318 s. HATTA : 



mesoblast presents almost the same feature of the Anlage as in 

 the fifth somite mentioned above. Fig. 5 represents a section 

 through the sixth somite, next behind the fifth ; when compared 

 witli fig. 2 no marked difference is detected in regard to the 

 structure of the mesoblast. But in some segments the development 

 of the Anlage is somewhat weaker than in others, as seen in 

 fig. 6, which shows a section throngh the seventh somite; while 

 in a segment posterior to this somite, we find the Anlage as much 

 pronounced as in the sixth somite. However, generally speaking, 

 the Anlage of the pronephros in an anterior somite develops 

 further than that in a posterior. It must be remembered that 

 the somite in which the Anlage has already become expressed 

 does not pass over suddenly into the somite in which no trace 

 of it is to be seen ; but its development gradually grows less and 

 less distinct from the anterior to the posterior part, until finally 

 no trace of it is perceived. 



In the present stage, therefore, the Anlage of ilie pronephros 

 is detected in more than 4 somites but is completely separated from 

 the 7nyotome only in one segment, viz. the fourth somite^\ and it 

 has no genetic connection either with the Anlage in the next 

 following somite or icith the epiblast ; and it must he noticed that 

 tve find the foremost Anlage not exactly beneath the fourth myotome, 

 but always underneath its hind border. 



Figs. 8-17 represent sections throngh a still older em- 

 bryo of this stage, having about 20 somites. The epiblast {ep.), 

 the neural cord (n.), and the chorda (ch.) are essentially the same 

 as before. Being cut through somew^hat obliquely, the myotomes 



l)Such a case is very rare. In most specimens examined, the Anlage separated from 

 the myotome is fcinnd in many segments, so that we can hardly decide in whicli segment 

 the separation takes place first. 



