306 PROFESSOR J. COSSAR EWART. 



7. The Allantois. 



In the rabbit the allantois is represented on the eighth day by a mass of mesoderm 

 cells at the posterior end of the embryo. On the ninth day a diverticulum from the 

 hind-gut extends into this cell mass and soon expands to form the allantoic vesicle. 

 In the pig and sheep a similar diverticulum (which extends into a mass of mesoderm 

 on or about the fifteenth day) gives rise at a comparatively early stage to a long 

 compressed allantoic vesicle (text-fig. 15). If the allantois appeared relatively as soon 

 in the horse as in the sheep, one would expect to find allantoic mesoderm investing 

 the posterior end of Martin's embryo (fig. 7), but, as already stated, there is no 

 indication of an allantoic diverticulum at the middle of the third week. 



In the 21-days embryo the caudal end was surrounded by a large fin-like expansion 

 (fig. 11), mainly composed of mesoderm, into which extended a diverticulum from 

 the hind end of the intestine (fig. 33). From the series of transverse sections of the 

 21-days horse embryo it was impossible to make out the exact form and relations 

 of either the allantoic diverticulum, the heart, or the pharynx. These difficulties, 

 together with the difficulty of accounting for the difference between Bonnet's and 

 Martin's so-called 21-days embryos and the still greater difference between Martin's 

 embryo and my 21-days embryo, ultimately led to a deadlock. Repeated attempts 

 to obtain 14-days horse embryos failed; nevertheless, progress eventually again 

 became possible. With the help of information gained from breeding experiments 

 I was in course of time able to account for the differences between my 21-days 

 embryo and the so-called 21-days embryos of Bonnet and Martin, and at an 

 opportune moment my colleague Professor Robinson was good enough to offer to 

 have a model of my 21-days embryo constructed by the wax-plate method. 



This model was in due time completed in the Anatomical Department of the 

 University of Edinburgh by Mr A. Gibson, M.B.* 



In the description of the model of the 21-days embryo, t Professor Robinson, in the 

 section dealing with the foetal membranes, points out that " the allantoic diverticulum 

 is a narrow-necked sac (fig. 33) so flattened dorso-ventrally that its cavity is reduced 

 to the dimensions of a narrow cleft" (fig. 52). He further states (a) that the 

 diverticulum expands as it passes caudally into the somatic mesoderm beneath the 

 caudal part of the amnion fold, unti] it attains a width of 235/a ; (b) that " the endo- 

 dermal diverticulum is surrounded laterally, ventrally, and caudally with mesoderm 

 (fig. 33), but dorsally it lies in relation with the ectoderm in the region of the caudal 

 end of the amnion " ; and (c) that " the allantoic blood-vessels consist of a number of 

 dilated capillaries which form a coarse network on each side. Each lateral network 



* Mr Gibson, before leaving Edinburgh to occupy the Chair of Anatomy in the University of Winnipeg, 

 was good enough to place at my disposal notes and drawings of the model he made under the supervision of 

 Professor Robinson. 



t " Description of a Reconstruction Model of a Horse Embryo Twenty-One Days Old," Trans. Boy. Soc. Edin. 

 vol. li, by Arthur Robinson, M.D., Professor of Anatomy, University of Edinburgh. 



