262 DEVELOPMENT OF THE GUINEA-PIG. 



Special types of development. 



The Guinea-pig, Cavia cobaya. Many years ago Bischoff 

 (No. 176) shewed that the development of the guinea-pig was strikingly 

 different from that of other Mammalia. His statements, which were at first 

 received with some doubt, have been in the main fully confirmed by Hensen 

 (No. 182) and Schafer (No. 190), but we are still as far as ever from explain- 

 ing the mystery of the phenomenon. 



The ovum, enclosed by the zona radiata, passes into the Fallopian tube 

 and undergoes a segmentation which has not been studied with great detail. 

 On the close of segmentation, about six days after impregnation, it assumes 

 (Hensen) a vesicular form not unlike that of other Mammalia. To the inner 

 side of one wall of this vesicle is attached a mass of granular cells similar to 

 the hypoblastic mass in the blastodermic vesicle of the rabbit. The egg still 

 lies freely in the uterus, and is invested by its zona radiata. The changes 

 which next take place are in spite of Bischoff's, Reichert's (No. 188) and 

 Hensen's observations still involved in great obscurity. It is certain, how- 

 ever, that during the course of the seventh day a ring-like thickening of the 

 uterine mucous membrane, on the free side of the uterus, gives rise to a kind 

 of diverticulum of the uterine cavity, in which the ovum becomes lodged. 

 Opposite the diverticulum the mucous membrane of the mesometric side of 

 the uterus also becomes thickened, and this thickening very soon (shortly 

 after the seventh day) unites with the wall of the diverticulum, and com- 

 pletely shuts off the ovum in a closed capsule. 



The history of the ovum during the earlier period of its inclusion in the 

 diverticulum of the uterine wall is not satisfactorily elucidated. There 

 appears in the diverticulum during the eighth and succeeding days a cylin- 

 drical body, one end of which is attached to the uterine walls at the mouth 

 of the diverticulum. The opposite end of the cylinder is free, and contains 

 a solid body. 



With reference to the nature of this cylinder two views have been put 

 forward. Reichert and Hensen regard it as an outgrowth of the uterine wall, 

 while the body within its free apex is regarded as the ovum. Bischoff and 

 Schafer maintain that the cylinder itself is the ovum attached to the uterine 

 wall. The observations of the latter authors, and especially those of Schafer, 

 appear to me to speak for the correctness of their view 1 . 



The cylinder gradually elongates up to the twelfth day. Before this pe- 

 riod it becomes attached by its base to the mesometric thickening of the 

 uterus, and enters into vascular connection with it. During its elongation it 



1 Schafer's and Hensen's statements are in more or less direct contradiction as to 

 the structure of the ovum after the formation of the embryo; and it is not possible to 

 decide between the two views about the ovum till these points of difference have been 

 cleared up. 



