HISTOGENESIS OF THE LIVER 259 



The direct production of new hepatic cyhnders from the 

 hepatic pouches contributes but little to the bulk of the hepatic 

 network and continues for only a short period after anastomoses 

 are formed. As described in section 2, there are numerous 

 tubules which arise at the bases of or between tubules which have 

 previously sprung from the hepatic pouches. These younger 

 tubules are at first small nipple shaped elevations arising singly 

 and not from ridges. So long as the earlier tubules remain simple 

 the later tubules follow them closely in their development, but 

 when anastomoses become common among the older tubules 

 and the sinusoidal circulation is well established, the younger 

 tubules anastomose almost immediately^ after their formation 

 either with one another or with cylinders of the already estab- 

 lished network. In later stages hepatic tubules may sometimes 

 arise as loops, the ends of which are attached to the wail of the 

 hepatic pouch. By the time the embryo reaches a length of 

 approximately 20 mm. (Normal plate Nos. 27-28, Balfour's stage 

 N) the hepatic pouches are transformed into veritable hepatic 

 ducts and thereafter no new hepatic tubules arise from them. 



Hepatic cylinders, as has been seen, give rise to secondary 

 buds while still in the form of single tubules, and this budding 

 process continues long after anastomosis. The process can be 

 most clearly demonstrated by the means of thick sections of 

 which figure 41 from an embryo 24 mm. in length is an example. 



* The method of preparation of the thick sections used in this study was as 

 follows: The embryo was infiltrated with celloidin and cemented to a piece of 

 infiltrated spleen or liver which in turn was fastened to a fiber block. The speci- 

 men was then cleared by Gilson's method, which makes the block almost trans- 

 parent. The block was then placed in the microtome clamp and a strong beam of 

 light from a condenser directed upon the object. A Greenough binocular micro- 

 scope equipped with low power lenses was set up over the object. With this 

 arrangement it was possible to follow in detail the process of section cutting. 

 Sections were then carefully cut away until the exact region desired was reached. 

 By focusing with the binocular it was possible to determine the approximate 

 thickness of the section needed to just include the desired structures, and this 

 section was then removed with a single cut. Sections made in this way are often 

 superior to reconstructions for the study of the form of very small structures. 

 They are best stained in a very dilute carmin solution and cleared for a long 

 period in. cedar oil, after which they can be observed with the binocular micro- 

 scope, or better with the aid of an Abby binocular eyepiece. 



