W. K. BROOKS ON THE GENUS SALPA. 53 



ages in a long calm, shows that the animals are most prolific, and the 

 complicated structure of the organs for nourishing the embryo shows 

 that every provision is made for rapid growth. 



The placenta is not the only nutritive organ, for, as we have seen, 

 the follicle also makes most important contributions to the supply of 

 material which is available for the construction and rapid completion 

 of the body of the embryo, and while I have spoken of the segmentation 

 and the formation of the blastodermic germ layers as retarded, the 

 retardation is probably not actual, but only relative, and the process of 

 development is, on the whole, accelerated by the presence of the follicle, 

 and by its share in the growth of the embryo. 



I have now shown that the ultimate fate of all the follicle cells is the 

 same, and that they may be found, in the sections, detaching themselves 

 and degenerating, first, in the somatic layer of the embryo ; secondly, in 

 the somatic follicular lining of the perithoracic structures ; third, in the 

 cavity of the pharynx ; fourth, in the visceral mass outside the digestive 

 cavity, and last, in that part of the placenta which is derived from the 

 somatic layer of the follicle. 



While it is not possible to trace the history of every cell from first to 

 last, we have as ample evidence as we could hope from sections, that 

 the function of the follicle of salpa is exclusively nutritive; that it is 

 transitory and embryonic, and that the tissues of the embryo are not 

 built up out of follicle cells, but from blastomeres, after the analogy of 

 all the rest of the animal kingdom. 



