b A CRITICAL PERIOD IN THE 



of failure, but from reports recently received it seems to be still 

 higher in certain districts in India. 1 Believing that some of 

 my mares have proved unfruitful for the time being because of 

 their frequently breaking service, and believing further that not 

 a few of the disappointments that fall to the lot of breeders 

 result from the same cause, I decided to examine material 

 collected for another purpose, with a view to shedding fresh light 

 on this difficult but important practical question. To admit of 

 the conditions which obtain in the brood mare being more easily 

 understood, I shall at the outset refer to the apparatus provided 

 for the nourishment of the chick, and to the corresponding 

 structures in one of the simpler mammals. 



The Chick's Foetal Appendages. 2 



Every one who has examined a hen's egg at, say, the end of the 

 ninth day of incubation, knows that the already well-formed 

 chick is, as it were, mounted on a large, well-filled forage bag — 

 the yolk sac — from which it hour by hour draws its nourishment. 

 But in addition to the familiar yolk sac there is a second sac, in 

 its way quite as important, though empty of food, viz., the thin 

 compressed sac lying in contact with the shell, which plays the 

 part of a breathing organ or lung. This second sac is known as 

 the allantois. The position and relations of the yolk sac and 

 allantois are indicated in fig. 1. Through the walls of both 

 sacs blood is constantly passing to and from the chick, collecting 

 particles of altered yolk from the one and fresh supplies of 

 oxygen (direct from the air which penetrates the shell) by means 

 of the other. Fig. 1 shows that the chick is connected to the 

 yolk sac by a short thick stalk, awd to the allantois by a long 

 slender stalk, and that it is invested by a special water-tight robe 



i Mr Pease, in his work Horse- Breeding for Farmers, says : — " Nine out of ten 

 inares that miss are barren simply from want of care in catching the mare at the 

 right time, or from subsequent neglect ;" that "half the mares that are geld are 

 so because the mare, having once or twice refused the horse, she is dismissed from 

 all further consideration," and further, that " many mares will refuse the horse 

 for several successive periods, and then come in season again perhaps without 

 any great show of their condition." 



2 The chick has been selected not because mammals are related to birds, but 

 that any one desiring to see the foetal structures dealt with may readily gratify 

 the wish by examining hen's eggs at the ninth or tenth day of incubation. 



