W. K. BROOKS ON THE GENUS SALPA. 285 



1891) has discovered and minutely described the migration of the follicle, 

 but he has failed to trace the history of the blastomeres, and believes 

 that these degenerate and disappear, and that the embryo is built up of 

 follicle cells. I find that all the follicle cells are ultimately used up as 

 food, and that the true embryo is formed from blastomeres after the 

 analogy of the rest of the animal kingdom. 



SECTION 2. The Aggregated Salpce. 



During their development the aggregated salpae undergo complicated 

 changes of position, which render the interpretation of sections very 

 difficult, and as both Salensky (Morph. Jahrbuch, 1877, III) and Seeliger 

 (Jena. Zeitsch., 1885) have totally failed to understand these changes, 

 their accounts of the origin of the aggregated salpae have no permanent 

 value. 



I pointed out, in 1886 (Studies from the Biological Laboratory, Johns 

 Hopkins University, 1886, pp. 398-414), that the salpa-chain is, morpho- 

 logically, a single row of salpae, all in the same position, with their dorsal 

 surfaces proximal or towards the base of the stolon, and their right sides 

 on its right. The account of the origin of the aggregated salpae, which 

 is given in this memoir, is simply an amplification and expansion of the 

 statement which, in 1886, I made briefly and in outline. 



The stolon is bilaterally symmetrical, its plane of symmetry is funda- 

 mentally identical with that of the solitary salpae, and the rudiment of 

 each aggregated salpa is bilaterally symmetrical in the same plane, 

 although the secondary changes begin very early and convert the single 

 row into a double row, which comes to consist of a series of right-hand 

 salpae and a series of left-hand ones placed with their dorsal surfaces out, 

 their ventral surfaces towards the ventral surfaces of those in the oppo- 

 site row, and with the left sides of those on the right, and the right sides 

 of those on the left towards the base of the stolon. In order to illustrate 

 these secondary changes of position let us represent the series of salpae 

 by a file of soldiers all facing the same way. Now imagine that each 

 alternate soldier moves to the right, and the others to the left, to form 

 two files still facing the same way. Now let them face about so that the 

 backs of those in one row are turned towards the backs of those in the 

 other row. They will now represent two rows of salpae in their secondary 

 positions. 



To make the illustration more perfect, suppose that, instead of step- 



