114 BULLETIN OF THE 
intermediate cell-mass as the beginning of a blood-vessel, which is 
formed as a solid mass, of which the peripheral cells constitute the 
venous wall, while its central cells float away as blood corpuscles. In 
later stages of the Salmon, Ziegler has traced the cells of the interme- 
diate cell-mass in their migration to the ventral side of the intestine and 
over the yolk, where they practically separate the yolk below them from 
the lateral layers above. 
Wiedersheim (’90, p. 131, Taf. VI.) has shown that in later stages 
of Proteus anguineus, the segmental duct (“ Vornierengang”’) lies in a 
“blood space” just below the aorta and above the intestine. These 
blood spaces come from the two sides and are united in the median line, 
hence their contour in a cross section shows a double or paired outline. 
They are densely packed with blood corpuscles, and, no doubt, represent 
the posterior cardinal veins. Some distance (fifteen sections) back of the 
nephrostome (Taf. VI. Fig. 10) the segmental ducts are distinctly 
formed, as seen in transverse sections, one on each side of this bilobed 
mass of cells. 
(b) The Origin of the “ Intermediate Cell-mass” in Fundulus. — In 
Fundulus the intermediate cell-mass originates in two ways ; it appears 
both as a primary and as a secondary formation. It first appears in 
about the middle of the trunk region, where it occurs as a cord of cells 
between the protovertebre and lateral layers, at the time the latter are 
cut off. In this region it probably retains no histological connection with 
either protovertebre or lateral layers. In a more anterior region it is 
also independent of the protovertebre from the time of its earliest 
formation, but here it remains for a time in histological union with the 
proximal margin of the lateral layers, from which it is subsequently cut 
off as a secondary process. 
This separation of the intermediate cell-mass from the proximal 
margin of the lateral layers proceeds forward from the anterior limit of 
its primary formation to the region of the nephrostome. It may be 
stated that in Fundulus the intermediate cell-mass on either side of the 
embryo moves ventrally toward the median plane, and that the two 
structures meet under the chorda ; but this statement must be modified 
to represent the exact conditions in the two regions. 
In the region of its primary formation, the intermediate cell-mass 
does, indeed, pass ventrally as a mass of cells, and take its position 
under the chorda; but in the region of its secondary formation, al- 
though it begins this ventral migration, it never reaches the median 
line as a compact mass. When it reaches the level of the future seg- 
