W. K. BKOOKS ON THE GENUS SALPA. 65 



The segmentation of the salpa egg is so much retarded that, in the 

 gastrula stage, the germ layers are represented only by the scattered 

 blastomeres, 3 and 9. There is no digestive cavity, as the space between 

 the scattered endodermal blastomeres, 9, is completely filled with migra- 

 tory follicle cells, as is the blastopore also ; while the ectoderm is repre- 

 sented by a few scattered ectodermal blastomeres, 3, external to the 

 follicle in the region of the blastopore. 



While the development of the true germ layers is thus retarded, the 

 follicle cells follow, in their migration, the paths which were established 

 for them before the retardation of the germ layers was initiated, and 

 they now shape themselves into the cast of a gastrula, and this pseudo 

 gastrula becomes converted into the simulacrum of a tunicate embryo, 

 carrying with it the retarded blastomeres, in the way which has already 

 been fully illustrated and described. The blastomeres thus reach their 

 final positions in the body of the embryo before they become differ- 

 entiated and arrange themselves in organs and tissues. Finally, the 

 follicular layers break up. Many of their cells are taken into the sub- 

 stance of the blastomeres and digested, while others are left in the body 

 cavity as free wandering cells, destined to degenerate at later stages and 

 to serve as food for the tissues of the body. 



In this section I have not been able to avoid the language of phy- 

 logeny, and I therefore wish to say once more that the account which 

 is here given is both hypothetical and diagrammatic. The actual salpa 

 embryo is complicated by the possession of a placenta, and by peculiari- 

 ties which may be due to the former presence of a food-yolk which has 

 disappeared. These features have been left out of consideration, and, 

 even with this qualification, I have no desire to assert that, during the 

 actual history of the evolution of the salpa embryo, it has moved along 

 the path which I have indicated. 



We have no history of its origin, and it is quite possible that the 

 course of development has been very different from this imaginary recon- 

 struction. 



I believe, however, that the comparison throws light upon the funda- 

 mental nature of the embryo, and upon its relation to the embryos of 

 other tunicates, and I claim that, so far, it is valuable, although I should 

 be the last to assert its definite phylogenetic significance. 



I have spoken of the history of the blastomeres as retarded, but it is 

 probable that the retardation is not actual but only relative, and that the 

 embryo is actually produced more quickly than it was before the folli- 

 cular migration became established. 



