218 DF.YELOPMENT OF THE GUINEA-PIG. 



placenta is capable of being used to some extent in classification, 

 yet at the same time the striking resemblances which can exist 

 between such essentially different forms of placenta, as for instance 

 those of Man and the Rodentia, are likely to prevent it being employed, 

 except in conjunction with other characters. 



Special types of development. 



The Guinea-pig", Cavia Cobaya. Many years ago Bischoff (No. 176) 

 shewed that tlie 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 Schiifer (No. 190), but we are still as far as ever from explaining 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 stiidied with great 

 detail. On the close of segmentation, about six days after impreg- 

 nation, 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 in- 

 volved in great obscurity. It is certain, however, 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 divei- 

 ticulum the mucous membrane of the niesometric side of the xiterus also 

 becomes thickened, and this thickening very soon (shortly after the seventh 

 day) unites with the wall of the diverticulum, and completely 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 

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

 mouth of the diverticuhim. 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 Schiifer 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 '. 



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

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

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

 it becomes hollow, and is filled with a fluid not coagulable in alcohol, while 

 the body within its apex remains unaltered till the tenth day. 



^ Schafer's and Heusen'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. 



