138 Mr. F. H. A. Marshall. On the (Estrous Cycle and 



been discharged. It is rather more than half as large as the ripe 

 follicle, and consequently does not protrude from the surface of the 

 ovary. Very little blood remains within the cavity, but corpuscles are 

 seen scattered through the membrana granulosa, these being derived 

 from vessels whose walls have broken down, not only near the point 

 of rupture of the follicle, but to a less extent around the whole theca 

 interna. The membrana granulosa is approximately twice the thick- 

 ness of that of the ripe follicle, some of the cells having increased 

 largely in size, while others, especially those nearest to the periphery, 

 retain the characters of the original follicular epithelial cells. The 

 central cavity contains a fluid resembling in all respects the liquor 

 folliculi. At this stage there is no sign of any growth inward of the 

 theca interna, and I have not observed any mitoses among the cells of 

 that layer. 



The corpus luteum of twenty-four hours has undergone considerable 

 changes. Its increase in size is well marked, its dimensions now ap- 

 proaching those of the ripe Graafian follicle. Its shape is generally 

 irregular, and its walls are much folded. The central cavity is smaller. 

 This cavity, which, as in the earlier stage, contains a fluid, communi- 

 cates with the exterior by a slit-like passage opening into a cup-shaped 

 depression on the surface of the ovary, from which the corpus luteum 

 now appreciably protrudes. The depression and slit-like passage 

 represent the point of rupture of the follicle. The epithelial wall of 

 the cavity is at this period at least twice as thick as that of the 

 seven-hour stage, this increase being due for the most part to the 

 simple hypertrophy of the individual cells composing it, these appear- 

 ing in section two or three times the size of those of the membrana 

 granulosa of the Graafian follicle. Division is, however, not very 

 infrequently to be observed among the epithelial cells. But the thick- 

 ness of this layer is also increased by the ingrowth of connective tissue, 

 strands of which, arising by cell proliferation of the theca interna, are 

 growing inwards and penetrating the epithelium. These connective 

 tissue strands present a radial appearance. The cells of which they 

 are composed are commonly fusiform in shape, and mitotic division is 

 very common among them. But although the connective tissue ele- 

 ment of the corpus luteum of the sheep is provided chiefly by the pro- 

 liferation of the cells of the theca interna, it is in part derived from 

 the more fibrous theca externa, from which layer strands of cells, 

 usually in close connection with those of the inner layer, are at this 

 stage beginning to grow inwards between the epithelial cells. Bed 

 blood corpuscles occur in scattered patches and streaks, as in the earlier 

 stage. 



In the corpus luteum thirty hours after coition, the inner theca layer 

 has all but disappeared, having been used up in the formation of the 

 inter-epithelial connective tissue. The epithelial cells, which have still 



