114 THE DEVELOPMENT OF THE CHICK 
was made, the concurrent testimony of which was_ perfectly 
conclusive.! 
We shall then proceed on the assumption that the first somite 
formed is also the first of the series, and that the remainder arise 
in succession behind it as transverse sections of the paraxial 
mesoblast. 
There is always a stretch of unsegmented paraxial mesoblast 
between the last somite and the anterior end of the primitive 
streak. 
The first four somites belong to the head, and enter into the 
composition of the occipital region. The more anterior part of 
the mesoblast of the head never becomes segmented in the chick. 
In the anamniote vertebrates, segmentation of the mesoblast 
extends farther forward, and there is a greater number of cephalic 
somites. This may be taken as evidence that a large part, at 
least, of the head was primitively segmented like the trunk. 
As we shall see later, the primitive metamerism of the head is 
also expressed in other ways: neuromeres, branchiomeres, etc. 
The segmentation of the mesoblast finally extends to the 
hind end of the tail, new segments being continually cut off 
from the anterior end of the paraxial mesoblast until it is all used 
up. This is not complete until the fifth day. The number of 
somites thus formed is perfectly constant, as is also the fate of 
the individual somites. 
Primary Structure of the Somites. Hach somite is primarily 
a block of cells arranged in the form of an epithelium around a 
small central lumen, towards which the inner ends of all the cells 
converge (Fig. 68 B). The central cavity (myoccele) is, however, 
filled with an irregularly arranged group of cells, and, though 
the cavity must be regarded as part of the primitive body-cavity, 
or ccelome, it has no open communication with it. After the 
somites are formed they rapidly become thicker so that their 
lateral boundary becomes very sharply marked; this is not due 
to a longitudinal constriction external to the paraxial mesoblast, 
as usually stated. Each somite has six sides, of which five are 
free, viz., dorsal, ventral, anterior, posterior, and median. The 
sixth or lateral side is continuous with the nephrotome. 
The Nephrotome, or Intermediate Cell-mass (Middle Plate). 
1Sinece the above was written, J. T. Patterson has obtained the same 
results (Biol. Bull. XIII, 1907). 
