324 s. HATTA : 



plainly dernonstrate a progressive change undergone by the 

 pronephric Anlage. The Anlage on the right side [a.pn.'Z) 

 presents a feature much like that seen in fig. 3, notwithstanding 

 some points of progress. The Anlage on the left side (a.pn.l), 

 however, shows a considerable progress ; it has become much more 

 compact by the active multiplication of its component cells. 

 Owing to mutual pressure, the cells are compressed and their 

 nuclei are regularly arranged, describing together an ellipsoidal 

 figure. The inside of the ellipse encloses a comparatively large 

 lumen, which is standing in connection with the body- cavity 

 represented, at the present stage, only by the boundary line of 

 the parietal and the visceral layer of the lateral plate (Im.). 



In a little more advanced embryo, the cross-sections of which 

 are rej)resented in figs. 20-31, the neural cord {n.), the myotomes 

 (mt.), and the Anlage of the pronephros show some progress as 

 compared with those described in the preceding pages. The epiblast 

 (ep.) consists, as in the embryo just described, of a single layer 

 of cubical cells and is limited by a sharp line against the struc- 

 tures l^elow. The component cells of the neural cord^^ become 

 arrariged in two layers, leaving, in the anterior secti(jn of the cord, 

 a vertical fissure-like lumen in the median line of the cord, which 

 represents the beginning of the central canal (figs. 20-21, &c.). 

 The posterior part of the cord is still solid, although the position of 

 the central canal is marked by a vertical line produced by cell- 

 boundaries (tig. 26) just as described in the foregoing pages. The 

 myotomes are, in the anterior region, likewise enlarged, probably 

 owing partly to multiplication of component cells and j^artly to 



]) Owing to liie same cause as the sections rejuesented in Figs. 2 and o, the vertical 

 (liaiueter of tlie neural cord in Figs. 20-23 is shown somewhat longer than it is in reality. 



