W. K. BROOKS ON THE GENUS SALPA. 225 



but cells, and the evidence all seems to show that they are neither 

 degenerating eggs, as Seeliger believes, nor eggs in process of trans- 

 formation into follicle cells, as Salensky holds, but migratory follicle 

 cells, undergoing degeneration and disintegration; and I regard them 

 as the food supply for the growing eggs. 



SECTION 6. Summary of tJie History of the Ovarian Eggs of Salpa. 



The first trace of the stolon of salpa is the germinal mass, which 

 makes its appearance in the young egg-embryo at the spot where the 

 stolon is afterwards developed. As this is formed, the germinal mass 

 is folded into it, and it elongates with the growth of the stolon. 



It is at first a simple aggregation of unspecialized embryonic cells, 

 multiplying by karyokinesis. While there is at first no differentiation 

 among its constituent cells, its later history shows that it represents the 

 testes, the eggs, follicles, and fertilizing ducts of the chain-salpae. 



It gives rise to nothing else; all its derivatives are concerned in 

 reproduction, and I have already given my reasons for believing that 

 its cells are morphologically or phylogenetically germ cells, although 

 the cells which give rise to the follicles and ducts are not germ cells in 

 the physiological sense. 



The root or proximal end of the germinal mass permanently retains 

 its embryonic character, and even in old stolons it is undifferentiated, 

 and its cells multiply by karyokinesis, but the distal portion soon 

 becomes differentiated into a peripheral layer of epithelial cells, which 

 retain their vegetative power, and continue to multiply by karyokinesis, 

 and a central mass of ova. As soon as the differentiation takes place, 

 and long before the young stolon shows any trace of the chain-salpa3, the 

 central cells lose the power of multiplication, and quickly assume the 

 optical characteristics of young eggs. They produce no "test cells," 

 and show no indications of cell multiplication. Their integrity as inde- 

 pendent cells is perfectly preserved from the time of their appearance 

 until they are fertilized in the bodies of the chain-salpae. 



They are at first immature, and they grow and ripen, but they are, 

 obviously, true eggs, physiologically as well as morphologically, from the 

 first. New eggs arise nowhere except at the base of the genital string, 

 and the outline of the egg nucleus is, from the first, sharp and uninter- 

 rupted, with no trace of nuclear buds. 



At first the eggs are crowded together, but as the stolon lengthens 



