WEYSSE. — BLASTODERMIC VESICLE OP SUS SCROPA. 283 



XII. 



CONTRIBUTIONS FROM THE ZOOLOGICAL LABORATORY OF 



THE MUSEUM OF COMPARATIVE ZOOLOGY, UNDER THE 



DIRECTION OF E. L. MARK, XLIII. 



ON THE BLASTODERMIC VESICLE OF SUS SCROFA 



DOMESTICUS. 



By A. W. Weysse. 



Communicated by E. L. Mark. Receiyed August 10, 1894. 



Page 



I. Introduction 283 



1. Subject 283 



2. Material 28i 



3. Technique 285 



11. Description of the Embryos . . . 287 



1. General Characteristics of the 

 Blastodermic Vesicle . . . 287 



2. Detailed Account of Observations 



on the Germinal Disk . . . 289 



1st Stage, Figs. 7-9, Plate II. . . 289 

 2d Stage, Fig. 1, Plate I. ; Fig. 10, 



Plate II 292 



3d Stage, Fig. 2, Plate I. ; Figs. 



21 and 26, Plate IV 293 



Page 



4th Stage, Figs. 3-5, Plate I. ; Figs. 

 11-13, Plate II. ; Figs. 14-19, Plate 

 111. ; Fig. 20, Plate IV. ... 294 

 5th Stage, Fig. 6, Plate I. ; Figs. 



22-25, Plate IV 299 



3. Summary of Observations . . . 300 

 lit. Historical and Theoretical .... 303 



1. Consideration of Observations on 

 the Blastodermic Vesicle in Gen- 

 eral 303 



2. Interpretation of the Bridge . 311 



3. Summary 315 



Literature cited 317 



Explanation of Plates 321 



L Introduction. 

 1. Subject. 



In the latter part of the summer of 1893 the opportunity was opened 

 to me of obtaining young embryos of Sus scrofa domesticus. So I let 

 the work rest which I had in hand on the development of some other 

 mammals and began investigations on this. I was fortunate enough 

 to secure a number of embryos several days younger than the youngest 

 of the six, from fourteen to fifteen days old, which Keibel ('93) has 

 recently described so elaborately ; and since these present phenomena 

 of development unusual in the ontogeny of the Mammalia, it has seemed 

 best to record them, in the hope that they may possibly lead to similar 

 discoveries in allied forms. 



The incompleteness of our knowledge of the early stages in the 

 ■embryology of the higher viviparous animals is due largely, of course, 



