MOEPHOLOGY OF CYCLOSTOMATA. 317 



Fig. 4 represents a section passing between the two somites 

 above mentioned (the fourth and fifth) and is ranch magnified 

 (Zeiss, E^ 2) to iUnstrate the finer structure of this portion. The 

 structural cells are all loaded with an enormous quantity of 

 ovoid corpuscles or yolk-granules. The epiblasi {ep.) consists 

 of a single row of cubical cells and shows a sharp limit against 

 the structures inside it. The irregularly polygonal mass of 

 cells [mi.V) is the anterior wall of the fifth myotome. Two 

 rows of variously shaped cells (Im.) constitute the lateral plate 

 which is histologically quite like that in the soraitic portion, being 

 composed of irregularly quadratic cells and tapering towards 

 the distal (ventral) extremity (compare with the lateral plate, 

 Im., in figs. 2 and 3). However, in the proximal portion, 

 where the Anlage of the pronephros consisting of a regular row 

 of tall columnar cells w^ould be found in the somitic portion, w^e 

 see here a group {x) of a few cells of faint appearance, forming 

 the proximal edge of the lateral plate. By a comparative study 

 of two or more series of sections, it is easily demonstrated that 

 these cells are a piece of the somite lying in front and have 

 nothing to do with the Anlagen. To elucidate this point still 

 further, I have drawn fig. 7 which represents a section through 

 the intersomitic plane between the sixth (fig. 5) and the seventh 

 somite (fig. 6). In this part the Anlage of the pronephros 

 is developed still more weakly, and the mesoblast remains in a 

 more primitive state. In the proximal edge of the lateral plate 

 {x), no special structure is detected, but the edge fades away without 

 a distinct limit. By comparison with Fig. 4, w^e can not find any 

 marked diiïerence ; thus, here likewise, there is no cellular con- 

 nection between the Anlagen in the two succeeding somites. 



From the fifth somite backwards for 9 or 10 somites, the 



