126 Mr. W. K. Brooks on the 
The Ectoderm of the Salpa Embryo.—At an 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 middie 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, 1., 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 pinnata in 
his plate xi. fig. 26, in Salpa pectinata in his pl. xxiii., and 
in Salpa fusiformis in his plate xxiv. fig. 8, where they are 
marked by the letters Hckb, which might be supposed to 
stand for “ectodermal blastomeres” if he did not fell us 
explicitly on p. 889 that the ectoderm of this species is derived 
from the epithelial capsule (“ Epithelhiigel”’). 
‘Lhe ectodermal blastomeres seem to be more conspicuous 
in Salpa fusiformis than in other species, for Salensky says 
(p. 845) 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 difficult 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 appearance of these cells suggests that they are 
blastomeres which have passed out from the cell-mass.’ 
Salensky believes that the ectoderm of Salpa democratica is 
