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VIII. — Description of a Reconstruction Model of a Horse Embryo Twenty-One 

 Days Old. By Professors Arthur Robinson, M.D., and A. Gibson, M.B., F.R.C.S. 



(MS. received June 7, 1915. Eead June 7, 1915. Issued separately January 17, 1916.) 



[Plate XIX, figs. 54-63. Text-figs. 22-26.] 



[Note. — The following account of a horse embryo twenty-one days old is based upon 

 a reconstruction and upon notes made by Mr A. Gibson, M.B., F.R.C.S., etc., Professor 

 of Anatomy in the Medical College, Winnipeg, when he was working in the 

 Anatomical Department of the University of Edinburgh, and he, therefore, is 

 responsible for everything except the mere writing. 



When the reconstruction and the notes were completed, Professor Cossar Ewart, 

 to whom the embryo belonged, was too fully occupied to proceed with his description 

 of the embryo, consequently Mr Gibson's notes were put aside for the time. Now, 

 when circumstances permit Professor Ewart to proceed with his part of the work, the 

 services of Professor Gibson are not available ; therefore, with the aid of the recon- 

 struction and Professor Gibson's original notes, I have put together the following 

 account of the embryo ; but, as Professor Gibson has not had the opportunity to 

 revise his notes, I must be held entirely responsible for any errors of description 



or interpretation which may be found in the account. 



Arthur Robinson. 



Feb. 1915.] 



The embryo, as represented by the model, is bent upon itself, at the level of the 

 cranial border of its umbilical orifice — that is, at the caudal end of the pericardial 

 region ; and, as a result of the bending, the general outline of the embryo, when 

 viewed from the side, is hook-shaped. 



The dorsal and longer limb of the hook is formed by the more caudal portion of 

 the embryo — that is, the part which would lie caudal to the cranial margin of the 

 umbilicus if the embryo were unbent. 



The shorter limb of the hook is formed by the pericardial and cranial regions 

 of the embryo with the included foregut and the stomatodseum. This part of the 

 embryo is of course reversed so that its true 'dorsal surface is directed ventrally, and 

 its ventral surface is turned dorsally and lies in relation with the ventral surface of the 

 more caudal portion of the embryo (figs. 54, 55, 56, PI. XIX # ; and 8 and 11, PI. IX). 



When Professor Ewart measured the embryo in the fresh condition, it was found 

 to be 11 mm. total length, measured round the bend, whilst the distance from the 

 bend to the most anterior part of the head was 3 '25 mm. and the distance from the 

 bend to the caudal extremity was 6 '25 mm. 



* Figs, referred to, other than those in Plate XIX, are in the Plates of Professor Cossar Ewakt's paper. 

 TRANS. ROY. SOC. EDIN., VOL. LI, PART II (NO. 8). 48 



