MORPHOLOGY OF CYCLOSTOMAïA. 321 



through the twelfth somite ; we can find no marked difference 

 between tlie Anlage in the seventh somite and in this. The 

 segments Ivins; still farther backwards are not cut throuo;h exact- 

 ly transversely in this same series of sections, owing to the cause 

 stated above (pp. ol2 and 315), so that we can not trace the dif- 

 ferentiation of the mesoblast from the anterior to the posterior 

 part in this one series. But 1 could demonstrate from several 

 other series of sections that the Anlage of the pronephros is, in 

 the present stage, found in no less than 15 somites. 



Figs. 9-11 represent the contiguous sections through the 

 intersomitic portion, on the right side, between the first and 

 second Anlagen, i.e. between that of the fourth, and that of the 

 fifth, somite. Fig. 9 is from the section next behind that shown in 

 lig. 8 ; the portion [cd.) lying j^roximal to the lateral plate {Im.) 

 presents no longer a weak appearance as in younger embryos 

 (see the statement on p. 317 and figs. 4 and 7), but is occupied 

 by a compact cellular structure (cd.) which suddenly passes over 

 into the loosely composed lateral plate {Ivi.). Fig. 10 is from the 

 section next posterior to fig. 9 and next anterior to the second 

 Anlage represented in Jig. 11 and shows almost the same con- 

 dition as in lig. 9, with respect to the structure in the proximal 

 portion of the lateral plate. In other words, in the intersomitic 

 portion between the first and second Anlagen, a cellular coj'd 

 has become established, which connects these two Anlagen. It 

 is this cord which gives rise to the collecting duct or Sammelrohr 

 of RÜCKERT ('88), putting all the pronephric tubules in communi- 

 cation. 



On the left side of figs. 9, 10, and 11, the contiguous sec- 

 tions through the intersomitic portion l)etween the Anlagen second 

 and third, are represented. In figs. 9 and 11, the condition of 



