STUDIES ON THE DEVELOPMENT OF THE HORSE. 317 



forming the larger trophoblastic discs (fig. 29) and cells with sac-like processes 

 (fig. 30), some of which surround the discs, while others line the shallow areas 

 scattered over the distal end of the blastocyst. Up to the end of the third week the 

 trophoblast in the horse consists of a single layer of cells, which are either quite free 

 or loosely connected with the uterine epithelium. 



In the sheep the trophoblast at the outset consists of a single layer of simple 

 polygonal cells ; by the tenth day the cells are wedge-shaped, and by the fourteenth 

 cubical. By the eighteenth day the cells which roughly correspond to the coty- 

 ledonary burrs projecting from the lining of the uterus, have assumed a columnar 

 form.* These columnar cells develop minute ridges which by fitting into slight 

 grooves in the cells of the burrs may assist in fixing the blastocyst. 



The Uterus. — In the sheep, when the ovum reaches the uterus " the mucous 

 membrane is that typical of an anoestrous period " ; but as development proceeds there 

 is a general increase in the complexity of the glands, with increased secretion, an 

 invasion of the epithelium by leucocytes, and towards the close of the third week a 

 thickening of the stroma. These changes are accompanied by a destruction of the 

 uterine epithelium over the cotyledonary areas by means of binucleate trophoblastic 

 cells insinuating themselves between the cells of the uterine epithelium to form a 

 layer between the epithelium and the sublying stroma. The nourishment for the 

 embryo, which mainly consists of " uterine milk," is taken up by the cells of the 

 trophoblast lying between the cotyledonary burrs. 



In the mare, even at the end of the third week of gestation the mucous membrane 

 is still that typical of the anoestrous period. The glands at the end of the third 

 week may be more active, and leucocytes may be more numerous in the uterine 

 cavity, but of this there is no evidence, neither is there any indication of erosion of 

 the uterine epithelium or of changes in the sublying stroma. f 



Martin states that the mucous membrane of the uterus from which his embryo 

 was obtained was congested and spotted. There was neither congestion nor spotting 

 of the lining of the uterus from which I removed my 21 -days embryo. In all pro- 

 bability, in the Equidse at the end of the third week the whole of the trophoblast 

 takes part in procuring nourishment for the embryo. 



Amnion. — In the sheep the lateral folds are conspicuous on the fourteenth day, 

 and extend well over the embryo on the following day. By the end of the sixteenth 

 day the amnion is complete and connected by a cord to the inner surface of the 

 chorion (text-figs. 13 and 14). 



Though in the sheep the amnion is well advanced on the fifteenth day, at the 

 stage characterised by two pairs of mesodermic somites (text-fig. 14), there is no indica- 



* Opposite the cotyledonary burrs Assheton says the trophoblast perhaps consists of two layers. 



f The pig closely agrees with sheep during the earlier weeks of gestation. The blastocyst begins to elongate on 

 the eleventh day. As it increases in length it is thrown into transverse folds, with the result that, though apparently 

 of no great length, it may measure when extended over 1000 mm. at the middle of the third week. The uterine 

 epithelium begins to degenerate on the fourteenth day, and is reduced to a thin layer by the eighteenth day. 



