314 s. H ATTA : 



neural cord is interposed in the median line between the right and 

 left halves of the mesoblast, underneath the epiblast composed by 

 of a single row of columnar cells. The epiblast is always limited 

 with a sharp contour against the mesoblast. The latter consists, on 

 each side, of a dorsal id.), a ventral {y.) and a median {m.) row 

 of cell.«. The dorsal row represents the parietal, and the median 

 and ventral rows together form the visceral, layer. Both the 

 visceral and parietal layers of the mesoblast show, in the prox- 

 imal portion, a regular arrangement of a high columnar epithel- 

 ium, while distally this arrangement is more or less disturbed. 



The above structures are localized in a small portion on the 

 dorsal aspect of the ovoid embryo, the remaining larger part 

 being taken up by yolk-cells compactly loaded with yolk-granules. 



In this early stage, we can detect, therefore, neither in the 

 epiblast nor in the mesoblast any structure whatever which is to 

 be regarded as the rudiment of the pronephros. 



Period 1. 



In the next following stage, which corresponds approximate- 

 ly to the early part of Stage ii {loc. cit., fig. 1, B), certain alter- 

 ations are met with in the mesoblast. The most important of 

 these is its metameric segmentation. This process first begins at 

 the neck^^ and proceeds both forwards and backwards. At the 

 present stage, there are found 16 or more mesoblastic somites.^^ 

 At about the time when this process has extended to the 

 anteriormost section of the mesoblast a second change arises in 

 the mesoblast, viz. the first appearance of the pronephros. 



l)The term neck is used for tlie sake of convenience to designate the slender region where 

 the liead-fold ])asses over into tlie hind ghjbiüar part. 



2) The exact number of tlie somites can not be reckoned, for the metameres become 

 indistinct posteriorly. 



