136 



s. HATTA. 



For the sake of convenience, I shall treat of the meso])last under 

 the following heads : — 



A. The mesoblast in the neighbourhood of tlie blastopore 

 (or the peristomal mesoblast of Rabl) 



1). The mesoblast on each side of the dorsal axial organs 

 (or the gastral mesoblast of Rabl) 



C. The lateral and ventral portions of the mesoblast. 



D. The metameric segmentation of the mesoblast. 



A. Tiie inesobhist in ihe neighbourhood of the blastopore, (or the 

 Peristomul Mesoblast of Rid:)l) has never yet been described in Petromyzon 

 Ijy ])revious writers. I have, however, obtained sections from which it 

 appears to me safe to conclude that meso])last cells are budded out from 

 all arouiid the lip of the blastopore except the dorsal median point where 

 the epiblast turns round to join tlie hypoblast. Fig. 5 represents the 

 median sagittal section through an egg in which the gastrulation is 

 already completed. In this ligure, we find tlie floor of the invaginated 

 tube fn'med, for a sh(n"t distance from tlie ])lastoporic opening, of 

 specially small cells. In continuation with this stretch of small cells, 

 there is seen a Avedge-sha[)ed aggregation of similar small cells (^mes. 

 p.) extendi iig ventrally between yolk-cells and the epiljlast. It is 

 marked off from the epibdast i)y a slit, while it lies in contact witli 

 yolk-cells and is marked off fr(3m these only by the small size of its <-oyi- 

 stituent elements. In this mass, as well as in the stretch of sn.ia 11 

 cells forming M.ie floor of the invaginated tube, many karyokinetic 

 figures are seen arid testify to the fact that cells are very raj^idly 

 mu]ti])lying in tliis region. This mass is in my opinion tlie pcristo- 

 iiuil iiicsoblas! (('f Rabl). 



Fig. G is a section from the same series passing through the 

 lateral li]) of the Idastopore. Mere we find a mas^ of similar small 



