72 THE CORPUS LUTEUM OF PREGNANCY IN SWINE. 



80 per cent alcohol for preservation, and blocks cut from them for sectioning were put for 

 24 hours in each of the following fluids: 85, 90, 95, 98 per cent alcohol, equal parts 98 per 

 cent alcohol and ether, celloidin solutions 2, 4, 6, 8, 10, 12, 14 per cent. Finally the 

 celloidin blocks were hardened with chloroform. As a routine, from one corpus luteum of 

 each ovary or pair of ovaries two sections were made, 10 or 15 microns in thickness (the 

 sections often passed through two adjacent corpora lutea), and in addition sections were 

 made of any unusual or puzzling structure. Of these two routine sections one was stained 

 with eosin and Ehrlich's hematoxylin, the other with Mallory's triple connective-tissue 

 stain (acid fuchsin, phosphomolybdic acid, aniline blue, and orange G) . The latter staining 

 method is particularly valuable, since it not only gives beautiful pictures of the connective 

 tissue, but also of those very structures of the lutein cells which are most important for the 

 present study. 



BREEDING HABITS OF THE SOW. 

 The females of the wild swine are monestrous and give birth to one litter a year, accord- 

 ing to Kaeppeli (1908). But in domestication the sow undergoes an estrous cycle lasting 

 2 to 4 weeks, generally 3 weeks (Kaeppeli, Fleming: Veterinary Obstetrics). If the animal 

 becomes pregnant, she may conceive again 5 weeks after delivery. The period of gestation 

 is 16 to 17 weeks, usually between 116 and 120 days. Farmers commonly arrange to have 

 litters produced in the spring and autumn, but in the material received at this laboratory, 

 from stock raised for slaughter, fetuses are found in all stages of development, without 

 great regard to the time of year. 



The domestic sow is a very prolific animal, sometimes giving birth to as many as 19 

 pigs in one litter. Wentworth (1914) mentions an extreme case in which a sow gave birth 

 to 23 pigs in one litter. John Hunter kept a record of the total progeny of one sow, in 

 order to have a control for another, one of whose ovaries he had removed. During her life 

 the normal animal gave birth to 13 litters, numbering in all 162 pigs, making an average 

 of 12.5 pigs for each birth. In animals sent to the city for slaughter such high figures do 

 not occur, for several reasons. The animals are sold young, often while bearing their first 

 litters, which are commonly small in number; or if they are multipara, they are sold for 

 the very reason that they are not profilic breeders. Moreover, the stock sent to the slaugh- 

 ter-house is the general product of the country-side, not always from selected droves. My 

 records show that 6 is the commonest number of pigs in one litter, the extremes being 

 1 and 10. 



GENERAL FEATURES OF THE CORPUS LUTEUM. 

 On examining the ovary of the pregnant sow, the first objects to strike the eye are the 

 extraordinarily prominent corpora lutea, which project nearly all their volume from the 

 surface of the* ovary. In each ovary of the domestic sow I have found from 1, or none, to 

 10 recent corpora lutea, and in both ovaries from 1 to 16, most commonly 8. For the 

 reasons which I have given, these figures are lower than would be found in well-bred, selected 

 stock. It will be noticed also that the pigs are commonly fewer than the corpora lutea; in 

 other words, that frequently not all the ova expelled at one time develop into pigs. The 

 corpora lutea are generally ovoid or spherical in form, with outside diameters of 8 to 10 mm., 

 most often about 10x10x10 mm. The full size is attained when the fetuses are about 

 10 mm. long, and does not change until retrogression of the corpus luteum is well advanced 

 after delivery ; in other animals the corpus luteum is said to increase slowly in size through- 

 out pregnancy. (For instance, the bat, as studied by Van der Stricht, 1912.) The custom- 



