THE PRINCIPLES OF EMBRYOLOGY 479 



muscle- cell from the very beginning, and on the other the manner 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 lnyoccele, 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 lcnger be all shed at one time. 



In such an animal as Vol vox gldbator 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. 



