THE PRINCIPLES OF EMBRYOLOGY 479 
muscle-cell from the very beginning, and on the other the manuer in 
which all the skeletal muscles of the adult are lined with a lymphatic 
endothelium, I am strongly inclined to believe that at the closing 
up of the myoccele, when the myomere separates from the mesomere, 
the lining cells remain scattered in among the forming muscle-cells 
and form the ultimate lymphatic tissue of the muscles. If this 
is really so, then the evidence in favour of the mesothelium being 
composed of free cells not connected with the nervous system 
would be much strengthened, for, on the one hand, an intimate 
relation exists between the connective tissue cells and the endo- 
thelium of the roots of the lymphatic vessels, a relation which, 
according to Virchow, has rendered it impossible to draw any sharp 
line of distinction between the two; and, on the other, the lymphatic 
endothelium merges into the lining cells of the great serous cavities 
of the body. 
It is impossible to conceive of an animal possessing a nervous 
system which is not in connection with sensory and muscular 
tissues; an isolated nerve-cell is a meaningless possession ; but it is 
equally natural to conceive of a germ-cell being isolated, capable of 
living an independent existence. Such a difference between the two 
kinds of tissues must have existed from the very commencement of 
the Metazoa, so that we must, it seems to me, imagine that in the 
formation of the Metazoa from the Protozoa the whole of the body 
of the latter did not break up into a mass of separate gonads, each 
capable of becoming a free-living protozoan similar to its parent, but 
that a portion proliferated into a multinucleated syncytium while 
the remainder formed the free-living gonads. This multinucleated 
syncytium, or host, as it might be called, would still continue to 
exist for the purpose of carrying further afield the immortal gonads, 
which need no lIcnger be all shed at one time. 
In such an animal as Volvow globator we have an indication of 
the very kind of animal postulated as connecting the single-celled 
Protozoa and the multi-cellular Metazoa, for it consists of a many- 
celled case which forms a hollow sphere, each of the cells being 
provided with flagella for the purpose of locomotion of the sphere, 
except a certain number which are not flagellated; the latter leave 
the case to swim freely in the fluid contained within the sphere, and 
forming spermaries and ovaries, conjugate, maturate, and then are set 
free by the rupture of the encircling locomotor host. 
