350 RICHARD E. SCAMMON 



The lateral pouches have continued their dorsal growth and 

 now extend as far upward as the dorsal surface of the gut. Their 

 upper parts are expanded, particularly posteriorly^, so that a 

 proximal constricted stalk and a distal expanded portion can be 

 distinguished. In earlier stages as shown by figures 33 and 37, 

 the origin of each lateral pouch was continuous with the entire 

 lateral edge of the median one, but at this stage it is confined to 

 the posterior four-fifths of this edge. The dorsal part of each 

 lateral pouch is curved a little medially. All the external lateral 

 surface of each lateral pouch is corrugated with rather irregular 

 longitudinal ridges which are somewhat broken by shallow trans- 

 verse fissures (fig. 40). Similar ridges are forming on the ventral 

 surface of the median pouch anterior to the gall 'bladder. The 

 dorsal edge of each median pouch is also rendered exceedingly 

 irregular by the several small pouches springing from it. All 

 of these structures are the anlagen of hepatic tubules the forma- 

 tion of which was referred to in the description of the preceding 

 embryo. 



The gall bladder is now a large thick-walled sac, ovoid in shape 

 and somewhat flattened dorso-ventrally. Distinct grooves sepa- 

 rate it from the median liver pouch and connecting piece behind 

 and along its dorsal edge. These grooves are however deeper 

 posteriorly than anteriorly as might be expected from their his- 

 tory in earlier stages. The extreme anterior tip of the gall blad- 

 der is drawn to a point and projects very slightly forward below 

 the anterior part of the median liver pouch. 



IV. CONCLUSIONS 



All the main divisions of the liver are now established and before 

 giving an account of their later history it may be well to sum- 

 marize the development of the organ up to this stage. The semi- 

 diagrammatic models shown in figures 9 to 12, illustrate this 

 process. They are of embryos 3.6, G.4, 7.5 and 9 mm. long 

 respectively, and are based upon wax reconstructions and meas- 

 urements of specimens described in the preceding pages. In 

 each a portion of the mid and fore gut is represented as resting on 

 a block of yolk. The dorsal half of the archenteron is cut away 



