A SKETCH OF HIS LIFE 27 



budding in Salpa with that in Pyrosoma and in the Clavelinidae 

 among the ascidians. The stolon is bilaterally symmetrical, its 

 planes of symmetry coinciding 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 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 outward while the 

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

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

 those of the other row are turned toward the base of the stolon. 



He showed that the placenta of Salpa does not resemble the 

 mammalian placenta in its method of nourishing the embryo, 

 but that certain cells in the placenta, taking nourishment from 

 the blood stream of the nurse (the chain. Salpa), grow to very 

 large size, then lose their connection with the placenta and wander 

 to different parts of the embryo, where they break down and nour- 

 ish the growing tissues of the embryo. 



Salensky's generally accurate work upon the embryology of 

 many species of Salpa contained one fundamental error, since 

 he described the embryos as arising not from true blastomeres 

 but from follicle cells, the blastomeres degenerating early in the 

 developmental history. Brooks, recognizing the improbability 

 of any such conditions, succeeded in tracing the development of 

 the egg itself until from its blastomeres the organs arise. He found 

 that the blastomeres develop very slowly; that the follicle cells, 

 on the other hand, proliferate very rapidly and take on the form 

 of the rudiments of the several organs, the organs being thus 

 blocked out in these extra-embryonic cells, while as yet the blas- 

 tomeres are very few in number. Later the blastomeres multiply 

 and pass into the different parts of the mold thus formed for them 

 by the follicle cells, and gradually use as food the degenerating 

 follicle cells that surround them. 



In his latest, unpublished work he traced the cleavage of the 

 egg; he found a clear gastrula arising by invagination from the group 

 of blastomeres; he observed the hollow dorsal nerve tube, finding 

 it at first considerably elongated; he found a postero-dorsal rod 



