Development o/Tyrosoma, 241 



myself in tlic case of the Salps, which, however, present by 

 no means unimportant differences. Cliief among these is the 

 fact that, wliereas in the case of Pi/roaoma both kalymocytes 

 and blastomores take an equal share in tlie 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 Fi/rosoma 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 ])henomenon 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 {Circimdium, Didemuium^Leptoclinium, 

 Amauracium) , 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 Pi/ro- 

 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 



