W. K. BROOKS ON THE GENUS SALPA. 59 



The embryology of Salpa furnishes, as I shall show in another sec- 

 tion, still further evidence regarding the phylogeny of the adult, but we 

 are here concerned only with the comparative study of the embryo. 



The Origin and Significance of the Follicle of Salpa. 



The most peculiar and anomalous feature in the life-history of Salpa 

 is the follicle, but even here we are not absolutely without means for 

 comparative study. 



In numerous, widely separated members of the animal kingdom, 

 follicle cells migrate into the substance of the ovarian eggs, and supply 

 them with food. Many instances are found among the Tunicates ; Cla- 

 velina for example ; and, as we have seen, Salpa must be added to the 

 list. Salpa undoubtedly inherits this peculiarity, which is also found in 

 Doliolum, from their common ancestor, and very probably from a still 

 more remote ancestor, common to the other Tunicates and to Appendicu- 

 laria. 



Kowalevsky (Arch. f. Mikros. Anat., Band XI) was the first to show 

 that the migration and degeneration of follicle cells goes on in Pyrosoma 

 after the egg is fertilized. He says that great numbers of follicle cells 

 separate from the follicular epithelium and wander into the space 

 between it and the developing egg, as free inner follicle cells. Some 

 of them gather in groups external to the germinal disk, while others 

 work their way in between the segments and are for a long time distin- 

 guishable from the blastomeres. Finally, he says (p. 607), these cells as 

 well as the yolk are surrounded and inclosed by the blastoderm and 

 turned to nutritive material or to blood corpuscles. 



While Salensky believes that instead of nourishing the embryo these 

 migratory follicle cells take part in its construction, his account of their 

 origin is essentially like Kowalevsky's, except that he shows that besides 

 those which make their way between the blastomeres, many others pass 

 under the edge of the blastoderm and accumulate in the segmentation 

 cavity, or the space between the blastoderm and the yolk, and that others 

 wander into the substance of the yolk. 



Cuts E and F are two early stages in the segmentation of Pyrosoma, 

 copied from Salensky's memoir. They are sections through the blasto- 

 derm, and the food-yolk is not represented. It should occupy the space 

 below the figures. In my pen-copy of his figures I have dotted the 

 migratory follicle cells to make them more conspicuous. In cut E they 



