NATIONAL ACADEMY BIOGRAPHICAL MEMOIRS VOL. VII 



germinal cells into the chain individuals, where they reach 

 maturity. Since these germinal cells give rise to both ova and 

 spermatozoa, both solitary and chain forms may be considered 

 potentially bisexual, though the solitary forms never contain 

 mature germ cells. 



2. In the formation of the salpa chain a stolon, or stalk, 

 which grows out from the solitary salpa is constricted at in- 

 tervals, thus giving rise to the members of the chain. The 

 stolon is bilaterally symmetrical, its planes of symmetry coin- 

 ciding with those of the solitary salpa which bears it, and, at 

 first, the planes of symmetry of every member of the chain 

 coincide with those of the stolon and of the solitary salpa. Very 

 soon, however, a twisting of the chain occurs which leads to 

 the formation of a double row of salpse, each row with the 

 dorsal surfaces of its members turned outwards, while the ven- 

 tral surfaces of the two rows are turned toward one another, 

 and the right sides of one row and the left sides of the other 

 are turned toward the base of the stolon. 



3. The salpa embryo, which develops from the fertilized egg 

 into the solitary salpa, "is blocked out in follicle cells which 

 form an outline or model of the general features of the embryo. 

 While this process is going on the development of the blasto- 

 meres is retarded, so that they are carried into their final posi- 

 tions in the embryo while still in a very rudimentary condition. 

 Finally, when they have reached the places which they are to 

 occupy, they undergo rapid multiplication and growth and 

 build up the tissues of the body directly, while the scaffolding 

 of follicle cells is torn down and used up as food for the true 

 embryonic cells. ,, 



4. The salpa embryo is connected by a placenta with one of 

 the members of the chain, and is nourished by placenta cells 

 which migrate from the placenta into the embryo. The pla- 

 centa is thus an organ which nourishes the embryo, not by the 

 transfusion of blood, but by means of giant cells which in the 

 placenta receive nourishment from the blood of the parent, and 

 then migrate into the embryo, there to break down and supply 

 food to the embryo. 



But Brooks' work on salpa is not merely a description of the 

 anatomy, embryology, and classification of these interesting 



56 



