W. K. BROOKS ON THE GENUS SALPA. 283 



reproductive cells and have been specialized for another function. I 

 have also shown that this function is nutritive, and that they all become 

 food for other cells and tissues. The follicle cells have great vegetative 

 power, and while some are used up as food very early, others continue 

 to grow and multiply until the embryo has reached an advanced stage 

 of development, although ultimately their fate is the same. 



I have shown also that while some of them, page 271, wander 

 through the body cavity of the embryo as amoeboid cells before they 

 become vacuolated and disappear, others, or their nuclei at least, page 

 273, push into the substance of the protoplasm of the blastomeres, 

 during the later stages of segmentation, and supply them with food, 

 while still others, page 223, enter the substance of the ovarian eggs, 

 before fertilization, and furnish the food which they assimilate and turn 

 into yolk. 



These latter cells, the follicle cells which migrate into the ovarian 

 ova, are "test cells," and as their history is only a small part of the 

 history of the migrating follicle cells of salpa, the extension of the 

 period of migration through the stages of segmentation into the embryo, 

 and its persistence up to a period when the embryo is well advanced in 

 its development, give us a new basis for discussing the nature of " test 

 cells." 



I have no desire to enter the interminable " test cell " controversy, 

 nor to make a new addition to its voluminous literature, and I shall not 

 venture to open anew the dispute which has already exhausted the 

 patience of embryologists, although I believe that the life-history of 

 salpa furnishes evidence of an entirely new sort. 



The history of the discussion up to the year 1889 may be found in 

 Davidoff 's memoir on Distaplia (16, pp. 138-190), and it is not necessary 

 to review it again. 



The wandering cells which I have described among the young ova 

 of salpa are "test cells" identical in their origin with those which 

 Ulnanin (pp. 7, 40) has figured and described in the young egg of doli- 

 olum, and the history of the later stages in the life of the egg and 

 embryo of salpa proves that they are follicle cells, and that their 

 appearance is the first stage in the follicular migration which after- 

 wards becomes so prominent. 



"Whether the " test cells " of all tunicate eggs are migrating follicle 

 cells or not, is a question upon which I shall express no opinion. 



