190 THE DEVELOPMENT OF THE CHICK 
muscles of these parts. The voluntary muscles of the head, on 
the other hand (excepting the hypoglossus musculature), arise 
in front of the somites; the mesoblast from which they arise is, 
however, part of the original paraxial meroblast, in large part 
at least. It is important to note that the voluntary muscles 
are epithelial in origin. The involuntary, or smooth, muscle 
fibers, on the other hand, are mesenchymal in origin. 
The dermatome remains epithelial in all the somites well 
into the third day; the cells then begin to separate and form 
mesenchyme; this process begins at the anterior somites and 
proceeds backwards. The mesenchyme thus formed is the 
foundation of the derma. 
The Intermediate Cell-mass. This is the cord of cells uniting 
somite and lateral plate; it reaches its typical development only 
from the fifth to the thirty-third somites, in which it contributes 
to the development of the excretory system. Behind the cloaca, 
that is in the region of the tail, there is no lateral plate and no 
nephrotome. 
Origin of the Excretory System. The history of the excretory 
system in Amniota is of particular interest, because it shows a 
succession of three separate organs of excretion or kidneys, the 
first of which is a mere functionless rudiment, the second is the 
principal organ of excretion during embryonic life (at least in 
reptiles and birds), and the third finally becomes substituted 
for the second, which degenerates and is mostly absorbed; 
however, parts of the second remain and contribute to the 
formation of the organs of reproduction. The first, known as 
the head kidney or pronephros, is probably homologous to the 
permanent kidney of Amphioxus; the second or mesonephros, 
is the homologue, in part, of the permanent kidney of Anamnia, 
and the third or metanephros is the permanent kidney. The 
secreting parts of all arise from the intermediate cell-mass, though 
not in the same manner. The development of the metanephros 
does not begin until the fourth day; it is therefore not considered 
in this chapter. 
Pronephros and Wolffian Duct. The pronephros extends 
over only eleven or twelve somites, viz., from the fifth to the 
fifteenth or sixteenth inclusive; it consists originally of as many 
parts or tubules as the somites concerned. Each tubule arises 
as a thickening of the somatic layer of the intermediate cell- 
