MOKPHOLOGY OF CYCLOSTOMAÏA. 389 



tii(3 myotome is not yet cut off from the lateral plate, just as in 

 Petroiinjzoii. The foremost of them is found in the hind part of 

 the third or fourth body-somite. The development of the Anlage 

 in each segment agrees also with that in Pelromyzon ; for he says: 

 " Der Segmentalwulst zeigt in vorliegende Stadium (Stad. i) 

 regelmässig die stärkste Entwicklung in seiner mittleren Ab- 

 schnitt, also ungefähr im Bereich des dritten des ihm gehörigen 

 Somiten, und verjüngt von da allmählich nach seinem vorderen 



und hinteren Ende zu " {loco cit., pp. 210). 



The account given by Ruckert is essentially confirmed by 

 later investigators such as van Wyhe ('89)^^ Rabl ('96), and others, 

 although they differ from one another in the interpretation of the 

 facts and in some unimportant points. Rabl looks upon the 

 Anlage of the pronephros (his Vornieren wulst) as the ventral 

 poition of the somite just as Ruckert does, ^Yhile it is, ac- 

 cording to VAN Wyhe, the product of the lateral plate (his 

 Hypomer). This is, as it seems to me, not a contradiction in the 

 facts, but in the terms used ; for van Wyhe states : " Da nun der 

 Pronephros, wie, spätere Entwicklungsstadien zeigen, ein Produkt 

 der Seitenplatte ist, während der unmittelbar dorsal davon liegende 

 Theil des Mesoderms zur Mittelplatte gehört, ist die Segment- 

 irung des Mesoderms bei Selachiern also nicht auf die jMyotom- 

 platte beschränkt, sondern erstreckt sich auch auf die Mittelplatte 

 und den dorsalen Theil der Seitenplatte " {loco cit., pp. 474-47Ö). 

 The fact is, therefore, no other than that the portion of the meso- 

 blast dorsal to the ventral limit of the Anlage of tlie pronephros 

 undergoes segmentation, and the portion ventral to this point 

 remains unsegmented, constituting the lateral plate. I will, in 



1) The em})i7o, in which tlie first traces of the pronephros is seen, is, according to 

 VAX Wyhe, in a stage witli 27 somite-, whereas Rabl has seen in an embryo o{ PriMiiiius 

 with 25 somites. 



