878 A Sketch of Comparative Embryology. (December, 
other structure. In nearly all animals the central nervous 
system (nerve ganglia, 
spinal cord, etc.) lies 
quite deep, well separated 
from the ectoderm or 
skin, yet in the embryo 
the nervous system arises 
from the ectoderm (Fig. 
25, cf. also Fig. 20) ap- 
Fic. 25—Transverse section of caste a - pearing at first as cells 
qus raped ri se ton ing eae nervous Very much like the rest 
system; v, ciliated b: “ally parating the two parts of of the ectoderm. They 
the, nervous system; c, c’, par f body cavity; 
. „mesoderm. Only the Krt ‘half ar the sec- Soon, however, separate 
tinje Sqered. from their first neighbors, 
moving inwards; the mesoderm then grows in between the half 
developed nervous cells and the ectoderm, so that they are com- 
pletely divided. 
The following table shows to which of the” germ layers the 
principal organs belong: 
aer DE MESODERM. 
ic pidanti or external skin. 1. Wandering cells, 
rust of arthropods. 2. Connective tissue, fat cells, etc. 
o 
3 
Horns, hairs and nails. 
sgt glands. 
of larva, etc. 
2. Prata system, 
Organs of, a Touch. 
b Taste. 
c Smell. 
d Hearing. 
e Sight, etc, 
4. Fore gut. 
5. Hind gut. 
6. Mouth gut (vertebrates). 
7. Gillis 
8. Tracheæ of insects. 
3. Internal skeleton. 
. Muscles. 
5. Genital products. 
6. Blood. 
7. Organs of circulation. 
8. Organs of excretion. 
9. Secondary g con 
10, Lymphatics (and spleen 
ENTODERM. 
f: Middle gut. 
id, pancreas, € 
hyroi 
5. ey appendages H the digestive 
_ As appears from this table the erne of each germ layer is 
predetermined. 
The eighth law is, that the simple cells formed ‘during segmen- 
tation change their character during embryonic growth, not only 
appearing EA but altering also their activity from general 
