124 Mr. W. K. Brooks on the 
features in the organization of the embryo. While this 
process is going on the development of the blastomeres is 
retarded, so that they are carried into their final positions in 
the embryo while still in a very rudimentary condition. 
Finally, when they have reached the places which they are 
to occupy they undergo rapid multiplication and growth, and 
build up the tissues of the body directly, while the scaffolding 
“of follicle cells is torn down and used up as food for the true 
embryonic cells. 
No other animal presents us with an embryonic history 
quite like that of Salpa, although other ‘Tunicata show some- 
thing similar but very much less pronounced. In the chapter 
of my memoir “On the Morphological Significance of the 
Salpa Embryo,” I attempt to show how the life-history of 
Salpa has come about, but we must now confine ourselves to 
the facts. 
An imaginary illustration may help to make the subject 
clear. Suppose that while carpenters are building a house 
of wood, brickmakers pile clay on the boards as they are 
carried past, and shape the lumps of clay into bricks as they 
find them scattered through the building where they have 
been carried with the boards. Now, as the house approaches 
completion, imagine that bricklayers build a brick house over 
the wooden framework, not from the bottom upwards, but here 
and there, wherever the bricks are to be found, and that, as 
fast as parts of the brick house are finished, the wooden one 
is torn down. ‘To make the analogy more complete, however, 
we must imagine that all the structure which is removed is 
assimilated by the bricks, and is thus turned into the substance 
of new bricks to carry on the construction. 
Salensky (“Neue Untersuchungen,” &c., Naples Mittheil- 
ungen, 1., 1882, and “‘ Embryonalentwicklung der Pyrosoma,” 
Zool. Jahrb. iv. and y., 1891) has discovered and minutely 
described the migration of the follicle; but he has failed to 
trace the history of the blastomeres, and believes that these 
degenerate and disappear, and that the embryo is built up of 
follicle cells. I find that all the follicle cells are ultimately 
used up as food, and that the true embryo is formed from 
blastomeres after the analogy of the rest of the animal 
kingdom. 
Lhe Aggregated Salpe—During their development the 
aggregated Salpe undergo complicated changes of position, 
which render the interpretation of sections very difficult, and 
as both Salensky (Morph. Jahrb. 1877, iii.) and Seeliger 
(Jena. Zeitschr. 1885) have totally failed to understand these 
