Development 0 / Pyrosoma. 241 
myself in the case of the Salps, which, however, present by 
no means unimportant differences. Chief among these is the 
fact that, whereas in the case of Pyrosoma both kalymocytes 
and blastomeres take an equal share in the formation of the 
embryo, in the case of the Salps the kalymocytes play the 
most important part in the development, in opposition to the 
blastomeres, which are of secondary importance. In the 
development of Pyrosoma and the Salps, however, we have 
to deal with a phenomenon which has already attained a 
tolerably high degree of perfection ; since in both cases the 
kalymocytes, which in the case of all other animals have no 
function at all, become all at once of great importance in the 
formation of the embryo. Somewhere or other the primitive 
stages of this singular phenomenon must exist, in which the 
adaptation of the kalymocytes to their new role of formative 
elements may be supposed to have begun. My own investi- 
gations, as yet unfinished, into the development of certain 
compound Ascidians ( Gircinalium , Didemnium ) Leptoclinium, 
Amaur oecium ) , as well as the already known, though but 
scanty, statements of other authors about the development of 
this interesting group, lead me to the conclusion that it is in 
them that we must look for the origin of this remarkable 
phenomenon, which reaches its culminating point in Pyro- 
soma and the Salps. The kalymocytes of the compound 
Ascidians take, it is true, as yet no part in the development 
of the embryo ; but they behave towards the blastomeres in 
precisely the same way as do the kalymocytes of Pyrosoma in 
the first stages of segmentation — that is to say, they penetrate 
between the blastomeres and remain in that position for some 
time, without mingling with the blastomeres and taking part 
in the development of the embryo. 
2. The Development of the Germinal Layers and 
Differentiation of the Mesoderm . 
The stages of segmentation and germinal-layer formation, 
the most important in development, are very sharply marked 
off from one another in the case of Pyrosoma . The seg- 
mented nucleus consists, as we have already seen, of a mass 
of similar cells, and appears as a solid cupola-shaped eleva- 
tion, resting on one pole of the oosperm. The earliest 
changes of all in the segmented nuclear cap are exhibited in 
the differentiation of a superficial layer of cells, which are 
distinguished by their cylindrical shape from the polygonal 
cells of which the remainder of the mass consists. This 
superficial layer represents the ectoderm, and in the later 
