44 



MESOBLAST IN PETROMYZON. 



The obscurity of the interrelation between the two cases is doubtless 

 caused by the modified mode of formation of the mesoblast in the higher 

 Craniota. If such be the case, there is some excuse in describing the 

 precise mode of the metamerie segmentation of mesoblast in Petromyzon, 

 which forms the subject of present communication. 



As discovered by v. Kupffer ') and confirmed by myself 2) as far back 

 as ten years ago, the mesoblast in Petromyzon is brought about, in the 

 anterior region, by a simple folding of the entoblast, just as in Amphioxus. 

 The question above referred to must be, therefore, decided by a careful 

 observation of the metamerie segmentation in this part of the mesoblast. 

 The harmony between the two cases, if such truly exist, ought to be 

 seen here. 



In the course of differentiation of the mesoblast in the head fold of 

 Petromyzon, I have found great harmony between the case in Potromyzon 

 and that in Amphioxus ; I will enumerate it in the following lines : — 



!) Before the metamerie segmentation, the mesoblast is a sack of 

 the cylindrical epithelium, which is, on a cross-section, of a triangular 

 shape and fills up the space on either side of the chorda and neural cord, 

 bordered upon laterally by the epiblast and ventrally by the roof of the 

 enteric canal. 



2 ) The metamerie segmentation of the mesoblast is not confined to 

 the proximal part of the layer adjacent to the axial organs, but goes 

 through the whole substance of it. 



3 ) In a mesoblastic somite thus formed, the proximal (dorsal) and 

 distal (ventral) portions are distinguished : they constitute respactively 

 the scleromyotome and a part which loses later the segmental character 

 and, by the subsequent growth of which the non-segmented lateral plate 

 is brought about. 



4 ) The scleromyotome is converted, having been cut off from the 

 rest of the mesoblast, into a polyhedral case which is interposed between 



1) V. Kupffer, Die Entwickelung von. Petromyzon Plaaeri : Arch. f. tnikrosk. Anat., 

 Bd. XXXV, p. 501, 1890. 



2) S. Hatta, On the Formation of the Germinal Layers in Petromyzon : Jouvn. Coll. 

 Sc.Imp. Univ., Tokyo, vol. v. 1891. 



