14 A CRITICAL PERIOD IN THE 



less powerful contractions of the numerous uterine muscular 

 fibres. The periodic disturbance is likely to be greater in mares 

 which have not previously bred. In mares which have had several 

 foals, or have recently foaled, the uterine vessels are readily 

 enlarged, and from the first tend to deflect blood which might 

 otherwise rush to the ovaries. At a time when the connections 

 between the embryonic sac and the uterus are at their weak- 

 est, this extra excitement of the reproductive system may very 

 readily lead to an arrest in the developmental process, and 

 eventually to the embryo being discharged. When this hap- 

 pens, evidence will soon be forthcoming that the mare has 

 broken service. 



The Seven- Weeks Horse Embryo. — If the storms which natu- 

 rally set in about the end of the sixth week are successfully 

 weathered, a period of calm sets in, not likely to be seriously 

 interrupted at the end of the ninth week ; for by that time the 

 uterine vessels will have sufficiently increased to carry off all the 

 extra blood that finds its way to the reproductive organs. The 

 ovaries will thus be but little excited, and the nervous system, as 

 a whole, will all but maintain its normal calm. There is, how- 

 ever, another danger ahead, for at the end of the seventh week 

 the supply of nourishment by means of the yolk sac has all but 

 come to an end, and the arrangements for providing for the 

 wants of the embryo, by means of the allantois, have not yet 

 been completed. To put it another way : at or about the end of 

 the seventh week the remote marsupial ancestors of the horse 

 were, in all probability, in the habit of leaving the uterus for the 

 shelter of the pouch or marsupium, already capable of imbibing 

 ordinary milk from teats, instead of absorbing uterine milk by 

 means of a yolk sac. 1 The seven-weeks' embryo (with its append- 

 ages and its inner and outer investing tunics) is represented in 

 fig. 6. This embryo, which is nearly half an inch longer than the 

 six-weeks' one, is at a most interesting stage in its development. 

 Had it been an opossum, it would have already been glued to a 

 teat in the marsupium, — not yet able to suck, but sufficiently 



1 Some would even go the length of saying that, in obedience to the law of 

 heredity, there is an attempt on the part of the horse embryo, at or about the 

 end of the seventh week, to slip its moorings and escape from the uterus, — to 

 mahe an attempt to repeat this particular episode in the ancestral history. 



