630 EXISTENCE OF A HEAD-KIDNEY 



section was filled by its own wall and the Mullerian duct. In 

 the following section the thickening in the wall of the Wolffian 

 duct has disappeared (Plate 28, series H, No. 3), and every trace 

 of the Mullerian duct has vanished from view. The Wolffian 

 duct is on one side in contact with the germinal epithelium. 



The stage during which the condition above described lasts 

 is not of long duration, but is soon succeeded by our third stage, 

 in which a fresh mode of termination of the Mullerian duct is 

 found. (Plate 28, series I.) This last stage remains up to about 

 the close of the sixth day, beyond which our investigations do 

 not extend. 



A typical series of sections through the terminal part of the 

 Mullerian duct at this stage presents the following features: 



A few sections before its termination the Mullerian duct 

 appears as a well-defined oval duct lying in contact with the 

 wall of the Wolffian duct on the one hand and the germinal 

 epithelium on the other (series I, No. i). Gradually, however, 

 as we pass backwards, the Mullerian duct dilates ; the external 

 wall of the Wolffian duct adjoining it becomes greatly thickened 

 and pushed in in its middle part, so as almost to touch the 

 opposite wall of the duct, and so form a bay in which the 

 Mullerian duct lies (Plate 28, series I, Nos. 2 and 3). As soon 

 as the Mullerian duct has come to lie in this bay its walls lose 

 their previous distinctness of outline, and the cells composing 

 them assume a curious vacuolated appearance. No well-defined 

 line of separation can any longer be traced between the walls of 

 the Wolffian duct and those of the Mullerian, but between the 

 two is a narrow clear space traversed by an irregular network of 

 fibres, in some of the meshes of which nuclei are present. 



The Mullerian duct may be traced in this condition for a 

 considerable number of sections, the peculiar features above 

 described becoming more and more marked as its termination is 

 approached. It continues to dilate and attains a maximum size 

 in the section or so before it disappears. A lumen may be ob- 

 served in it up to its very end, but is usually irregular in outline 

 and frequently traversed by strands of protoplasm. The Mtiller- 

 ian duct finally terminates quite suddenly (Plate 28, series I, No. 

 4), and in the section immediately behind its termination the 

 Wolffian duct assumes its normal appearance, and the part of 



