320 s. HATTA : 



section throngli tlie anterior border of the somite. This phase 

 of constriction is doubtless earlier than that shown in fis;. 3. The 

 right half of fig. IG and the left of fig. 14 is from the section through 

 the mid-plane of the next following somite, i.e. the seventh. 

 The Anlage of the pronephros {a.pn.4) is not yet cut off from the 

 myotome {mi.VII), but the process is beginning as shown by a slight 

 constriction and an inclination to arch out, while the suddenly 

 weakened lateral plate {Im.) forms the distal (ventral) continuation 

 of it. This feature of the mesoblast reminds us of the youngest 

 stage of the Anlage described above (compare with figs. 2 and 5). 

 In the section passing through the anterior or the posterior border 

 of the somite too, the same condition of the Anlage, as on the 

 rio'ht side of fio". 14, is observed. 



From the facts mentioned above, it is easily understood that 

 the separation of an Anlage from a myotome begins with the 

 constriction which takes place at the anterior and the postej'ior 

 border of the somite, and that the middle portion is the last to 

 be cut off, so that the cavity of the myotome {myocoelome) com- 

 municates with the peritoneal cavity, during some time, through a 

 narrow passage at the middle point. 



Myotomes when cut off from the Anlage of the pronephros 

 assume a pentagonal form (see fig. 3) constructed of a dorsal, a 

 lateral, a ventral, and two median sides, each of which is composed 

 of a regular row of tall cylindrical cells. The dorsal and lateral 

 rows of cells constitute the parietal layer of the myotome, while 

 the three other sides represent the visceral layer (compare with 

 the description on p. 314). 



For about ten segments behind the seventh somite, the 

 Anlage of the pronephros shows the same condition as that seen 

 on the right side of fig. 16. Fig. 17 represents a section 



