340 RICHARD E. SCAMMON 



are no definite boundaries, except the dorsal one to the hepatic 

 area at this time. The arrangement of nuclei just described 

 extends ventrally nearly to the point where the lateral walls of 

 the arclienteron turn laterad as a part of the blastoderm. Longi- 

 tudinally the hepatic region extends forward a little past the 

 posterior end of the fore gut to become indistinguishable in the 

 general ventral enlargement of the pharynx already referred to. 

 Its characteristics are less noticeable as we follow the gut poste- 

 riorly and 100 M behind the point of union of fore and mid gut the 

 hepatic area is indistinguishable from the other entoderm. There 

 is but a slight indication of evagination of the liver area. Al- 

 though the lumen between the walls of the ventral zone is nearly 

 twice as wide as that above, this width is due mainly to the 

 decrease in the thickness of the walls themselves, the entire trans- 

 verse diameter of the gut being a little greater dorsally than 

 ventrally. 



A slightly older embryo having 24 trunk segments and 3.6 mm. 

 in length, which is a little more advanced than the Normal plate 

 No. 20 (Scammon '11), gives a clearer picture of the liver anlage. 

 The pharynx, from which two well formed gill pouches pro- 

 ject and fuse with the skin ectoderm, is followed by a short seg- 

 ment of gut which represents both the oesophagus and the ante- 

 rior part of the stomach. This segment is elongately oval in cross 

 section with very much thickened lateral walls. It is somewhat 

 produced ventrally as it approaches the anterior wall of the yolk- 

 stalk, and in this ventral region shows lateral expansion. The 

 archenteron extends forward forming a large anterior recess above 

 the yolk in front of the anterior wall of the yolk-stalk. Immedi- 

 ately behind the point of union of the fore gut with the yolk- 

 stalk the archenteron has the same form as in the younger 

 embrj'o, being greatly elevated and flattened transversely. The 

 distinction between dorsal and ventral zones is fairly marked, 

 and the para-archenteric grooves can be traced along the gut 

 above the hepatic region, although in places they are very faint 

 and shallow. The ventral zone in this region is now distinctly 

 curved outward, forming a pair of shallow, lateral diverticula 

 which extend approximately lOO/z posterior to the anterior vitello- 



