MOKPHOLOGY OF CYCLOSTOMATA. 315 



Fig. 2 represents a section through the middle of the fifth 

 somite^^ of the embryo mentioned above. The general features of 

 tlie germinal layers and of other primitive organs are essentially 

 the same as before. The epiblast {ep.) is a single row of columnar 

 cells and is sharply bounded from the structures beneath it ; the 

 neural cord (71.) remains still solid.^^ In the mesoblast, however, 

 two portions are distinguishable : the proximal portion composed 

 of high columnar cells {mt.V and a.pn.2) which undergoes 

 metameric segmentation, and the distal portion consisting of 

 a loose group of somewhat irregularly shaped cells (Im.) which 

 remains unsegmented and constitutes the lateral plate. It 

 is noteworthy that the former takes up the largest portion of 

 the mesoblast, while the latter is represented by a small portion ; 

 these two portions represent respectively the parts of the same 

 name in the mesoblast of AmpMoxus. However, between them 

 there exists no distinct limit in the lamprey ; the one passes 

 gradually over into the other. Although the visceral layer shows 

 no sign of constriction, the parietal layer is notched at about the 

 middle of the proximal segmented portion (.i). The parietal layer 

 distal to this notch is composed of a regular cylindrical epithelium 

 {a.pn.2), which is slightly arched against the epiblast, so as 

 to cause an indentation in the latter, wdiile the visceral layer 

 of the corresponding portion consists of a more or less disturbed 

 row of high columnar cells. As the subsequent history teaches, 

 the proximal half of this extent {mt. V) represents the myotunie"^ 



1) The somites are reckoned from the anterior end. Tlie first, i.e. the foremost lies 

 immediately behind the auditory vescicle when the vescicle comes into view. 



2) The vertical diameter of the neural cord in tigs. 2 and 3 is shown greater than it 

 really is, the sections passing obliquely owing to the bending of the longitudinal axis of 

 the embryo, as noted in the introduction (p. 312). 



3) This term liere means the Sclero-rayotom of German authors. 



