322 PROFESSOR J. COSSAR EWART. 



that a 21-days blastocyst 50 mm. in length has been described, presumably Bonnet 

 would state that in the horse the blastocyst at the end of the third week varies from 

 13 mm. to 50 mm. 



By studying the oestrous period in the mare and the condition of the ovaries 

 before and after service, I arrived at the conclusion that the difference between my 

 50-mm. blastocyst and Bonnet's 13-mm. blastocyst was not due to arrested develop- 

 ment but to a difference in age — a conclusion supported by drawings of horse 

 embryos (text-figs. 19-21) published in 1840 by Hausmann. 



Further, it has been taken for granted that during the earlier weeks the horse 

 closely agrees in its development with other typical Ungulates. It is conceivable 

 that up to the end of segmentation, perhaps up to the formation of the embryonic 

 shield, there may be almost complete agreement between even-toed and odd-toed 

 Ungulates ; but by comparing young horse embryos with sheep embryos at a corre- 

 sponding stage, evidence has been obtained in support of the view that from the 

 time the mesodermic somites make their appearance the two main groups of the 

 modern hoofed mammals follow different routes — the sheep, pig, and other even-toed 

 Ungulates especially differing at the end of the third week from the horse in the form 

 of the blastocyst, yolk-sac, and allantois, and in the structure and relations of the 

 trophoblast. 



E. Summary. 



During the last fifty years so much progress has been made in clearing up the 

 pedigree of the horse that we are now familiar with every chapter in the ancestral 

 history between the 14-inch four-toed Hyracotherium of the London Clay and the 

 60-inch Equus sivalensis of the Indian Pliocene. 



But while our knowledge of the remote ancestors of the Equidse is well-nigh com- 

 plete, we still know surprisingly little about the more interesting chapters of the life- 

 history of the horses now living under domestication. We are acquainted with the 

 ancestral history because Cope, March, Osborn, Scott, and other American 

 palaeontologists made a systematic and exhaustive study of " fossil " horses ; we know 

 little about the more interesting phases of the life-history of the living Equidee 

 because embryologists have not yet succeeded in working out the development of 

 either horses, asses, or zebras. 



In America several expeditions have been sent out in search of material for 

 the study of extinct horses, but until recently only one serious attempt seems to 

 have been made to obtain material for a systematic study of the development of 

 recent horses. 



Unfortunately, this attempt (made in Hanover by Hausmann some eighty years 

 ago) yielded meagre results. Professor Bonnet of Giessen, famous for his memoirs 

 on the development of domestic animals, when referring in 1889 to previous work 

 on the fastal membranes of the horse, pointed out that about the first stages of 



