HISTOGENESIS OF THE LIVER 267 
The tubules which are formed from the dorsal parts of the pouches 
and of the pars hepatica medialis arise singly or in small clusters 
as do those in the same situation in Acanthias, but in the forms 
under discussion the dorsal or marginal tubules form a much 
greater part of the whole number produced than is the case in 
Acanthias. In Torpedo and in Mustelus the tubules from the 
ventral portion of the lateral pouches arise as in Acanthias from 
tubule ridges. These ridges are but slightly elevated and the 
corresponding grooves on the internal surfaces of the pouches 
are wide and. shallow. The individual tubules which form from 
these ridges do not remain as mound-like structures arranged in 
rows but grow out almost at once and like the dorsal tubules appear 
as slender tubes extending outward between the veinous sprouts. 
In most cases the hepatic tubules which are first formed come in 
contact with the splanchnic mesothelium covering the liver. 
The omphalo-mesenteric veins in these forms increase in size 
very rapidly and before tubule formation has progressed very 
far the anterior, ventral and lateral surfaces of the liver pouch 
are practically surrounded by a venous lake. All tubules which 
arise after this period project into this sinus and in their growth 
they push its endothelial wall before them. This process is 
illustrated in figure 44, Which is of a thick frontal section of the 
liver of an embryo of Mustelus canis approximately 12 mm. long. 
The number of tubules arising directly from the pouch walls is 
small in these forms as compared with Acanthias for the surface 
area of the pouches is much reduced from the first. The large 
majority of the later tubules is formed by the branching of the 
earlier ones. 
In Acanthias the hepatic tubules in the earlier period of their 
development are short, broad and pouch-like and the branches 
which arise from them are nipple-like projections. In the other 
forms studied both the primary and secondary tubules are slender 
elongated tubes which are early united in a complex branching 
and anastomosing network, the meshes of which are separated 
by sinusoids of large size. Figure 44 shows an anastomosis of a 
young hepatic tubule in Mustelus, and figure 45 of a similar 
