EMBRYOLOGY. 
Tun. 3 
from follicular cells. He worked with various forms of Simple Ascidians, 
especially species of Cynthia and Clavelina. 
There are two periods in the development of Pyrosoma — (1) the con- 
version of the fertilized ovum into the cyathozooid ; (2) the formation 
of the first four Ascidiozooids by budding. Salensky' (11, 12) tries to 
clear up especially two points not dealt with by Huxley and Kowalevsky: 
1. The part played in the development of the cyathozooid by the inner 
follicle cells, which are now called “ kalymocytes.” 2. The origin and 
changes of the mesoderm. 
The kalymocytes are the “test cells ” of ordinary Ascidians. They 
are follicle cells which have separated from the follicle wall and have 
wandered to the surface of the yolk. They are pyriform to begin with, 
but those that migrate into the yolk become amoeboid. After passing 
through the yolk, some of them help to form the wall of the mesenteron. 
During the mesoblastic segmentation some of the kalymocytes aggregate 
themselves round the upper (protoplasmic) pole of the embryo, and get 
botwoon, and even into, the blastomeres. They are easily distinguished 
from the blastomeres by their size and pyriform shape, and their vacuo- 
lated protoplasm for some time ; but in the end these two sets of cells 
become exactly alike, and take an equal part in the formation of the 
cyathozooid. Therefore we have in Pyrosoma non -fertilised elements, 
the kalymocytes, uniting with the blastomeres, the derivatives of the 
fertilized ovum, and becoming formative elements. 
In Salpa, , Salensky has shown that the kalymocytes play the most 
important part in development, while the blastomeres are of secondary 
importance. Investigations into the development of Compound Asci- 
dians show that it is in that group that the beginning of that remarkable 
new function of the kalymocytes is to be found. The kalymocytes of the 
Compound Ascidians take as yet no actual part in the formation of the 
embryo, but otherwise they behave towards the blastomeres in the same 
way that the kalymocytes of Pyrosoma do. They penetrate between the 
blastomeres, and remain there for some time, though taking no part in 
the development of the embryo. 
In regard to the germ-layers and mesoderm, the blastomeres first 
differentiate into an epiblast layer (which afterwards gives rise to the 
atrial tubes and the nervous system), and a lower meso-endodermal mass 
which afterwards splits into a many-layered mesoderm and a one-layered 
endoderm. The ccelomic spaces appear as several lacuna-like cavities, 
which eventually convert the mesoderm into two coelomic sacs, lying one 
at each side of the longitudinal axis of the germinal disc. These sacs 
open into the enteric cavity between the germinal disc and the upper 
surface of the yolk. The relations between these cavities are the same 
as those described by Yan Beneden and Julin. There is a median longi- 
tudinal ridge projecting into the upper wall of the enteric cavity and 
traversed by a canal, which is probably the representative of the noto- 
chord. This is a transitory structure and only lasts a short time. The 
coelomic sacs of Pyrosoma have at first many layered walls, while in 
