MOEPHOLOGY OF CYCLOSTOMATA. 369 



tiiiguislied, first of all, in the histological structure from the 

 distal portion : the former is composed of columnar cells, and 

 the latter of irregularly shaped cells. Only the proximal portion 

 Avhicli occupies the largest part of the mesoblast undergoes the 

 metameric segmentation and gives rise to the scleromyotome and 

 the nephrotome (in the sense of Kuckert) ; the distal smaller 

 portion remains unsegmented and is later converted into the 

 flattened epithelium of the peritoneum. 



2. The earliest traces of the pronephros are noticeable in 

 exceedingly young stages, that is, in the early part of Stage ir, 

 in which the embryo has about 16 somites. 



3. They are expressed in the form of a diverticulum of the 

 parietal layer of that section in each mesoblastic somite, which 

 forms the ventral half of the segmental part of the mesoblast 

 and is called the nephrotome. This is the Anlage of the pro- 

 nephric tubule and not of the segmental duct. 



4. The pronephric diverticulum or the Anlage is brought 

 about by the evagi nation of the parietal layer in each nephrotome, 

 enclosing a part of the primary coelomic cavity. 



5. The nephrotome is separated from the proximal portion 

 of the segmented mesoblast and forms awhile the proximal portion 

 of the unsegmented mesoblast or the lateral plate. The separation 

 begins with an indentation in the anterior and posterior borders 

 of the mesoblastic somite ; the myocoelome communicates for some 

 time by a narrow passage with the general coelomic cavity. 



6. The Anlao-e has no histological connection either with 

 the preceding or the following Anlage or Avitli the other 

 germinal layers ; it is, tlierefore, segmental in origin and myo- 

 meric in positiou. 



7. The Anlagen are developed, in Stage ii, in about 12 



