STUDIES ON THE DEVELOPMENT OF THE HORSE. 327 



Plate X. 



Figs. 12 and 13. The right and left ovaries of the Highland mare from which the 21-days embryo was 

 obtained. Natural size. 



Fig. 14. The uterus, oviducts, and ovaries of the same mare, half natural size, with part of the wall of 

 the body of the uterus and of the left uterine horn removed to expose the 21-days blastocyst. Note the 

 longitudinal folds of the uterine mucous membrane cut by transverse furrows into tongue-like processes, and 

 the opening leading from the right uterine horn. 



Fig. 15. The left uterine horn, blastocyst, and oviduct, and the ovary sectioned to show ovarian follicles 

 at various stages of growth. Had the blastocyst escaped about the end of the third week the large follicle 

 on the right (o.f.) would have discharged a ripe ovum on or about the day the mare was killed. Had the 

 gestation gone on to full time the size of the large follicles would have been gradually reduced. There is no 

 evidence of erosion of the lining of the uterus by trophoblastic cells, or of epiblastic processes from the 

 trophoblast projecting into the spaces between the cells of the uterine mucosa. Natural size. 



Plate XI. 



Fig. 16. Section through the ovary of a mare which died on the second day of oestrus. Note that the 

 outer wall of the large follicle (o.f.) is thinner than in the large follicle in fig. 15. Had this mare survived 

 four or five days longer this follicle would have doubtless discharged a ripe ovum. Natural size. 



Fig. 17. Section through the ovary of a mare which died three months after giving birth to a foal. 

 Note the corpus luteum (c. Im.) above the circular follicle near the middle of the ovary. Natural size. 



Fig. 18. The corpus luteum found in the left ovary of the mare from which the 21-days embryo was 

 taken. Natural size. The corpus luteum in the mare from which Martin obtained his so-called 21-days 

 embryo was about double the size of a lentil. 



Fig. 19. The "trumpet" of the oviduct and upper end of the left uterine horn represented in fig. 14- 

 Note the small opening (o.i.) leading from the horn into the oviduct, the numerous lamellee lining and 

 extending over the edge of the trumpet, and the interrupted lamellae lining the expanded part of the oviduct 

 adjacent to the trumpet. Natural size. 



Fig. 20. Transverse section through the upper end of the left uterine horn represented in fig. 14. 

 Natural size. 



Fig. 21. Part of the left uterine horn in contact with the 21-days blastocyst represented in fig. 15. 

 It shows the appearance assumed by the ridges when the horn is pinned out on a flat surface. As the horn 

 expands the ridges become less prominent and the spaces between the ridges are gradually increased. 

 Natural size. 



Fig. 22. Part of the sinus terminalis (s.t.) taking an irregular course through the unsplit mesoderm 

 of the 21-days embryo : v.a., vessels proceeding from the sinus towards embryo; b.v., vessels in mesoderm 

 beyond the sinus ; t.d., trophoblastic disc, x 28. 



Plate XII. 



Fig. 23. Section showing the highly congested condition of the uterine mucosa on the second day of 

 oestrus. x 40. 



Fig. 24. Part of the same more highly magnified, showing congested capillaries, distended glands, and 

 yellow bodies resembling haematoidin crystals. There was no evidence of the escape of blood corpuscles into 

 the cavity of the uterus or of destruction of the uterine epithelium. x 250. 



Fig. 25. Section through a ridge from the uterine horn containing the 21-days embryo. x 14. 



Fig. 26. Part of the same section more highly magnified ( x 100). At the end of the third week of 

 gestation the glands are decidedly less active and there is less congestion than is the case during oestrus. 



Fig. 27. The 21-days embryo, 15 times natural size. The head and front portion of the trunk are 

 bent backwards round the large heart, v.v., the large vitelline veins reaching the sinus venosus from the 

 yolk-sac; r.v.a., small right vitelline artery; l.v.a., large left vitelline artery; the left aorta and the 

 proximal part of the left vitelline vessel (text-fig. 8) are not represented in the drawing. 



TRANS ROY. SOC. EDIN., VOL. LI, PART II. (NO. 7). 47 



