STUDIES ON THE DEVELOPMENT OF THE HORSE. 



315 



for some days sheep embryos come into intimate relation with the lining of the 

 uterus. 



In the horse the zona pellucicla, according to Bonnet, has a thickness of 4ju when 

 the blastocyst measures 13 mm. (fig. 3), i.e. at the end of the second week. Further, 

 during the first half of the third week the blastocyst acquires an albumen coat, which 

 at the middle of the third week has a thickness of 3 or 4 mm. (fig. 4, alb.). 



In the sheep as segmentation proceeds it soon becomes evident that certain 

 cells will give rise to the trophoblast (text-fig. 9, tr.), and that others will form the 

 inner cell-mass (text-fig. 9, i.c.m.) which is eventually differentiated into endoderm 

 and embryonic shield. In text-fig. 10 the embryonic shield (e.s.), trophoblast (tr.), 



a-lbn 



Text-fig. 11. — Diagram to indicate the stage reached in the horse about the middle of the third week. 

 albn., thick (3-4 mm.) albumen coat; am., amnion fold ; ecd., ectoderm ; end., endoderm ; ms., 

 mesoderm ; pr., primitive groove. Albumen layer is lost before end of third week. 



and endoderm (end.) of a deer are represented. Like differentiations and phases 

 in all probability occur in the Equidse. 



The Blastocyst. — In the horse the blastocyst, nearly spherical at the end of the 

 second week (fig. 3), is egg-shaped at the middle of the third week (fig. 4) and pear- 

 shaped at the end of the third week (fig. 5). At this stage it has a length of 

 50 mm., and occupies the lower part of one of the uterine horns (fig. 15). 



In the sheep soon after the rupture of the zona pellucida the blastocyst loses its 

 spherical outline (text-fig. 13) and soon elongates into a narrow tube which occupies 

 both uterine horns. This tubular blastocyst may have a length of 240 mm. at the 

 end of the twelfth day, and measure 400 mm. (nearly 16 inches) at the end of the 

 third week (text-fig. 16). 



The Trophoblast. — In the horse up to the middle of the third week the tropho- 

 blast, except in the vicinity of the embryo (text-fig. 11), is in contact with the 

 endoderm, but by the end of the third week, over and immediately beyond the embryo, 

 it is in contact with non-vascular somatic mesoderm (text-fig. 12). Beyond the 

 exoccelom up to the sinus terminalis the trophoblast forms the outer wall of a space 



