MUSEUM OF COMPAEATIVE ZOOLOGY. 271 



fereut criteria for determining the boundaries of the protovertebrae. 

 There can be no doubt that the earlier pronephric thickening is made 

 up of metameric constituents ; but I should be unwilling to regard 

 all segmented mesoderm as belonging to the protovertebrae. On the 

 contrary, I am of opinion that the ventral extent of the protovertebrse 

 is for the first time defined when the longitudinal constriction appears 

 which divides the primitive coelom into protovertebral cavity and pleuro- 

 peritoneal or (secondary) body cavity. When such a definite line of 

 demarcation has been established, the remnant of the pronephros in 

 Selachii, as well as the functional pronephros in Amphibia, remains con- 

 nected with the latter space. The remaining points of difference relate 

 to the number of tubules involved, — which, as we have seen, varies 

 even within the class of Amphibia, — and to their position with refer- 

 ence to the somites. The latter feature seems to me to be at once 

 difficult to determine and of minor importance. 



Before the conclusion of this paper I shall endeavor to indicate how 

 the glomus of Amphibia may possibly have been derived from the 

 type of structure which is described by Eiickert for Selachians and by 

 Boveri ('90) for Amphioxus. 



The results gained by van Wijhe ('89) do not seem to me to differ 

 from those of Riickert in many respects which are of importance for a 

 comparative study. The great divergence of their descriptions in the 

 case of many details seems to me to be occasioned mainly by the peculiar 

 conception which Ruckert holds of the relations between the protover- 

 tebral and the lateral mesoderm. For these details and for the hotly 

 contested questions of priority, I must refer to the original papers (van 

 Wijhe, '86, '87, '88% '88^ '89, Ruckert, '88, '89), and consider here those 

 features only which merit special attention because of their bearing on 

 the general questions of homology. Van Wijhe denies positively the 

 participation of the ectoderm in the formation of the pronephric thicken- 

 ing ; and he claims that the ostium abdominale is formed from the pro- 

 nephros by the fusion of the nephrostomes. Finally, structures which are 

 supposed by him (pp. 480-482) to represent the pronephric glomeruli 

 of Eiickert are described as occurring on both sides of the body, not, 

 as affirmed by Riickert, on the right side alone, and van Wijhe inclines 

 to the view that they are actually equivalent to the glomi of Amphibia. 

 The body described by van Wijhe consists of a vascular rod, which passes 

 obliquely from the dorsal to the ventral lip of the pronephric pouch, and 

 represents the last trace of the partition between two peritoneal open- 

 ings, which have not yet fused. RUckert's description is not entirely 



