STUDIES ON THE DEVELOPMENT OF THE HORSE. 319 



begun at the end of the twelfth or the beginning of the thirteenth day is completed 

 at or about the end of the sixteenth day. When the yolk-sac endoderm is completely 

 invested by splanchnic mesoderm a typical yolk-sac (i.e. a free vesicle separated by 

 a distinct space from the chorion) is the result (text-fig. 13). As the blastocyst 

 assumes a tubular form the yolk-sac elongates : by the twenty-first day it may reach 

 a length of over 350 mm. (14 inches) (text-fig. 16). As the mesoderm splits the 

 splanchnic layer undergoes vascularisation, without, hoivever, a sinus terminalis being 

 established. 



In the horse the splitting of the mesoderm begins on the seventeenth or eighteenth 

 day, but proceeds so slowly that it has only extended a short distance beyond the 

 embryo on the twenty-first day (text-fig. 12). As the unsplit mesoderm extends 

 between the trophoblast and endoderm it undergoes vascularisation, and the left 

 vitelline artery bifurcates at an early stage to form a sinus terminalis (fig. 34). # As 

 the splitting of the mesoderm is never complete in the horse, the most distal part of 

 the yolk-sac endoderm retains throughout foetal life a slight connection with the 

 trophoblast. This implies that the yolk-sac in the horse is never, as in the sheep, a 

 free vesicle. 



In the horse at the end of the third week tubercles project into the cavity of the 

 yolk-sac (fig. 32), each endodermic tubercle being related to a trophoblastic disc. 

 There are apparently neither trophoblastic discs nor endodermic tubercles in the sheep. 



Allantois. — In the sheep there is a rudiment of the allantois at the end of the 

 second week (text-fig. 14) ; a like stage is not reached in the horse until the end of 

 the third week (fig. 33). On the sixteenth day the allantois in the sheep is a two- 

 horned sac 27 mm. in width; on the seventeenth day it measures 16 mm. (text- 

 fig. 15) ; the following day the width is nearly double ; and on the twenty-first day, 

 as text-fig. 16 shows, it has a length of nearly 300 mm. (nearly 12 inches). 



In the horse at the end of the third week the allantois is still a minute diverti- 

 culum 235/a in width ; in the sheep at a corresponding stage of development the width 

 between the horns is over 30 mm. 



The Embryo. — The youngest sheep embryo figured by Bonnet was 3 mm. in 

 length (text-fig. 14). It represents the phase reached 14 days and 22 hours after 

 service, and is characterised by two pairs of mesodermic somites. In Bonnet's figure 

 (text-fig. 14) the mesodermic somites, primitive streak, neural groove, allantoic rudi- 

 ment, and parts of the amnion are represented. 



The horse embryo figured by Martin (fig. 7) differs but little from Bonnet's 

 14-days 22-hours sheep embryo (text-fig. 14). Martin's embryo, which represents the 

 stage reached at the middle of the third week, measured 3'25 mm. in length and had 

 four mesodermic somites. Though in this embryo the primitive and neural grooves 



* Id the pig, in which the yolk-sac is a free vesicle as in the sheep, there is a temporary sinus terminalis followed 

 by general vascularisation ; hence in a sense the arrangement of the yolk-sac vessels in the pig is intermediate between 

 that of the sheep and that of the horse. 



TRANS. ROY. SOC. EDIN., VOL. LI, PART II (NO. 7). 46 



