THE DEVELOPMENT OF THE MOLE. 4i7 



Comparison between the Early Stages of Development 

 of the Mole and Mouse^ &c. 



Until a few months ago there had been no satisfactory ex- 

 planation of the manner in which the extraordinary phenomenon 

 of the inversion of the layers in the Guinea-pig had been brought 

 about, although the fact that such an inversion really existed 

 had been described many years ago by BischofF (Nos. 8 and 9), 

 Reichert (No. 23), and Hensen (No. 12). 



KupfFer (No. 18), Selenka (No. 27), and Fraser (No. 10), 

 have, however, recently worked at the development of the 

 Field Mouse, House Mouse, and Rat, and have found that the 

 position of the layers in these animals is also inverted. 

 Further, they each discovered the method by which the inver- 

 sion was accomplished, and at the same time Hensen (Nos. 13 

 and 14) arrived at somewhat similar results for the Guinea- 



From these papers and from that of Spec (No. 28) I gather 

 it is probable that the fully-segmented ovum of these various 

 animals is similar to that of the Mole. 



The changes which take place after segmentation are, how- 

 ever, somewhat different in each, and show a gradually 

 increasing difference from the normal type to that one most 

 specialised, viz. the Guinea-pig ; while the phenomena exhibited 

 during the development of the mole supply the connecting link 

 between the. two types. 



These facts have not been, as far as I know, hitherto brought 

 forward, and I venture to think merit some attention. 



In the Field Mouse a blastodermic vesicle of flattened outer 

 layer cells is formed, at one place on the circumference of which 

 a solid inner mass is attached. 



A layer of hypoblast is formed on the lower side of the inner 

 mass, and the two shortly after flatten out ; a thickening of 

 the outer layer then takes place above the inner mass, and the 

 flattened plate, with the hypoblast on its inner side, becomes 

 involuted within the vesicle, and in this way an arched plate 

 is formed, the circumference of which rests upon the outer layer 



