STUDIES ON THE DEVELOPMENT OF THE HORSE. 301 



cells of the uterine mucosa. While the simple columnar cells of the trophoblast are 

 probably mainly concerned with taking in nutritive fluids, the cells with sac-like 

 processes seem to be truly phagocytic. In one of the cells in fig. 30 minute bodies 

 are seen in the sac-like projection. 



It is probable that, aftes the albumen layer (text-fig. ll) disappears, phagocytic 

 cells are comparatively common, but that, as the sinus terminalis migrates towards 

 the distal pole, their number is gradually reduced. Seeing that a membrane-like 

 coagulum is not present within the layer of simple columnar cells forming the 

 trophoblast up to the sinus terminalis, it may be presumed that the coagulum under 

 the trophoblast beyond the sinus is mainly formed from material taken in by the 

 epiblastic cells with sac-like processes. 



Though the trophoblastic discs and the internal hydrostatic pressure may together 

 be sufficient at the end of the third week to retain the blastocyst in the same position 

 in the uterine horn, they are evidently not able to prevent it escaping from the 

 horn when by accident or otherwise the passage leading to the uterus becomes 

 dilated. At the end of the third, as at the end of the sixth week, the moment the 

 blastocyst is exposed it seems to detach itself from the lining of the uterus ; to 

 what extent this result is due to uterine contractions induced by incisions made to 

 expose the embryo it is difficult to say, but the fact remains that until the allantoic 

 villi begin to appear at the end of the seventh week such adhesions as exist between 

 the blastocyst and the uterus are easily broken down. Structures at all comparable 

 to the trophoblastic discs of the 21 -days horse blastocyst have not, as far as I am 

 aware, been met with either in other Eutheria or in the Marsupialia. The discs 

 differ from the irregular epiblastic villi which in the rabbit during the second week 

 of gestation acquire a close attachment to the mucous membrane of the uterus and 

 thus aid in fixing the blastocyst. They differ still more from the multinuclear giant 

 cells which in some cases erode the uterine mucosa, and from the large trophoblastic 

 masses (syncytia) which, e.g. in Spermophilus, not only penetrate the epithelium but 

 extend some distance into the connective tissue of the uterus. 



4. The Yolk-sac. 



Just as a typical amnion consists of an inner layer of ectoderm and an outer 

 layer of somatic mesoderm, a typical yolk-sac consists of an inner layer of endoderm 

 and an outer layer of splanchnic mesoderm. In the horse at the middle of the third 

 week the extra-embryonic endoderm consists of polygonal epithelial cells separated 

 by many gaps of a considerable size. In the absence of mesoderm, except in the. 

 vicinity of the embryo, this imperfect layer of endoderm lies in direct contact with 

 the trophoblast. Hence at the middle of the third week the yolk-sac, except in the 

 neighbourhood of the embryo, consists only of an imperfect layer of endoderm. 



Before the end of the third week is reached the mesoderm has greatly increased, and, 

 more important still, has split within and for some distance beyond the embryo (fig. 33 



