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VII. — Studies on the Development of the Horse. I. The Development during 

 the Third Week. By J. Cossar Ewart, F.R.S., Regius Professor of Natural 

 History, University of Edinburgh. 



(MS. received June 7, 1915. Read June 7, 1915. Issued separately December 14, 1915.) 



[Plates IX-XVIII and Twenty-one Text-figures.*] 



CONTENTS. 



A. Introduction and Review of Previous Work on 



Young Horse Embryos — 



1. Bonnet's so-called 21-days embryo 



2. Martin's so-called 21-days embryo 



3. Hausmann's 19-days embryos 



B. Reproductive Organs and Fcetal Membranes 



at end of Third Week— 



1. Ovaries and Corpus luteum, Oviduct and 



Uterus ...... 



2. Blastocyst 297 



3. Trophoblast 297 



4. Yolk-sac 301 



5. Mesoderm 303 



292 

 292 

 294 



294 



6. Amnion 



7. Allantois 



l'AOK 



305 

 306 



C. The Embryo at end of Third Week— 



1. Size and External Characters . . 307 



2. Nervous System and Sense Organs . 308 



3. Alimentary Canal .... 309 



4. Notochord 311 



5. Heart and Blood-vessels . . .311 



D. Early Developmental Stages in the Horse and 



Sheep 314 



E. Summary 322 



F. Explanation of Plates 326 



A. Introduction and Review of Previous Work on Young Horse Embryos. 



Soon after the publication of The Origin of Species it was realised by Huxley 

 and others that convincing evidence of the fact of evolution might be obtained by 

 a systematic investigation of the ancestral history and development of the Equidse. 

 From studying material in the British and other Museums Huxley announced at 

 the end of the 'sixties that he believed " the Anchitherium, the Hipparion and the 

 modern horses constitute a series in which the modifications of structure coincide 

 with the order of chronological recurrence in the manner in which they must 

 coincide if the modern horses really are the result of the gradual metamorphosis in 

 the course of the Tertiary epoch of a less specialised ancestral form." t But this 

 conclusion was soon profoundly modified. When in 1876 Huxley had the opportunity 

 of examining the Yale and other collections of the fossil horses of America, he was 

 satisfied that "we must look to America rather than to Europe for the original seat 

 of the Equine series," \ and " that the European Hipparion is rather a member of a 

 collateral branch than a form in the direct line of succession." § 



Huxley directed his attention to the development as well as to the ancestral 

 history. Convinced that modern horses passed through a three-toed stage, he 



* The cost of reproduction of the plates and of certain of the text-figures has been defrayed by a grant from 

 the Carnegie Trust f»r the Universities of Scotland. 



t American Addresses, p. 83, 1877. J Loc. cit., p. 86. § Loc. cit., p. 87. 



TRANS. ROY. SOC. EDIN., VOL. LI, PART II (NO. 7). 42 



