ORIGIN OF THE CORPUS LUTEUM 133 



The author has presented ('17 c) a brief study of the matura- 

 tion of the pig's ovum, based upon some of these specimens, 

 which indicate that the sequence of events is the same as in other 

 mammals. The first polar body and the second polar spindle are 

 formed in the ovary just before rupture. After the entrance of 

 the spermatozoon, which occurs in the tube, the second spindle 

 completes its division, and the presence of two polar bodies is 

 therefore a sign of fertilization. If the ova are not fertilized, they 

 degenerate in the tube with the second spindle undivided. Just 

 how long they survive is not known, but by analogy with the 

 smaller and better-known mammals, we may assume that after 

 three or four days they are no longer capable of segmentation; 

 the degenerating ova may be found in the tubes a few days longer. 

 It is said that pregnancy is more likely to result when the sow is 

 served on the second day of oestrus. The number of ova ex- 

 truded at one ovulation, and consequently the number of fresh 

 corpora lutea in one animal, may be quite large. One prolific 

 sow is known to have given birth to twenty-three pigs in one 

 litter. However, in the mixed stock, not especially adapted for 

 breeding, which is found in the abattoirs, small litters are the 

 rule. Records of 128 sows raised in Maryland, presented in my 

 paper of 1915, show that the corpora lutea of pregnancy in both 

 ovaries numbered one to sixteen, averaging eight, and that the 

 number of foetuses in the uteri of the same sows varied from one 

 to ten, averaging six. Failure of fertilization, abortion, and re- 

 sorption of embryos dying in utero account for the fact that not 

 all the eggs of one ovulation proceed to full development. 



About 133 embryos of the sow younger than two weeks, taken 

 from twenty-three sows, have been observed and described in the 

 literature. The youngest of all are the three ova found in three 

 different sows by the present writer and Amsbaugh ('17), in which 

 conjugation of the pronuclei had not occurred. It was not possible 

 to know the exact time of insemination in these animals, but in one 

 case it is believed that the animal had not been in heat, and conse- 

 quently had not copulated, more than forty hours before kill- 

 ing. R. Assheton ('99) studied about 100 specimens during the first 

 ten days, the youngest stage being that of two blastomeres. It 



