W. K. BROOKS ON THE GENUS SALPA. 259 



Plate XXV, Fig. 6, H, m, and Plate XXXII, Figs. 7 M and 8 N, while an 

 outgrowth from the follicle in the middle line of the body of the chain- 

 salpa gives rise to the fertilizing duct, Plate XXV, Fig. 7, H, m, by means 

 of which the egg becomes attached to the wall of the atrium of the 

 chain-salpa, as shown in Plate XXXIX, Figs. 2, 3, 4 and 5 at x. 



Salensky's observations show that, before the development of the 

 embryo begins, a part at least of this duct becomes merged with the 

 follicle, so that the capsule of the fertilized egg consists of the follicle 

 plus part of the duct of younger stages. 



SECTION 2. The Nutrition of the Eggs. 



The follicle is bathed directly by the blood which circulates in the 

 ventral blood space of the stolon and in the bodies of the chain-salpa3, 

 and, at a very early stage, some of its cells become differentiated from 

 the others, as shown in Plate XXXI" Fig. 7. Their nuclei become 

 enlarged, and the chromatin loses its definite form and becomes vaguely 

 diffused, so that the nucleus now takes an uniform color from staining 

 fluids. These modified cells I regard as yolk cells, which, having become 

 charged with food absorbed from the blood, and having lost their vege- 

 tative power, wander in among the egg cells, where they become irreg- 

 ular and amoeboid, as shown in Plate XXXI, Fig. 5, and soon degen- 

 erate and break up into irregular, deeply stained globules and granules, 

 Plate XXXI, Fig. 6, which are assimilated by the growing egg cells, and 

 again laid down in their protoplasm as yolk granules. 



These migratory follicle cells are most abundant near the root of the 

 genital string, where the young eggs are growing rapidly, but they 

 become less numerous as the eggs grow larger, and they are seldom 

 found in the budding section of the stolon. 



No one could examine sections of the young stolon of any of the 

 species which I have studied without finding them, although they have 

 received little notice. They are quite different from the abortive eggs 

 which occur in older stolons of Salpa democratica and in its young buds. 



The migration and degeneration of follicle cells around a growing 

 egg as a provision for its rapid nutrition is not at all unusual nor 

 remarkable, and the evidence that it takes place in salpa is clear and 

 conclusive. It would not be necessary to dwell upon or to defend this 

 simple and natural interpretation of the facts, were not every part of the 

 life-history of tunicates a source of dispute and of contradictory and 



