allantois are at first of small dimensions, the allantois considerably exceeding the amnion in 

 length. These relations, however, soon begin to be equalized and then gradually reversed, so that the amnion is consideralily 

 longer than the allantois, the latter however, during the entire embryo life, occupying a considerable space. The further developement 

 of the chorion consists chiefly in a growth of villi with an increasing formation of capillary net-work. When the fætus has 

 attained to about 'Ao of the length of the animal at birth, the villi appear to have assumed their definite shape. In the 

 mean time, more and more villi gradually make their appearance on the cornua of the chorion, almost up to their ends. It 

 is only in an early stage that there is a distribution of villi, corresponding to the "placenta zonata". The chorion, however, 

 preserves its primitive, "diffuse placenta" type until the end of the period of gestation, so that there is no question of a 

 placenta in its true sense. 



