W. K. BROOKS ON THE GENUS SALPA. 217 



in among the eggs and nourish them, as will be described in the section 

 on the maturation of the eggs. 



SECTION 4. The Homology of the Germinal Mass. 



As soon as the germinal mass has become differentiated into a super- 

 ficial layer of cells and a central core, the central cells lose the power of 

 vegetative multiplication, and thus become set apart as eggs, as I shall 

 show more in detail in the next section. 



While they continue to grow and mature, they undergo no vegetative 

 change until they have been fertilized, and as the differentiation takes 

 place long before the bodies of the chain-salpre are formed, there does not 

 seem to be any escape from the admission that the egg, as a definite 

 independent cell, is older than the body of the chain-salpa which carries 

 it, and that the central cells are true eggs at the stages shown in Plate 

 XLI, Fig. 9, Plate XXI, and Plate XXXIV, Figs. 2 and 4, and in See- 

 liger's Plate IV, Figs. 1, 2, 3, 4 and 5. 



We must not permit this truth to be obscured by the fact that, at an 

 earlier stage, the embryonic, undifferentiated germinal mass, Plate XLI, 

 Fig. 7, unquestionably contains the potency of many tissues besides the 

 eggs. We cannot, at this early stage, term it the ovary, although my 

 studies of Salpa pinnata show that all the structures which are derived 

 from it are in one way or another concerned in reproduction. It gives 

 rise on the one hand to the essential reproductive elements, the ova, and 

 the cells of the testes of the chain-salpae, and on the other hand to the 

 follicle and the fertilizing duct, structures which, while necessary to 

 reproduction, are out of the line of genetic succession. 



The nature and homology of the undifferentiated germinal mass has 

 caused much discussion, and while this has no bearing upon the subse- 

 quent, although very early, differentiation of the eggs, the questions to 

 which it has given rise are both interesting and important. 



Two views of its nature are possible, and both have advocates. 



We may on the one hand regard it as part of the soma, as a con- 

 stituent part of the body of the embryo, destined to become differentiated 

 into its organs, or those of the animals formed from it by budding. On 

 the other hand we may regard it as a body of cells set apart for repro- 

 duction and secondarily differentiated into essential and accessory repro- 

 ductive cells. 



Assuming the integrity and continuity of germinal plasma, the first 



