THE CORPUS LUTEUM OF PREGNANCY IX SWINE. 87 



The process of destruction, which is called atresia, consists of the degeneration of the 

 granulosa cells, shrinkage and disappearance of the ovum, and filling of the cavity by 

 enlarged cells of the theca interna, so that the atretic follicle resembles a corpus luteum, and 

 like the latter is finally replaced by a hyaline scar. On the other hand, the follicle may 

 proceed to ripeness, in which case the granulosa tends to range its cells in rather marked 

 layers, and the cells of the theca interna increase in number, show mitoses, become polyg- 

 onal, swell in size, and have deposited in them numerous fat granules, so that altogether 

 they show a notable resemblance to lutein cells (fig. IS, b). Next, through causes not yet 

 understood, the follicle bursts, the ovum is extruded, and a corpus luteum is formed. 

 Should rupture not occur, atresia may take place in the ripe as well as the unripe follicle. 

 As one of the causes of the non-ovulation during pregnancy, it has been pointed out, 

 especially by De Sinety (1877), Stratz (1898), and Schulin (1881), that atresia of the 

 follicles tends to be very marked during pregnancy. Sandes has confirmed this in the 

 marsupial Dasyurus (1903). In the pig it is certainly true. I have not a sufficiently large 

 number of non-pregnant ovaries to make a numerical comparison, but it can be stated that 

 a great part of the larger follicles in pregnant animals are degenerating. Yet in very many 

 ovaries of pregnancy fairly large intact follicles are found. Very interesting in this con- 

 nection is the observation of Pearl and Surface (1914) that ovulation in the domestic fowl 

 can be inhibited by the administration of corpus-luteum extracts. 



It has been suggested by some that the follicles do not grow to full size during preg- 

 nancy, and that hence the crop of follicles which gave rise to the embryos in uU ro is not 

 followed by another crop until pregnancy is terminated. This view has been studied by 

 Leo Loeb (1906). He reports that in many of his animals the follicles went on to full size 

 during pregnancy, but that they did not rupture, as they would in the absence of pregnancy. 

 It is unfortunate that microscopic studies were not made, for, as pointed out by Robert 

 Meyer, in the human ovary mere size of the Graafian follicles is a faulty criterion of their 

 state; those which appear large and ripe may in fact be in a stage of cystic atresia. 



The diameter of the ripe follicle of the sow is said by Kaeppeli to be 5 to 8 mm., and in 

 this statement he is followed by Schmaltz (1911), but I believe that normal ripe follicles 

 may grow much larger than 8 mm. Eenckiser (1884) included in his studies on the origin 

 of the corpus luteum a perfectly normal ripe follicle 1 1 mm. in diameter. I collected 24 

 pairs of ovaries at random, without selection except to exclude specimens containing cor- 

 pora lutea and cystic formations. The largest follicles in each ovary varied in diameter 

 from 1 to 10 mm. ,On microscopical examination even the largest follicle of these ovaries 

 proved to be normal and ripe. The lower limit of ripeness is difficult to set. I have seen 

 3 mm. follicles which were apparently ripe; and a recent corpus luteum of ovulation may 

 be as small as 5 mm., showing that the follicle from which it proceeded must have been 

 no larger. In the hundreds of ovaries I have examined I have seen many such large follicles, 

 and I have no doubt that had they been examined microscopically they would have 

 proved normal. 



As a contrast to this, in my entire series of ovaries from pregnant sows no follicles were 

 found having an outside diameter of more than 6 mm. with two exceptions, and I did not 

 find ripe follicles in any ovary of the series. The conclusion is simple: in pregnancy the 

 Graafian follicles may attain or maintain a size at which, in the absence of pregnancy, 

 ripening and rupture might occur. But they do not usually attain the larger diameters 

 reached by follicles in the non-pregnant ovary; and when they do happen to reach large 

 size, they are found to be either unripe or atretic. 



