ox THE FORMATION OF THE GERMINAL LAA'ERS IN FETROMYZOX. 143 



roof of the enteri«- cüNity. ^Fy ol).ser\;!ti()ij.s lead nie to coucliide that 

 the process of the inesoblast formation begins in the region round the 

 blastopore and proceeds forwards as the gastral mesobJast. This is 

 contrary to the conclusion of Knpffer and of Shipley. 



('. The lateral and ventral portions of the mesnhla.'^t. In the 

 ce})ha]o-eervical region which is free from yolk-cells, the lateral and 

 ventral portions of the mesoblast are Ibrmed Ijy the simple downward 

 growth of tlte distal or outer extremity of the mesoblastic fold. In the 

 region of the trunk, tlie process is somewhat moditied on account of 

 the presence of yolk-cells. The gastral mesoblast in this region 

 does Jiot extend itself laterally and ventrally as a sohd slieet (jf cells. 

 The distal border of the gastral mesol)last gives rise to free and 

 separate cells, which penetrating l)etween tlie \'olk-cells and the epiblast 

 establish in time a complete layer of a single row of cells. 



This has been sufficiently demonstrated by Shipley Clor, oil.) At 

 the same tune, the jK'i-istomal mesoblast becomes free from the li[)s (»(" 

 the l)lastopor('. and as separate scattered cells pu>h their wav between 

 the epiblast and the yolk-cells and contrilnite to the forma.tiou of the 

 mesoblast in the part ^•entral io the Ijlastojxjre. Tin's eircumstauce 

 may be the reason why the previous investigators haxc, o\ erlooked the 

 peristomal mesoblast. Thus, at about the stage given in tig. 18, the 

 whole mesoblast has no connecti()n with any part of the primary lay- 

 ers. The subse(|uent growth (^f the layei' is solely effected by cell- 

 multi])lication within the layer itself. The posterior growth of the 

 mesoblast is liroiight aljout not Ijy any fresh addition from the hypo- 

 blast or any other struct ure but (jnly 1)\' the extension of its own 

 territory. 



I). TIte metamerie segmentation of the mesohlast. The metameric 

 seo'mentation of the inesoblast begins first in tlie doi-sal region and 

 proceeds ijoth bafdvwards and Ibi-wards. When the seginentation has 



