the Formation of the Corpus Luteum in the Sheep. 137 



question was separated from the rest, and a day afterwards killed, 

 when it became evident at once from the blood-clot on the surface of 

 one of the ovaries that ovulation had recently taken place. Sections 

 through this ovary showed the point of rupture of the follicle. This 

 fact, that ewes need not be served in order to induce ovulation, is of 

 considerable importance, as it indicates the possibility of obtaining 

 successful results from the artificial insemination of sheep. 



When ovulation takes place, one follicle only may rupture at a time, 

 or one follicle in each ovary, or two in the same ovary. I have never 

 observed any greater number of discharged follicles of the same age in 

 the ovaries of a sheep. * 



The period of " heat " in sheep is further marked by the distension of 

 the blood-vessels of the Fallopian tubes, which may throughout almost 

 their entire length be coloured a deep purple. The increased size of 

 the vessels is also seen in section, but there is no breaking down of 

 vessels. There is too some evidence of increased blood supply to the 

 ovaries, apart from the region of the ruptured follicle. 



The changes which take place in the metoestrous period have not as 

 yet been fully worked out, but at a period three days after coition, red 

 blood corpuscles in a state of haemorrhage, and arranged in streaks 

 below the epithelium, have been observed. It would also appear that 

 new capillaries have been formed. Metcestrum is succeeded by a 

 period of rest (dioestrum), which after not many days is followed by 

 another procestrum, and so on, until the sheep becomes pregnant or 

 the breeding season is over. The complete dicestrous cycle in the 

 sheep in the only case which came under my observation was fifteen 

 days, but from the observations of others with whom I have spoken it 

 would appear to vary from about thirteen to eighteen days. 



The Formation of the Corpus luteum. — The age of the corpus luteum 

 in this investigation was in each case reckoned, either from copulation, 

 or, where copulation did not or was not known to have taken place, 

 from the time when oestrus was observed. Of course it is possible that 

 ovulation does not always take place during oestrus, but the observed 

 relation between the state of development of the corpus luteum and 

 the time that had elapsed between oestrus and the killing of the animal 

 is by itself strong evidence that in the sheep the two phenomena are 

 approximately coincident. In no case after a sheep in which oestrus 

 had been observed, was killed to obtain a stage in the development of 

 the corpus luteum, was the corpus luteum not found. It could 

 usually be at once readily detected by the blood-clot which remains on 

 the surface of the ovary for several days after the rupture of the 

 follicle. 



The corpus luteum of seven hours differs from the unburst follicle 

 in its size and in the fact that the ovum and discus proligerus have 

 * Triplets are, however, not uncommon in some breeds of sheep. 



