46 



MESOBLAST IN PETROMYZON. 



As I have already mentioned 1), it is a fixed fact that the line 

 of the separation of the scleromyotome (the Urwirbel of Hatschek) from 

 the rest of the mesoblast agrees with each other in both cases : it divides 

 the segmented part of the mesoblast into the dorsal (Urwirbel) and the 

 ventral (Seitenplatten) halves. The Urwirbel of Amphioxus undergoes 

 exactly the same fate as the scleromyotome of Petromyzon. In Pet- 

 romyzon, the ventral half forms, as above stated, the Anlage of pronephros 

 and the coelomic projection ; this point has not yet been made clear in 

 Amphioxus. But the reasonable suggestion of Boveri 2) makes it intel- 

 ligible that this section of the mesoblast should give rise to the formation 

 of the Nierencanälchen, that is, the excretory organ of segmental 

 character. 



The results arrived at in Petromyzon differ from those in the higher 

 Craniota concerning the extent, to which the process of the segmentation 

 takes place. In the Craniota the segmentation is confined to a small 

 proximal part, while in Petromyzon the whole breadth of the layer 

 undergoes the process. But this difference is, I think, easily explained 

 by a more precise examination of the stage, in which the segmentation 

 takes place. In the higher Craniota, this process comes to an end only 

 in later stages when the mesoblast, i. e. lateral plate, has grown consid- 

 erably. While the segmentation is going on in the proximal part, the 

 lateral plate itself grows distally. And thus a phenemenon which leads 

 one to think of the process confined exclusively to a small proximal part 

 alone comes into view. The segmented and non-segmented parts uudergo 

 essentially the same fate as in Petromyzon. 



The differentiation of the mesoblast in Petromyzon holds, in this 

 respect, very important position in the morphology of the Chordata. In 

 earlier stages, it is exactly parallel with that in Amphioxus, and in later 

 stages, it shows the feature seen in the higher Craniota. We can thus 

 observe in one and the same animal, Petromyzon, the stages, through 

 which the primitive state of the mesoblast in Amphioxus has developed 



1) S. Hatta, Contrib. Morphol. Cyclost. ii. : Journ. Coll. Sc. Imp. Univ., Tokyo, vol. 

 xiii. 1900. 



2) T. Boveri, Die Nierencanälchen des Amphioxus : Zool. Jahrb. vol. v. 1892. 



