DEVELOPMENT OF THE ELASMOBRANCH LIVER 



339 



terior to the first pouch. Its ventral portion is not as yet 

 evaginated. 



As the fore gut approaches its connection with the blasto- 

 dermic entoderm it is at first broadly ovoid in cross section with 

 the narrower end of the oval upward and its vertical diameter 

 increases posteriorly. Back of the fore gut the archenteron is 

 flattened transversely until it is little more than a high, narrow 

 fold of entoderm, the transverse diameter of which is less than 

 one-fourth of the vertical diameter. The lateral walls of the gut 

 for a short distance behind and also a httle in front of the point 

 of union of the fore gut and the mid gut are differentiated into 

 dorsal and ventral zones. The epithehum of the dorsal zone is 



Fig. 1 Lateral view of the archenteron of an embryo of 19 to 20 segments, 4.0 mm. 

 in lengtji (H.E.C. 930). X 20. F.g., fore gut; G.p., gill pouches; Hep.a., hepatic 



area. 



from 25 to 30 n in thickness and contains two or more rows of 

 more or less interlocking oval nuclei. This is the primitive con- 

 dition found throughout the walls of the archenteron, both dor- 

 sally and ventrally in earlier stages. Close to the lower border 

 of the dorsal zone there is on either side a shallow and not always 

 continuous longitudinal groove which later becomes a definite 

 and important landmark. For these I suggest the name para- 

 archenteric grooves. The ventral zone in this region, as is seen 

 from following its later history, represents the liver anlage. Here 

 the epithelium is approximately half as thick as that of the dorsal 

 zone, and the nuclei, which form a single row only, lie in the basal 

 portion of the epithelium. Ordinary stains do not bring out definite 

 cell walls in either the dorsal or ventral zones at this stage. There 



