S BALFOUR AND SEDGWICK. 



In our preliminary account we stated/ in accordance with the 

 general view, that the Miillerian duct was formed as a groove, 

 or elongated involution of the peritoneal epithelium adjoin- 

 ing the Wolffian duct. We have now reason to believe that 

 this is not the case. In the earliest condition of the Miillerian 

 duct which we have been able to observe, it consists of three suc- 

 cessive open involutions of the peritoneal epithelium, connected 

 together by more or less well-defined ridge-like thickenings of the 

 epithelium. We believe, on grounds hereafter to be stated, that 

 the whole of this formation is equivalent to the head-kidney of 

 the Icthyopsida. The head-kidney, as we shall continue to call 

 it, takes its origin from the layer of thickened epithelium situated 

 near the dorsal angle of the body cavity, close to the Wolffian 

 duct, which has been known since the publication of Waldeyer's 

 important researches as the germinal epithelium. The anterior 

 of the three open involutions or grooves is situated some little 

 distance behind the front end of the Wolffian duct. It is simply 

 a shallow groove in the thickest part of the germinal epithelium, 

 and forms a corresponding projection into the adjacent stroma. 

 In front the projection is separated by a considerable interval 

 from the Wolffian duct; but near its hindermost part it 

 almost comes into contact with the Wolffian duct. The groove 

 extends in all for about five of our sections, and then terminates 

 bv its walls becoming gradually continued into a slight ridge- 

 lil^e thickening of the germinal epithelium. The groove arises 

 as a simple depression in a linear area of thickened germinal 

 epithelium. The linear area is, however, continued very con- 

 siderably further forward than the groove, and sometimes exhibits 

 a slight central depression, which might be regarded as a forward 

 continuation of the groove. The passage from the groove to 

 the ridge may best be conceived by supposing the groove to be 

 suddenly filled up, so as to form a solid ridge pointing inwards 

 towards the Wolffian duct. 



The ridge succeeding the first groove is continued for about 

 six sections, and is considerably more prominent at its posterior 

 extremity than in front. It is replaced by groove number two, 

 which appears as if formed by the reverse process to that by 

 which the ridge arose, viz., by a hollowing out of the ridge on 

 the side towards the body cavity. The wall of the second 

 groove is, after a few sections, continued into a second ridge or 

 thickening of the germinal epithelium, which, however, is so 

 faintly marked as to be hardly visible in its middle part. In its 

 turn this ridge is replaced by the third and last groove. This 

 vanishes after one or two sections, and behind the point of its 

 disappearance we have failed to find any further traces of the 

 ^ 'Proceedings of Royal Society,' 1878. 



