18 A CRITICAL PERIOD IN THE 



efficient than the primitive apparatus in operation during the 

 earlier weeks. 



The Eight-Weeks Horse Embryo. — This embryo is twice the 

 length and four times the weight of the seven-weeks' embryo. 

 As fig. 7 shows, at eight weeks we have a miniature horse. The 

 mouth is now closed, the eyes are provided with eyelids, the ear 

 projects some distance from the head, and the tail, compared with 

 the legs, is relatively short. The amnion (am.) still forms a com- 

 plete robe or mantle, and, by means of the fluid it contains, to the 

 last is of the utmost use both for the protection of the foetus and 

 the dam. The two stalks — yolk and allantois — have already 

 blended for some distance (sta.), but the yolk sac (y.s.), though 

 now of little or no use, still persists. 1 The absorbing area (a-c) 

 is very small, but still surrounded by a prominent ring (an.), 

 external to which the girdle (t.g.) may still be seen. The 

 allantoic sac is considerably larger than at seven weeks, and there 

 projects from it a countless number of villi. Some of these villi 

 are represented, but on too large a scale, in fig. 7 (all. villi). 

 The majority of them are still simple processes, but in a few 

 branching has already set in. As they increase in size they form 

 miniature vascular trees (7 b , 7 C )> which occupy equally complex 

 pits in the lining membrane of the uterus. 2 That the new mode 

 of catering for the wants of the embryo is an efficient one will 

 be still more evident when I mention that a four-months' em- 

 bryo, i.e., an embryo twice the age of the one represented in fig- 

 7, may be over ten inches in length. 



Having shortly described the foetal appendages of four horse 

 embryos, I trust those interested in the development of the horse 

 will now be better able to realise the conditions under which 

 the embryo lives during the earlier weeks, the provisional and 

 more permanent arrangements for its nourishment, and especially 

 that a critical stage is reached as the yolk sac ceases to convey 

 nutriment, and the allantoic villi come into action. As I have 

 found breeders and others practically interested in our domestic 

 animals only too glad to learn all that science has to say on 



1 At six months the yolk sac has about the same capacity as at six weeks, but 

 it is folded longitudinally, and completely concealed in the umbilical cord. 



2 The villi in the mare are never more than an eighth of an inch in length, 

 and at birth they are simply withdrawn from their pits. 



