^{page 287 note *} The cost of reproduction of the plates and of certain of the text-figures has been defrayed by a grant from the Carnegie Trust for the Universities of Scotland.

^{page 287 note †} American Addresses, p. 83, 1877.

^{page 287 note ‡} Loc. cit., p. 86.

^{page 287 note §} Loc. cit., p. 87.

^{page 288 note *} Ewart, “The Second and Fourth Digits in the Horse,” Proc. Roy. Soc. Edinburgh, 1894; “The Limbs of the Horse,” Journ. Anat. and Physiol., Jan. and Feb. 1894.

^{page 288 note †} “The Morphology of the Ungulate Placenta,” Phil. Trans. Roy. Soc., vol. c, Ser. B, 1906.

^{page 288 note ‡} Hausmann, , Über Zeugung und Enstehung des wahren weiblichen Eier bei den Süugetieren, Hanover, 1840.Google Scholar

^{page 288 note §} Martin, Paul, “Ein Pferdeei vom 21 Tage,” Schweizer Archiv für Thierheilkunde, Zurich, 1890.Google Scholar

^{page 288 note ║} Bonnet, , “Die Erhaute des Pferdes,” Verhandlungen der anat. Gesellschaft, Jena, 1889.Google Scholar

^{page 289 note *} “Ein Pferdeei vom 21 Tage,” Schweizer Archiv für Thierheilkunde, Band XXXIII, 1890.

^{page 289 note †} Bonnet, , Grundriss der Entwickelungsgeschichte der Haussäugethiere, Berlin, 1891.CrossRefGoogle Scholar

^{page 289 note ‡} Assheton found a difference in the size and in the state of development in twin germinal areas of a sheep. Journ. Anat. and Physiol., April 1898.

^{page 289 note §} I once found in a rabbit doe eight young (alike in size) in the right uterus, and four young (also of uniform size) in the left uterus; but when the eight were placed in one scale of a balance and the four in the other, the four weighed a few more grains than the eight; nevertheless, the eight small fœtuses were as well developed as the four large ones. EWART, 27th Report of the Bureau of Animal Industry, Dept. of Agriculture, U.S.A., 1911.

^{page 290 note *} Mr C. M. Douglas of Auchlochan informs me that Shetland pony mares sometimes take the horse regularly all through the period of gestation and yet produce a normal foal to the first service. Further, I am informed that in both Shetland and Clydesdale fillies œstrus may occur once and again during the earlier months of pregnancy without interfering with the normal development of the foal; and I have heard of a Clydesdale mare that came in season and was served three weeks before giving birth to a fully developed but dead foal.

^{page 290 note †} This mare belonged to a herd in the possession of the late Lord Arthur Cecil, a very competent and trust-worthy observer.

^{page 291 note *} In all probability service sometimes induces ovulation; but, as a rule, no matter how often the mare is served, the follicle remains intact until the seventh or eighth day of œstrus.

^{page 292 note *} A 19-days horse embryo (text-fig. 19) in Hausmann'S collection had ten mesodermic somites. Seeing that Martin's so-called 21-days embryo had only four somites, it was probably under rather than over 18 days.

^{page 292 note †} It is on record that the average gestation period for thirty-three thoroughbred mares of the Middle Park stud, Eltham, was 335·5 days; but in Shires and Clydesdales the gestation period seems to approach that of the wild horse of Mongolia (357 days), while in the ass it may run to 385 days.

^{page 292 note ‡} Assheton has pointed out that in the sheep, goat, and pig “there is a close parallelism in time with reference to the development of the embryo.” Guy's Hospital Reports, vol. lxii.

^{page 292 note §} That the zona pellucida of a 13-mm. equine blastocyst has a thickness of 4μ wants confirmation.

^{page 292 note ║} Assheton, , “Segmentation of the Ovum of the Sheep,” Quart. Journ. Micro. Sci., vol. xli, 1898.Google Scholar

^{page 293 note *} Heape, , “The Development of the Mole,” Quart. Journ. Micro. Sci., vol. xxiii, 1888.Google Scholar

^{page 293 note †} In Marsupials, as Professor Hill states, the ovum during its passage down the oviduct “becomes surrounded by a transparent layer of albumen ·015 to ·022 mm. in thickness, composed of very delicate concentric lamellæ, and having normally numbers of sperms embedded in it,” and that this albumen layer is invested by a double-contoured membrane comparable to the shell membrane of the Monotreme egg. Hill, J. P., “The Early Development of the Marsupialia,” Quart. Journ. Micro. Sci., vol. lvi, December 1910.Google Scholar

^{page 294 note *} Under normal conditions fillies reach maturity—begin to discharge ripe ova—about the end of the second or beginning of the third year, but under unfavourable conditions ovulation may only begin at the end of the third year. On the other hand, when fillies are well fed during their first winter, maturity may be reached at the end of the first or the beginning of the second year. Evidence of early maturity we have in a member of the Auchlochan herd of Shetland ponies. This pony, born on May 7, 1907, had a well-developed vigorous foal on May 23, 1909: assuming the gestation period was 336 days, this filly became pregnant at the age of 1 year and 45 days. Seeing that a heifer may become pregnant when only 5 months old, it is not surprising that a filly sometimes reaches maturity before she is a year old.

^{page 296 note *} Experiments by Marshall and Jolly seem to show that the corpus luteum provides a secretion essential for the attachment of the embryo and for its nourishment during the first stages of pregnancy. “Contributions to the Physiology of the Mammalian Reproduction,” Phil. Trans., Ser. B, vol. cxcviii, 1905.

^{page 296 note †} Further inquiries may show that in the case of mares that come in use during the period of gestation ovulation may occasionally take place.

^{page 296 note ‡} The function of the corpus luteum in the mare is dealt with in The Physiology of Reproduction, by F. H. A. Marshall, Longmans, 1910.

^{page 297 note *} Marshall, , “The Œstrus Cycle in the Sheep,” Phil. Trans., Ser. B, vol. cxcvi, 1903.Google Scholar

^{page 298 note *} Figures of the blastocyst at the end of the fourth, fifth, sixth, and seventh weeks are given in the writer's pamphlet, A Critical Period in the Development of the Hores, A. & C. Black, 1897.

^{page 298 note *} Weber, Max, “Beiträge zur Anatomie und Entwiekelung der Genus Manis,” Zoologische Ergebnisse einer Reise in Niederländisch Ost-Indien, Leiden, 1892.Google Scholar

From the appearance of some of the round bodies seen in the disc represented in fig. 29 it is extremely probable that had the 21-days blastocyst been fixed with osmic acid, fatty globules like those found by Jenkinson in the sheep would have been met with in the cells forming the trophoblastic discs. Jenkinson, , “Notes on the Histology and Physiology of the Placenta in Ungulata,” Proc. Zool. Soc., 1906.Google Scholar

^{page 302 note *} In the sheep and pig the mesoderm is soon completely split into somatic and splanchnic layers. The result of this splitting is the formation of a free yolk-sac vesicle (text-fig. 13). The yolk-sac is nearly, but never quite, a free vesicle in the horse.

^{page 305 note *} With a water-jacket (the amnion) above and a water-bed (the yolk-sac) underneath, the embryo horse is as well protected from jars and pressure as a chick all but completely surrounded by amniotic fluid.

^{page 306 note *} Mr Gibson, before leaving Edinburgh to occupy the Chair of Anatomy in the University of Winnipeg, was good enough to place at my disposal notes and drawings of the model he made under the supervision of Professor Robinson.

^{page 306 note †} “Description of a Reconstruction Model of a Horse Embryo Twenty-One Days Old,” Trans. Roy. Soc. Edin. vol. li, by Arthur Robinson, M.D., Professor of Anatomy, University of Edinburgh.

^{page 308 note *} Owing to the slight obliquity of the section, only the external opening of one of the vesicles is seen in the figure.

^{page 311 note *} The notochordal canal, unlike the neural canal, is closed, does not open into the amniotic cavity.

^{page 311 note †} Professor Robinson in his description of the model recognises (1) a sinus venosus, (2) a sinu-atrial canal, (3) a ventricle, (4) an atrio-ventricular canal, (5) a bulbus cordis, and (6) a truncus aorticus.

^{page 313 note *} In only having two pairs of aortic arches the 21-days horse agrees with a human embryo of about 15 days figured by HIS. In a figure by HIS of a human embryo of about 3 weeks all five aortic arches are present. If the age of these human embryos is approximately accurate, the aortic arches appear later in the horse than in man.

^{page 314 note *} For information about the development of the sheep I am mainly indebted to papers by Assheton and Bonnet, more especially to “The Morphology of the Ungulate Placenta,” Assheton, Phil. Trans. Roy. Soc., 1906; “The Segmentation of the Ovum of the Sheep,” Assheton, Quart. Jour. Med. Sci., 1898; “Beiträge zur Embryologie der Wiederkäuer,” R. Bonnet, Archiv f. Anat. u. Physiol., Anat. Abth., 1889.

^{page 317 note *} Opposite the cotyledonary burrs Assheton says the trophoblast perhaps consists of two layers.

^{page 317 note †} 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.

^{page 319 note *} In the pig, in which the yolk-sac is a free vesicle as in the sheep, there is a temporary sinus terminalis followed by general vascularisation; hence in a sense the arrangement of the yolk-sac vessels in the pig is intermediate between that of the sheep and that of the horse.

^{page 321 note *} Hausmann's 19-days horse embryo lends strong support to the view that Martin's so-called 21-days embryo represents the stage reached at the middle of the third week.

^{page 321 note †} “Eier vom 21-Tage schwanken zwischen 1·3 cm.−3·5 cm. Länge,” Bonnet, , Grundriss des Entwickelungsgeschichte, 1891, p. 240.Google Scholar

^{page 329 note *} The figures in Plates XV–XVIII are all magnified 75 diameters.