Preliminary Note on the Development of the 

 Pronephros in Petromyzon. 

 By S. Hatta. 



Biological Laboratory, Nobles' School, Tokyo. 

 I have recently studied the history of development of the pronephros 

 in Petromyzon, and the results arrived at throw, I believe, some light 

 on this subject. I will enumerate the chief points obtained in the 

 following pages. 



1). In Petromyzon, the pronephros becomes apparent at a com- 

 paratively early stage, that is, the stage approximately corresponding to 

 the early section of stage II in the list given by me*. When the 

 metamerie segmentation of the mesoblast in the anterier region of the 

 body has been finished, we can distinguish, in each of a few somites in 

 this part, a piece of mesoblast which lies between the proximal somatic 

 and the distal unsegmented portion (the lateral plate). The component 

 cells of this piece first assume the regular arrangement of a columnar 

 epithelium, and the parietal row of them is a little elevated aganist 

 the epiblast. It is this piece which develops, as the subsequent history 

 teaches, into the pronephric tubules and the nephrostomes ; I will, 

 therefore, hereafter call it the milage of the pronephros. All the anlagen 

 of the pronephros are cut off, with the lateral plates, from the segmented 

 portion of the mesoblast, and are still more thickened by the repeated 

 multiplication of cells. These thickened parts, however, is never a solid 

 knob ; for they acquire a lumen as soon as the anlagen come into view. 

 The lamina of the pronephric anlagen communicate, of course, with the 

 body cavity, which is a mere fissure in the stage in question. The cells 

 forming the the anlagen are high and columnar in shape, while the cells 



* Hatta, S. — Contrib. to the Morph, of Cyclostoiii. I. On the Formation of the Heart 

 in Petromyzon. Journ. Coll. Sci., Imp. Univ., Tokyo, vol. X, pt. II. 



