W. K. BROOKS ON THE GENUS SALPA. 211 



Plate XX, Figs. 5, 6 and 9, the ectoderm and endoderm of the stolon are 

 represented by a' and d' respectively. In the youngest of these stages, 

 Fig. 6, the thickened columnar ectoderm, a', of the region of the stolon 

 barely reaches to the edge of the germinal mass n, and this does not lie 

 in the stolon, but in the body cavity of the embryo. 



The series from which this figure was drawn was complete, and the 

 section is squarely in the middle of the region of the stolon, and shows 

 the greatest development which it lias reached at this stage. 



In the next older stage, Fig. 5, the thickened ectoderm, a', reaches 

 over something more than half of the germinal mass, and this is grad- 

 ually brought into the cavity of the stolon, not by migration, but by the 

 extension of the fold of the ectoderm until, at the stage of Fig. 7, it is 

 shut into the stolon between the endoderm d' and the ectoderm a'. 



In Plate XVI, Fig. 5, the fold of ectoderm has run back beyond the 

 germinal mass, and this is now in the cavity of the stolon, as is shown 

 by the transverse sections in Plate XX, but the process by which it 

 reaches this position is infolding and not migration, and the figures show 

 that it is set apart as a definitely specialized group of cells before it is 

 shut into the stolon. 



SECTION 2. Seeliger'' s Account of the Germinal Mass. 



The most extensive account of the germinal mass is the one given 

 by Seeliger (p. 11), and so far as the later stages in its history are con- 

 cerned, my work is no more than a verification of his, but as regards its 

 origin my observations are totally irreconcilable with his. 



I am confident that this disagreement is due to the fact that the 

 species which I have studied most thoroughly, Salpa pinnata, is excep- 

 tionally favorable, while the one Seeliger studied is peculiarly unfavor- 

 able for observations upon the early stages in the history of the stolon. 

 This is symmetrically placed in the embryo of Salpa pinnata, and sec- 

 tions may be made in any desired plane, even when it is too small to 

 isolate. As it is perfectly straight, any section in a series may be 

 directly compared with any other without difficulty. Furthermore, all 

 its parts are fully developed without any crowding, as the absence of a 

 twist prevents one-sided pressure. 



In Salpa democratica,- the twisting of the stolon begins at a very 

 early stage, and twists all its parts out of the plane of symmetry in such 

 a way that no two consecutive sections are morphologically parallel to 



