54 W. B. SCOTT AND HENllY P. OSBORM. 



of the more important types which have not as yet been 

 observed, and the clearing up of the doubts which still 

 envelop other types, will fix the derivation of the notochord 

 from the hypoblast as a law, rather than as a feature posi- 

 tive in some cases, and with an exceptional origin from the 

 mesoblast in others. 



The Hypoblast. 



In Stage c the notochordal cells are continuous at the 

 sides, with the layer of hypoblast lining the yolk (see PI. 

 IV, fig. 6). In Stage d this continuity is completely 

 broken, the layer appears as a long narrow row of cells, 

 flattened against the sides of the notochord, but not enclos- 

 ing it below. Elsewhere this layer shows no new features. 

 In Stage e, however (see PL V, fig. 8), the cells have 

 grown down and meet below, completely surrounding the 

 alimentary canal and shutting it off from the notochord. This 

 process is interesting, as it shows that, while the original 

 upper lining is mainly absorbed by the notochord, the per- 

 manent upper lining is formed from the yolk hypoblast cells, 

 and that now almost the entire layer is formed of this 

 secondary hypoblast, the bulk of the primary or invagina- 

 tion hypoblast having gone to the notochord. The hypo- 

 blast grows under the notochord, in much the same way in 

 the Lamprey, but at a somewhat earlier stage. In most of 

 the other forms there remains throughout, a thin layer of 

 cells intervening between the notochord and the yolk. 



Body Cavity and Somites of the Head. 



As already mentioned, the growth of the mesoblast is 

 from behind forward, and in Stage a (PI. IV, fig. 4) we see 

 that in the head region the mesoblastic plates do not meet 

 ventrally. They gradually thin out forwards and end near 

 the blind termination of the alimentary canal. At this 

 period the mesoblast is quite thick, and is composed of nu- 

 merous cells of spherical shape, but exhibits no tendency to 

 become divided into somatic and splanchnic layers. In 

 Stage B, however, the cells have arranged themselves into 

 two layers, and quite a cavity has appeared between them 

 (PI. IV, fig. 5). As yet this change is confined to the head, 

 and so there is a cavity in the head on each side of the mid- 

 dle line, contained between the somatic and splanchnic 

 layers of the mesoblast. These cavities, therefore, are parts 

 of the pleuro-peritoneal cavity, and when that is formed in 

 the body, will be directly continuous with them. As in the 



