THE DEVELOPMENT OF THE EMBRYO ITSELF. 93 
happens that an organ is made up of cells derived from more 
than one layer. Structures may, accordingly, be classified as 
epiblastic, mesoblastic, or hypoblas- 
tic; for, when two strata of cells 
unite in the formation of any part, 
one is always of subordinate impor- 
tance to the other: thus the digestive 
organs are made up of mesoblast as 
well as hypoblast, but the . latter 
constitutes the essential secreting 
cell mechanism. As already indi- 
cated, the embryonic membranes 
are also derived from the same 
source, 
The epiblast gives rise to the skin 
and its appendages (hair, nails, feath- 
ers, etc.), the whole of the nervous 
system, and the chief parts of the or- 
gans of special sense. 
The mesoblast originates the skel- 
eton, all forms of connective tissue, 
including the framework of glands, 
the muscles, and the epithelial (en- 
dothelial) structures covering serous 
mem branes. 
The hypoblast furnishes the se- 
creting cells of the digestive tract 
and its appendages—as the liver and 
pancreas—the lining epithelium of 
the lungs, and the cells of the secret- 
ing mucous membranes of their 
framework of bronchial tubes. 
It is difficult to overrate the im- 
portance of these morphological gen- 
eralizations for the physiologist; for, 
once the origin of an organ is known, 
its function and physiological rela- 
tions generally may be predicted with 
considerable certainty. We shall en- 
Die 
Fie. 103.—Embryo fowl 3mm. long, 
of about twenty-four hours, seen 
from above. 1x39. (Haddon. 
after Kélliker.) Mn, union of 
the medullary folds in the region 
of the hind-brain ; Pr. primitive 
streak ; Pz, parietal zone; Rf, 
posterior portion of widely-open 
neural groove ; Rf’, anterior part 
of neural groove ; Rw, neural 
ridge ; Stz, trunk-zone ; vAf, an- 
terior amniotic fold ; vD, anterior 
umbilical sinus showing through 
the blastoderm. His divides the 
embryonic rudiment into a cen- 
tral trunk-zone, and a pair of 
lateral or parietal zones. 
deavor to make this prominent in the future chapters of this 
work. 
Being prepared with these generalizations, we continue our 
study of the development of the bird’s embryo. Before the end 
