342 RICHARD E. SCAMMON 



A distinct step in development is shown by an embryo 5 

 mm. in length (K.U.E.C. 449^). This embryo has segments and 

 three gill pouches, none of which open to the exterior. The gut 

 is still connected with the yolk entoderm by a yolk-stalk, which has 

 an antero-posterior diameter equal to nearly half the length of 

 the archenteron. A lateral view of the liver region is outlined in 

 figure 4, which is a graphic reconstruction, with the hepatic area 

 outlined with broken lines. The lateral hepatic diverticula are 

 somewhat better defined, both dorsally and ventrally, than in 

 the preceding stage and are somewhat deeper. They show in a 

 more marked way the histologic differentiation described for the 

 preceding embryo, as is illustrated by figures 5 and 6, for the 

 cells have lengthened so rapidly that epithelium is one-fourth to 

 one-half thicker than that of the dorsal zone of the gut above it. 

 Numerous mitoses indicate the rapid growth now taking place 

 in this region. It will be noticed from figure 4 that the lateral 

 liver diverticula now extend forward distinctly beyond the ante- 

 rior wall of the yolk-stalk. Here the right and left pouches are 

 fused, forming a single median and ventral pouch in the posterior 

 part of the fore gut. The finer structure of this anterior part is 

 illustrated in figure 5. It shows the same character and dis- 

 tinction from the remainder of the gut wall as does the posterior 

 part of the anlage. The right and left omphalo-mesenteric veins 

 are now present, although of small caliber. 



A slightly older embryo 6.4 mm. long shows somewhat the same 

 stage of development and the liver region has been reconstructed 

 in wax. Figures 30 and 31 show anterior and right lateral views 

 of this object. The entire embryo is inclined markedly to the 

 left. The fore gut, ovoid in cross section, becomes immediately 

 flattened and triangular after passing the anterior wall of the yolk- 

 stalk. The lateral hepatic diverticula are distinctly outlined and 

 fuse together, forming anteriorly a ventral pouch in the floor of 

 the fore gut. The antero-posterior length of the hepatic anlage 



' The following designations of embryos are employed in this paper: H.E.C. = 

 Harvard Embryological Collections. K.U.E.C. = Kansas University Embryo- 

 logical Collection. S.C. = Author's Collection. 



