No. 3.] THE GASTRULATION OF AMPHIOXUS. 58 1 



ventral plane and shows the difference in the ectoderm at the 

 dorsal and ventral lips of the blastopore. At the dorsal lip 

 the endodermal cells are, as before, lighter and contain less 

 yolk and differ little from the ectodermal of the outer surface ; 

 in fact, there is really no histological difference between the 

 cells just outside and just within the lips of the blastopore 

 over the dorsal side. It is to be noticed that there are more 

 of the lighter cells on the dorsal wall than in the preceding 

 stage. This might result either by a continued rolling in of 

 outer cells, or by a multiplication of cells already inside, or by 

 both processes combined, or even by differentiation of the cells 

 anterior to the clear region. It is certain that the endodermal 

 cells continue to divide during development, and we have seen 

 no evidence of an actual inrolling. Whether the division of 

 the cells in this region is, in itself, sufficient to account for 

 the increase in the number of cells is a point almost impossible 

 to determine. 



The same section (PI. XXXIII, Fig. 2) shows also the char- 

 acteristic shape of the embryo at this stage ; over the dorsal 

 side the embryo is less curved than over the ventral side. The 

 deepest part of the invaginated endoderm is somewhat toward 

 the ventral side. The effect produced is that the ventral part 

 of the endoderm is thrown over toward the ventral side of the 

 embryo. Surface views of entire embryos show the same 

 asymmetry in the gastrula. On the ventral side of this sec- 

 tion (PI. XXXIII, Fig. 2) there is an abrupt transition between 

 the large yolk-bearing cells and the smaller cells that continue 

 out into the ectoderm. Whether these smaller cells that are 

 partly turned into the blastopore belong to the endoderm or to 

 the ectoderm can only be determined by their fate. They will 

 be considered later. 



There is generally present on the ventral edge a dilatation 

 of the segmentation cavity that is characteristic for this period ; 

 on the dorsal side the ectoderm and the endoderm are almost 

 in contact. 



An older stage is shown in PI. XXXIII,. Fig. 3. The 

 embryo is now cup-shaped and the archenteron much deeper 

 than before. The deepening is, in part, the result of the 



