358 THE AMERICAN NATURALIST [Vol.LVI 



three ten-day conceptuses, d.5 X 3 5 ^-^ X 4-5 > 

 ' (5.5 X 4-5 mm. respectively. Draper gave the estimated 

 length of an 11-day embryo measured under magnifica- 



Stockard and Papanicolaou (1918) likewise reported 

 that a ' ' slightly cystic ovary ' ' has frequently been diag- 

 nosed by palpation through the abdominal wall of the 

 guinea pig. In my observations 23 out of 75 ovaries were 

 found to be cystic; but the largest cyst measured only 

 1.6 mm.X 1-68 mm. and not even this, could by any chance 

 have been palpated through the abdominal wall. Hence, 

 it would seem that Stockard and Papanicolaou must have 

 been dealing with markedly large and unusual, rather 

 than with slightly, cystic ovaries. 



From a study of a large series of gestations in the 

 domestic pig, Corner, '21, concluded that internal migra- 

 tion of ova is relatively common. This small series of 

 pregnancies in the guinea pig furnishes very little evi- 

 dence upon this question, for such a possibility is sug- 

 gested only by No. 17, a pregnancy of 35 days in which 

 there were 2 corpora and 2 implantations on the right 

 side and no corpora but one implantation on the left side. 

 Since the total number of implantations in this case ex- 

 ceeds that of corpora, one must assume that one ovum 

 divided or that one follicle contained ova and that one 

 of the ova arising from the right then migrated to the 

 left cornu. However, since this pregnancy was so far 

 advanced, this assumption implies that a corpus luteum of 

 pregnancy in the guinea pig can not be wholly resorbed 

 in 35 days and that it never fails to form. 



It is of special interest in this connection that a second 

 case of this kind has been observed in this laboratory by 

 Miss Clark. In this case there were two corpora in the 

 left ovary and none in the right, with oile implantation 

 on each side. Since this pregnancy was only 17 days old, 

 the question of early resorption of the corpus luteum 

 probably can be excluded with considerable certainty but 

 that of failure of the corpus luteum to form, remains. 



