324 PROFESSOR J. COSSAR EWART. 



week varies in length from 13 mm. to 35 mm., i.e. from half an inch to over 1'5 inches. 

 Bonnet arrived at the conclusion that a 21 -days blastocyst may vary from 13 mm. 

 to 35 mm. because he received in 1889 a 13-mm. horse blastocyst taken from a mare 

 twenty-one days after the first service, while in 1890 Paul Martin of Zurich obtained 

 a 35-mm. blastocyst also taken twenty-one days after service. I find that at the end 

 of the third week of gestation, i.e. twenty-one days after service, the blastocyst 

 measures 50 mm. ; that Bonnet's so-called 21 -days embryo probably represents the 

 stage reached at the end of the second week ; and that Martin's so-called 21 -days 

 embryo represents the stage reached at the middle of the third week of gestation. 

 The age of Bonnet's and Martin's embryos having been dealt with, attention is next 

 directed to the condition of the ovaries, oviducts, and uterus at the end of the third 

 week, and to the position of the 50-mm. (2 1-day s) blastocyst in the uterine horn 

 (fig. 14). The trophoblast at the end of the third week is unusually interesting; in 

 several essential points it differs from that of the sheep and pig and other Ungulates 

 hitherto examined. Up to the sinus terminalis it consists of typical columnar cells 

 (fig. 28) ; beyond the sinus there are (l) groups of very tall columnar cells arranged 

 to form trophoblastic discs (fig. 29) which probably help to fix the blastocyst to the 

 lining of the uterus, (2) groups of columnar cells in the act of elongating to form 

 additional discs (fig. 30), and (3) columnar cells with sac-like processes (fig. 30) — 

 these phagocytic cells occur around the margins of the discs and in the grooves 

 surrounding the discs, but especially in shallow depressions beyond the sinus 

 terminalis which probably lie opposite the openings of the uterine glands and are 

 concerned in taking up the more solid particles of the " uterine milk." 



In the region of the embryo the trophoblast by uniting with the somatic layer of 

 mesoderm forms the outer wall of the exoccelom (text-fig. 12). From the exocoelom 

 to a short distance beyond the sinus terminalis the trophoblast forms the outer wall 

 of a narrow space occupied by highly vascular but still unsplit mesoderm (fig. 28). 

 The distal portion of the trophoblast is intimately related to the yolk-sac endoderm 

 (fig. 34). Sections of the blastocyst afford no evidence that solid particles penetrate 

 the cells of that part of the trophoblast extending between the embryo and the sinus 

 terminalis, but they afford abundant evidence that granules of various sizes enter 

 both the tall columnar cells of the discs and the cells with sac-like processes which 

 form a considerable part of the trophoblast beyond the sinus terminalis (figs. 29 

 and 30). The sections further demonstrate the presence of a continuous sheet of 

 coagulum between the trophoblast and the yolk-sac endoderm (fig. 30). Between 

 the discs this coagulum (which probably consists of nutritive material derived from 

 the " uterine milk ") resembles a basement membrane, but under the discs it reaches 

 a considerable thickness and is continuous with similar coagulated material entering 

 into the formation of yolk-sac tubercles (fig. 32). 



In the vicinity of the embryo the yolk-sac consists of endoderm and a layer of 

 splanchnic mesoderm, but from the exocoelom to the sinus terminalis it is only 



