252 RICHARD E. SCAMMON 



expansions of the primary ones. The secondary tubules are short 

 conical projections rarely over one and one-half times as long as 

 their greatest diameter. They arise from the sides of the distal 

 expansion and almost always extend out at right angles to the 

 axis of the primary tubule. Those primary tubules which arise 

 from the sides of the hepatic pouches and are packed in among 

 their fellows commonly assume a T-shaped form with the limbs 

 of the T directed antero-posteriorly. The primary tubules which 

 arise from the margins of the hepatic pouches become somewhat 

 larger than those formed from the tubule ridges and give rise to 

 from three to seven secondary tubules from their expanded 

 distal chambers. These secondary branches may again sub- 

 divide into branches of the third order and upon these in turn 

 there may occasionally be found nipple-like projections which 

 represent the fourth order of tubules. In the large majority 

 of cases, however, anastomosis' takes place before tubules of the 

 fourth or even the third order are formed. Figures 19 and 20 

 are of wax reconstructions of tubules from the lateral hepatic 

 pouch of an embryo 13.3 mm. in length (S.'C. No. 18). In figure 

 19 one sees the beginning of the outpouching of the distal ex- 

 pansion into secondary tubules. These structures are well 

 marked proj ections in the older tubule shown in figure 20. Figures 

 22 and 23 are two views of a hepatic tubule from the dorsal 

 margin of the left hepatic pouch of an embryo 15 mm. long 

 (H.E.C. 227). The latter figure shows tubules of the fourth 

 order. Figure 21 is a wax reconstruction of two young primary 

 tubules from the ventral surface of the pars hepatica medialis 

 of the same 15 mm. embryo. One of these shows the beginning 

 of tubules of the second order. 



When the tubule ridges first appear on the hepatic pouches 

 they are due to the increased thickness of the epithelium in these 

 places and not to an evagination of the pouch wall. In cross 

 sections of the ridges (see fig. 28, a cross section of the lateral 

 wall of the left hepatic pouch of an embryo of 8 mm. K.U.E.C. 

 542) it is noticeable that the nuclei which in other parts of the 

 wall are arranged in two interlocking rows tend to be reduced 



