Embryology. 141 
other such forms in nature, the first locomotive and 
transitory, the second fixed and permanent (Figs. 
43, 44). 
Here, then, we leave the lower forms of Metazoa in 
their condition of permanent gastrule. They differ 
from the transitory stage of other Metazoa only in 
being enormously larger (owing to greatly further 
growth, without any further development as to matters 
of fundamental importance), and in having sundry 
tentacles and other organs added later on to meet 
their special requirements. The point to remember 
is, that in all cases a gastrula is an open sac composed 
of two layers of cells—the outer layer being called the 
ectoderm, and the inner the endederm. They have 
also been called the animal layer and the vegetative 
layer, because it is the outer layer (ectoderm) that 
gives rise to all the organs of sensation and move- 
ment—viz. the skin, the nervous system, and the 
muscularsystem ; while it is the inner layer (endoderm) 
that gives rise to all the organs of nutrition and 
reproduction. It is desirable only further to explain 
that gastrulation does not take place in all the Metazoa 
after exactly the same plan. In different lines of 
descent various and often considerable modifications 
of the original and most simple plan have been intro- 
duced; but I will not burden the present exposition 
by describing these modifications!. It is enough for us 
that they always end in the formation of the two 
primary layers of ectoderm and endoderm. 
The next stage of differentiation is common to all 
the Metazoa, except those lowest forms which, as we 
1 The most extreme of them is that which is mentioned in the last 
foot-note. 
