EXISTENCE OF HEAD-KIDNEY IN THE EMBRYO CHICK. 11 



Wolffian duct has disappeared (Plate II, series h, No. 3), and 

 every trace of the Miillerian 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 Miillerian 

 duct is found. (Plate II, series i) . This last stage remains 

 up to about the close of the sixth day, beyond which our investi- 

 gations do not extend. 



A typical series of sections through the terminal part of the 

 Miillerian duct at this stage presents the following features ; 



A few sections before its termination the Miillerian 

 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. 1). 

 Gradually, however, as we pass backwards, the Miillerian 

 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 Miillerian duct lies (Plate II, series i, Nos. 

 2 and 3). As soon as the Miillerian 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 Miillerian, 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 Miillerian duct may be traced in this condition for a con- 

 siderable number of sections, the peculiar features above de- 

 scribed 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 

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

 outline and frequently traversed by strands of protoplasm. The 

 Miillerian duct finally terminates quite suddenly (Plate II, 

 series i, No. 4), and in the section immediately behind its ter- 

 mination the Wolffian duct assumes its normal appearance, and 

 the part of its outer wall on the level of the Miillerian duct 

 comes into contact with the germinal epithelium (Plate II, 

 series i, No. 5). 



We have traced the growing point of the Miillerian duct with 

 the above features till not far from the cloaca, but we have not 

 followed the last phases of its growth and its final opening 

 into the cloaca. 



