W. K. BROOKS ON THE GENUS SALPA. 251 



placenta in position, and for maintaining a free open channel for the 

 blood which circulates into and out of the placenta. In Salpa pinnata it 

 not only serves this purpose, but it is also a most remarkable mechanism 

 for the nutrition of the embryo, although it effects this indirectly ; fur- 

 nishing by means of a most noteworthy and remarkable process of 

 migration and degeneration, the food for the follicle cells of the roof of 

 the placenta, which, in their turn, migrate into the body cavity of the 

 embryo and there degenerate. 



The nutrition of the salpa embryo is a most interesting field for 

 research, and I have not been able to devote to it enough space nor a 

 sufficient number of figures to give more than a general outline, but I 

 hope that this will incite some one to more exhaustive study of the 

 subject. 



In Salpa pinnata, as I have stated, the supporting ring soon bends 

 inwards towards the roof of the placenta, losing its continuity with the 

 epithelial capsule, and becoming very intimately united to the roof of 

 the placenta, as shown in Plate XXII, Fig. 1. Its cells, which are bathed 

 by the blood which circulates in the placenta, grow and multiply with 

 most remarkable rapidity by direct division. Cell multiplication goes 

 on so fast that every nucleus in each section is in some stage of division, 

 and no resting nuclei are to be found. 



This is well shown at 23 in Plate XXXV, and more highly magnified 

 in Plate XLIV, Fig. 3. Growth takes place with equal rapidity, and as 

 the cells grow they move upwards towards the roof of the placenta, 

 their nuclei, which keep on dividing, growing to an enormous size, and 

 becoming so filled with chromatin granules that in stained specimens 

 they are almost opaque. At the upper edge of the supporting ring the 

 cells separate from each other and degenerate, giving rise to a stream 

 of deeply stained granules, irregular particles, and fragments of nuclei, 

 which are swept along, by some means which I have failed to discover, 

 towards the central region of the roof of the placenta, where they serve 

 as food for the enormous placenta cells which are shown at 29 in Plate 

 XVIII, Fig. 5, and in many of the other figures. It seems probable that 

 after the embryo of Salpa hexagona is born the cells of its supporting 

 ring are used up as food in the same way, but I have seen no indication 

 of this before birth, although the growth and migration of the follicle 

 cells, Plate XLV, Figs. 2, 3 and 4, 29, are as conspicuous in this species as 

 they are in Salpa pinnata. 



