STUDIES ON THE DEVELOPMENT OF THE HORSE. 805 



6. The Amnion. 



In Martin's embryo the amnion was only represented by indistinct lateral ridges 

 (text-fig. 1, am.) ; in my 21-days embryo the amnion was complete (figs. 33, 34, and 

 text-fig. 12). In the sheep and pig the amnion, well advanced on the fifteenth day, 

 may be complete on the sixteenth day, i.e. at the stage characterised by from 8 to 

 10 mesodermic somites. As there are neither head nor tail folds in the horse on the 

 seventeenth day, the amnion is probably only completed on the twenty-first day, i.e. 

 at the stage characterised by about twenty somites. It may hence be assumed that 

 the amnion appears relatively later in the horse than in the sheep and pig — the early 

 development of the amnion in even-toed Ungulates is doubtless correlated with the 

 early disappearance of the zona pellucida. Whether in the Equidee the amnion is 

 mainly derived from a head fold, as in the chick, or from a tail fold, as in the rabbit, 

 will doubtless be ascertained when a more complete series of embryos is available. 

 But seeing that the head is sharply bent backwards (fig. 33) at the end of the third 

 week, the probability is that in the horse, as in the pig, the head fold contributes 

 most. In a longitudinal section of the 21-days embryo the amnion (in contact at 

 its origin with the allantoic diverticulum) is seen to arch upwards and forwards over 

 the external opening of the spinal cord (fig. 33). In a transverse section on a level 

 with the minute diverticulum (fig. 33, cl.) which seems to represent the cloacal chamber, 

 the crescent-shaped amniotic cavity is seen to lie immediately above the lateral 

 extensions of the allantoic diverticulum (fig. 50, al.d.). In the semi-diagrammatic 

 drawing of the blastocyst (fig. 33) the amnion is represented as having a large cavity. 

 As a matter of fact, there is very little amniotic fluid at the end of the third week, 

 with the result that in transverse sections the amnion is seen to be only separated by 

 a narrow space from the embryo (fig. 8). 



The structure and relations of the amnion and the extent of its cavity will be 

 gathered from figs. 36 to 53. It will be observed that in the horse, as in the sheep 

 and pig, the amnion is directly continuous with the somatopleure, and that in some 

 of the sections the mesodermic layer of the amnion is thrown into more or less 

 distinct longitudinal ridges (fig. 45). It will be further observed that owing to the 

 head projecting into the amnion as into a cowl or sac, the facial portion of the embryo 

 is completely enveloped by the amnion (figs. 36 and 37). 



In front of the vitelline veins (figs. 39 and 40) the amnion again assumes the form 

 of a sac or hood, with the result that in the most anterior part of the flexed embryo 

 the dorsal portion of the amniotic cavity is no longer separated from the ventral 

 portion. It will be noticed that many of the sections figured suggest that the one 

 and only object of the somatopleure is to provide a water-jacket over the back of 

 the embryo.* 



* With a water-jacket (the amnion) above and a water-bed (the yolk-sac) underneath, the embryo horse is as 

 well protected from jars and pressure as a chick all but completely surrounded by amniotic fluid. 



