126 Mr. W. K. Brooks on the 



The Ectoderm of the Salpa Embryo. — At au early stage of 

 segmentation some of the blastomeres move upwards and pass 

 out of the follicle on the middle line of the dorsal surface, 

 where the two layers of the follicle are continuous with each 

 other. I have given reasons for believing that this is the 

 spot which was once occupied by the blastopore. These 

 ectodermal blastomeres thus become extra-follicular, although 

 they are covered for a time by the capsule of epithelium, 

 which Salensky has called the " Ectodermkeim." They give 

 rise by cell-division to the ectoderm, which spreads from the 

 dorsal middle line downwards and outwards over the embryo, 

 pushing off and replacing the cells of the capsule. The ecto- 

 derm has a growing edge, like that of meroblastic embryos, 

 and it does not close in completely on the ventral middle line 

 until after birth. 



Salensky (" Neue Untersuchungen," Mitth. a. d. Zool. 

 Station zu Neapel, i., 1882) has figured the migration of 

 blastomeres to an extra-follicular position on the dorsal sur- 

 face of the embryo in several species, although he regards 

 them as discarded blastomeres and derives the ectoderm from 

 other sources. They are clearly shown in Salpa lyimiata in 

 his plate xii. fig. 26, in Salpa pectinata in his pi. xxiii., and 

 in Salpa fusij'ornns in his plate xxiv. fig. 3, where they are 

 marked by the letters Eckh^ which might be supposed to 

 stand for " ectodermal blastomeres " if he did not tell us 

 explicitly on p. 889 that the ectoderm of this species is derived 

 from the epithelial capsule (" Epithelhiigel"). 



'J he ectodermal blastomeres seem to be more conspicuous 

 in Salpa fusiformis than in other species, for Salensky says 

 (p. 345) that while the epithelial capsule (" Ectodermkeim ") 

 is generally separated very sharply from the embryonic cell- 

 mass, it is at one end of the embryo so intimately related to 

 the follicle cells ("gonoblasts") that it is difiicult to determine 

 the boundary between them, and the blastomeres which lie 

 directly at this spot are covered only by the epithelial capsule 

 ("Ectodermkeim "). At a later stage he says (p. 350) that 

 the epithelial capsule (" Ectodermkeim ") contains cells which 

 differ greatly among themselves in both size and form. Some 

 of them are similar in appearance to the cells of the epithelial 

 capsule, as already described, at an earlier stage, and differ 

 from them only in being more flattened. "The others (Eckb) 

 are very much larger and very different in structure, and 

 contain a nucleus which is very similar to that of the blasto- 

 meres. The apijearance of these cells suggests that they are 

 blastomeres ichich have passed out from the cell-mass.^ 



Salensky believes that the ectoderm of Salpa democrafica is 



