ABSTRACT
The topography of the presumptive rudiments of the endodermal organs in the neurula of R. nigromaculata nigromaculata was studied by local vital staining. The results obtained are summarized in Text-fig. 22.
The main portion of the fore-gut develops into the pharynx, oesophagus, stomach, and the anterior half of the duodenum.
The lateral and ventral walls of the liver diverticulum give rise not only to the liver, but also to the gall-bladder, the bile-duct, and the ventral pancreas. The dorsal pancreas is derived from the dorsal and lateral archenteric walls at the boundary between fore-gut and mid-gut.
The yolk mass subjacent to the mid-gut is the material for the small intestine and the posterior half of the duodenum. The whole endoderm originally lining the mid-gut is incorporated into the dorsal wall in consequence of the closure of the archenteron and the perforation of the definitive cavity of the intestine through the yolk mass.
The hind-gut mostly forms the rectum; only a small portion at the margin of the blastopore becomes the post-anal gut.
