:\rORPHOLOGY OF CYCLOSTOMATA. oZO 



the loosening of the composition of the tissue'^ and assnme the 

 lijhape of a scalene triangle (figs. 20-23) ; tlie median side of the 

 triangle (mus.) represents the visceral, and the two otlier sides 

 [ruf.) the parietal, layer of the myotome. In the posterior region, 

 they are yet of a compact structure of a pentagonal form, en- 

 closing a cavity (figs. 25-31, mt.VII-X). 



The anteriormost Anlage of the pronephros is found as before 

 under the hind part of the fourth myotome, the section of which 

 is represented in fig. 20 {apii.l). It shows a considerable 

 development : the component cells, which are of high columnar 

 character are no longer compressed, but the tissue is more or less 

 loosened. Thus the Anlage itself is distended, and its upper 

 (dorsal) angle becomes acute and grows in between the epiblast 

 and the myotome. The internal cavity of the Anlage also be- 

 comes conspicuous. The Anlage of the pronephros under the 

 next posterior myotome (the fifth) is not so advanced as in 

 the last somite (the fourth). In fig. 22 is shown the section 

 through the hind part of this somite and of the pronephric An- 

 lage belonging to it {mt.V and a.pn.2), a section through the 

 mid-plane being unfortunately wanting in this series of sections. 

 The next posterior Anlage is found just under the sixth myotome 

 and represented in fig. 24 {a. pa. 3) together with the hind border 

 of the myotome {mt.VI). The Anlage shows a compact structure 

 wdiich is probably due to a rapid multiplication of the constitu- 

 ent cells. The next following Anlage of the pronephros is found 

 beneath the seventh myotome (fig. 26, a.j^n.é). It shows no 

 further development than the separation of it from the myotome 

 and the fusion at the retrenched ends of the two lavers of 

 mesoblast : it is in the same stage of constriction as that in the 



l)See the foot note in p. 319. 



