STUDIES ON THE DEVELOPMENT OF THE HORSE. 323 



development we have nothing to depend on except the statements and drawings of 

 Hausmann, who had a stud including fifty-two mares at his disposal for his researches. 

 These statements, which relate to the development at the end of the third week, 

 owing to the mistaken views of their author about the ovum and its membranes, 

 Bonnet points out, have caused more confusion than enlightenment, while the 

 drawings which illustrate Hausmann's contributions are almost incomprehensible 

 and worthless. 



In the home of the thoroughbred racehorse no systematic attempt has been 

 made by embryologists to work out the life-history of the Equidse, and sporadic 

 attempts to elucidate special points in the early history have hitherto been well- 

 nigh as fruitless in England as the ambitious attempt made in the 'thirties of last 

 century in Hanover. 



From first to last the difficulty in England has been lack of material. This was 

 especially true of an attempt made by Huxley to find rudiments of the second and 

 fourth digits. When preparing the lecture on fossil horses delivered in 1876 in 

 New York, it occurred to Huxley that strong evidence in support of the fact of 

 evolution would be forthcoming if it were proved that the modern horse passes 

 through a Hipparion or 3-toed stage during development. 



Armed with all the available material Sir William Flower could place at his 

 disposal, Huxley proceeded to search for rudiments of the phalanges of the second 

 and fourth digits. To Huxley's great disappointment the search was in vain. 

 Huxley failed, not because- the 3-toed stage is absent in the recent horse, but because 

 it appears, comes and goes, much earlier than he expected. Up to the end of the 

 Miocene period all horses had throughout life three complete toes both in front and 

 behind. For this reason it seems to have been assumed that the 3-toed phase, if 

 it actually occurred in the modern horse, would persist for a considerable time and 

 be preceded by a 5 -toed phase. As a matter of fact, as I shall show in a subsequent 

 paper, the abbreviation in the development in the Equidae is so marked that the 

 3-toed stage is reached during the fifth week of gestation and practically comes to 

 an end during the seventh week. 



It seems to have been taken for granted that during the earlier weeks horse 

 embryos develop along the same lines as other Ungulates. I find, however, that from 

 the second week onwards the horse follows a route different from that of all the other 

 mammals hitherto studied. 



Since 1891 it has been assumed that during the earlier weeks the development of 

 the horse is retarded in some cases, accelerated in others. In the roe deer the 

 development is arrested as soon as the cleavage stage is reached, with the result that 

 little or no progress is made for several months. There is, however, no evidence 

 that at any stage the development is arrested in the horse. The belief that in the 

 horse progress is sometimes retarded is based on a statement by Bonnet, who in 

 1891 asserted without any reservation that the blastocyst at the end of the third 



