EARLY STAGES IN THE DEVELOPMENT OF VERTEBRATES. 223 
Mr. Lankester, who has already entered into this line of 
speculation, even suggests (‘Q. J. of Micr. Science,’ April, 
1875) that this applies to all higher animals. It might then 
be supposed that the muscular system of part of the alimen- 
tary canal took the place of the primitive muscular system 
of the body; so that the whole muscular system of higher 
animals would be primitively part of the muscular system of 
the digestive tract. 
I put this forward merely as a suggestion, in the truth of 
which I feel no confidence, but which may perhaps induce 
embryologists to turn their attention to the point. If we 
accept it for the moment, the supplanting of the body mus- 
cular system by that of the digestive tract may hypothetically 
be supposed to have occurred in the following way. 
When the diverticulum or rather paired diverticula were 
given off from the alimentary canal they would naturally 
become attached to the body wall, and any contractions of 
their intrinsic muscles would tend to cause movements in 
the body wall. So far there is no difficulty, but there is a 
physiological difficulty in explaining how it can have hap- 
pened that thissecondary muscular system can have supplanted 
the original muscular system of the body. 
The following suggestions may lessen this difficulty, though 
perhaps they hardly remove it completely. If we suppose 
that the animal in which these diverticula appeared had a 
hard test and was not locomotive, the intrinsic muscular 
system of the body would naturally completely atrophy. But 
since the muscular system of the diverticula from the stomach 
would be required to keep up the movement of the nutritive 
fluid, it would not atrophy, and were the test subsequently to 
become soft and the animal locomotive, would naturally form 
the muscular system of the body. Or even were the animal 
locomotive in which the diverticula appeared, it is conceivable 
that the two systems might at first coexist together ; that 
either (1) subsequently owing to the greater convenience of 
early development, the two systems might acquire a develop- 
ment from the same mass of cells and those the cells of the 
inner or hypoblast layer, so that the derivation of the body 
muscles from the hypoblast would only be apparent and not 
real, or (2) owing to their being better nourished as they 
would necessarily be, and to their possibly easier adaptability 
to some new form of movement of the animal, the muscle-cells 
of the alimentary canal might become developed exclusively 
whilst the original muscular system atrophied. 
I only hold this view provisionally till some better explana- 
tion is given of the cases of Sagitta and the Echinoderms, 
VOL, XV.—NEW SER, Q 
