230 BULLETIN OF THE 



On passing backwards, the dorsal half broadens and finally exhibits 

 two lateral processes, or cornua, the walls of which are composed of a 

 layer one cell deep. The ducts open into the distal ends of these cornua 

 (see Fig. 27, showing the condition in Stage VI.). Behind the outlets 

 of the segmental ducts, the lumen of the intestine has a nearly circular 

 outline, and descends rapidly to the anus, or, as it may now more cor- 

 rectly be called, the cloacal aperture. I was able to see in Stage IV. faint 

 traces of these intestinal cornua. The cells of the dorsal roof of the 

 intestine showed in this region a looser structure, and a line of pigment 

 indicated the region of the outfolding. The cells of the duct and those 

 of the cloaca are histologically very different from each other, so that 

 it is for a long time possible to draw a line sharply separating the two 

 constituents where they have come in contact. 



The pronephric system of tubules presents in this stage quite uniform 

 histological characters. I shall therefore describe its typical condition, 

 and then consider the modifications that are to be found in certain of 

 its regions. The walls of the tubules are very thick, measuring on the 

 average about 25 //. in thickness. They accommodate themselves readily 

 to the structures with which they come in contact, becoming thinner 

 opposite elevations in neighboring surfaces, and thicker next to sinuses. 

 The size of the lumen varies greatly. In the segmental duct proper, the 

 diameter of the lumen is about 25 /x ; it is usually somewhat greater 

 in the region of the convoluted tubules. The walls of the tubules are 

 composed of an epithelium, consisting of a single layer of columnar cells. 

 The radial dimension of the cells in the case of thick walls is approxi- 

 mately three times their width. "Where the plane of the section cuts 

 the wall of a tube tangentially, the cells may be seen to have a polygo- 

 nal outline. The nuclei invariably occur close to the central lumen of 

 the tube ; each is large, and is usually provided with a single distinct 

 nucleolus. The eccentric position of the nuclei is attended with a 

 corresponding distribution of the cell protoplasm. By the picro-carmin 

 method which I have employed, the yolk spherules take a bright yellow 

 stain, and the nucleus a light red. The active protoplasm has a faint 

 pink coloration, which, however, is wholly invisible if too much picric 

 acid be left in the preparation. In young cells, where only a small 

 amount of yolk has been consumed, the delicate tint of the protoplasm 

 cannot be seen, since all the light passing through the section encounters 

 yellow yolk spherules. As the consumption of the yolk progresses, the 

 protoplasmic matrix comes into view. In the wall of the tubules, the 

 yolk is crowded to the outer surface of the cell, and a sheet of protoplasm 



