380 DK D. BEERY HART. 



abdomiual sections. The Wolffian ducts thus open into the 

 stalk of the allantois, according to this view of bladder origin. In 

 regard to the allantois of the human embryo, we now know that 

 there is no vesicular or free part, as in some of the lower 

 mammals, or in the chick. The existence of the urachus, or of 

 patent urachus, with its necessary bladder connection, shews, 

 however, that the allantois takes a certain part in bladder 

 formation, but the valuable diagram of Allen Thomson, which 

 has so long illustrated our anatomical text-books, indicated a 

 comparatively late stage of bladder development — about the 

 seventh week — where the bladder, urinogenital sinus, and 

 rectum have been formed after prior changes then unknown 

 (fig. 10, PL XXXV.). 



This view was first controverted by Eathke in 1832, but 

 received its death-blow from the work of Lieberkiihn and Keibel. 



I have not as yet had access to Lieberkilhn's memoir, and 

 therefore take Keibel's monograph. The guinea-pig has only a 

 transient allantois, and develops its bladder when no allantois 

 is present. Further, transverse sections of the caudal end of 

 the guinea-pig shew that the primitive gut at this end becomes 

 divided by coronal folds into an anterior and posterior cavity, 

 from the former of which the bladder and urinogenital sinus 

 arise. A peritoneal duplicature completes the separation. 



The same is found to happen in the human embryo. For 

 the study of the facts bearing on the development in the human 

 embryo, we may take Graf Spec's embryo, 2 mm. in length, and 

 three specially considered by Keibel — viz., E.B., His' embryo, 

 3 mm. ; a second one, HsJ., 3*2 mm. ; and a more advanced 

 one, Hsf., 6'5 mm. 



Graf Spec's requires to be considered from the dorsal aspect 

 and also in sagittal mesial section. 



On viewing the dorsal aspect with the amnion removed, we 

 see posteriorly the body stalk, then comes the primitive groove 

 (two ridges with the primitive streak between), while the 

 neurenteric canal lies at the anterior margin of the primitive 

 groove. The primitive streak is present as an ectodermic 

 thickening (fig. 4, PL XXXV.). 



This diagram shews clearly that the neurenteric canal is a com- 

 munication between entoderm and ectoderm, running between 



