350 GEORGE A. BATES 



OBSERVATIONS 



The object of the present paper is to present what the writer 

 believes to be conclusive evidence of the development of the pro- 

 nephric duct, from its origin in the pronephros to its union with 

 the cloaca, from cells derived originally from the mesoderm. No 

 attempt has been made to describe the development of the pro- 

 nephros, except in the merest outline which may be of use in in- 

 troducing the account of the development of the pronephric 

 duct itself. 



The investigation was undertaken at the Harpswell Laboratory 

 at South Harpswell, Maine, at the suggestion of Dr. H. V. Neal, 

 who also kindly furnished the material, ready prepared, and to 

 whom I gratefully render my thanks, not only for the material 

 but for the kind advice and helpful criticisms. The material used 

 was Acanthias embryos prepared by the vom Rath picro-osmo- 

 platinic method. This renders cell outlines and limiting mem- 

 branes very distinct, and makes it possible to differentiate be- 

 tween different cells and cell layers. In the writer's opinion, it 

 is the sharp differentiation of cell outlines that has made it pos- 

 sible to distinguish with certainty, in all cases, the layer to which 

 any individual cell belongs. 



The pronephros arises from the middle plate, or mesomere, of a 

 number of body somites — in Acanthias, six^ — by a proliferation of 

 cells from the somatic wall. These proliferations soon fuse and 

 the pronephric ridge (Vornierenwulst) thus formed extends later- 

 ally until it comes into contact with the ectoderm. At the line 

 of contact the cells of the outer surface of the ridge form a cord, 

 which later acquires a lumen and becomes converted into the 

 pronephric duct. The cord of cells which is the anlage of the duct, 

 rapidly grows caudally from the pronephl-ic body, between the 

 somites and the ectoderm, until it reaches the cloaca. 



During the process of backward growth the tip or growing end 

 of the anlagal duct is at first entirely free from the ectoderm. As, 

 however, the line of growth is obliquely backward and outward, 

 it almost immediately comes into contact with the adjacent ecto- 

 derm. The relation of the two structures, duct and ectoderm, is 



