STUDIES ON THE DEVELOPMENT OF THE HORSE. 289 



published an account of a horse embryo, believed, like Bonnet's, to be 21 days old.* 

 This embryo was obtained from an eight-year-old mare, served on the 2nd of April, 

 and killed on the 23rd of the same month. 



Martin's blastocyst, instead of being, like Bonnet's, globular and 12 to 13 mm. in 

 diameter, was egg-shaped (fig. 4) and measured 25 mm. by 35 mm. This blastocyst 

 was very much smaller than a 2 8 -days blastocyst figured by Bonnet. Hence Martin 

 came to the conclusion that the embryo from the eight-year-old mare had been 

 arrested in its development. On the other hand, Bonnet, on realising in 1891 that 

 the blastocyst of his damaged embryo was very much smaller than Martin's blasto- 

 cyst, came to the conclusion that the rate of development in the Equidse varies con- 

 siderably during the earlier weeks, and unreservedly stated that in the horse the 

 blastocyst at 21 days varies from 13 mm. to 35 mm.t 



From the observations made during the last fifteen years, I have arrived at the 

 conclusion that Bonnet's 13-mm. blastocyst (fig. 3) represents the stage reached at 

 the end of the second week of gestation ; that Martin'^ 35-mm. blastocyst (fig. 4) 

 represents the stage reached at the middle of the third week ; and that at the end of 

 the third week the blastocyst, in a breed about the size of the wild horse of Mongolia, 

 measures at least 50 mm. (fig. 5). If I succeed in giving good reasons for these 

 conclusions, in showing that Bonnet's so-called 21-days embryo is 14 or 15 days old, 

 and that Martin's so-called 21-days embryo is 17 or 18 days old, and that at the end 

 of the third week a horse embryo is nearly as well developed as a 2-days chick and 

 an 18 -days sheep embryo, the more important facts about the development of the 

 Equidse during the first three weeks of gestation will have been established. 



It is obvious that a difference between blastocysts and their contents may be due 

 to several causes : e.g. to (1) arrested development, or (2) arrested growth, or (3) a 

 difference in age. When in one of the two embryos derived from the same fertil- 

 . ised ovum progress is retarded, we have an example of arrested development ; % when, 

 on the other hand, the embryos in one uterine horn, though as well developed, are 

 smaller than those in the other horn, we have an example of arrested growth ; § 

 but when two embryos from, say, two different mares, belonging it may be to different 

 breeds, differ in size, before assuming that the difference is due to arrested develop- 

 ment or arrested growth, one must make sure that it is not due to a difference in 

 age, to the large embryo being some hours, or it may be several days, older than 

 the small one. 



To be in a position to obtain horse embryos of a definite age during the earlier 



* "Ein Pferdeei vom 21 Tage," Schweizer Archiv fur Thierheilkunde, Band xxxiii, 1890. 



f Bonnet, Grundriss der Entivickelungsgeschichte der Haussaugethiere, Berlin, 1891. 



+ 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. 



§ 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 foetuses 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. 



