DEVELOPMENT OF SPELERPES BILINEATUS ' 205 



The notochord cells, n, are distinctly differentiated, but in the 

 lateral roof, the differentiation into mesoderm and endoderm is 

 scarcely discernible in sections and does not show in the figures. 

 Fig. 40, the 56th section in 105, shows an advance in the differ- 

 entiation of germ layers. The archenteron, a, has begun to de- 

 crease in size and its roof to thicken. In the mid-dorsal line the 

 notochord cells, n, are distinct. As the section w^as somewhat 

 oblique, it shows the state of affairs in two parts of the egg. On 

 the left (the more posterior), the mesoderm cells, m, form a dis- 

 tinct layer of small polygonal cells. The endoderm, en, is a single 



Figs. 40 and 41 



layer of cells whose long axes are, as a whole, directed radially 

 to the archenteron, in contrast to the end-to-end arrangement, 

 previously existing. Near the chorda the two layers fuse with 

 one another and also with the notochord. There is no trace of 

 a cleft leading from the archenteron into the mesoderm, therefore 

 no indication of archenteric pouches. On the right side of the 

 egg, it is difficult to separate endoderm from mesoderm cells, 

 except, perhaps, by their position. In the anterior parts of the 

 embryo, conditions are not very different from that shown in 

 fig. 39, except that the archenteron has become smaller and its 

 roof thicker. 



At the next period, fig. 41, the 49th section of 102, the surface 

 views correspond to plate 1, fig. 7. The mesoderm has extended 

 ventrally and is entirely distinct from the other germ layers, 



THE AMERICAN' JOURNAL OF ANATOMY, VOL. 12, NO. 2 



