Normal Plates of the Development of Lepidosiren paradoxa and Protopterus annectens. y 



Digestive tract. (Graham Kerr, 1909.) 



The comparatively undifferentiated stomach arises from the hinder part of the foregut. The yolk- 

 laden region behind this becomes gradually converted by a process of modelling — associated with the 

 ingrowth of mesenchyme into the solid yolk along a spiral line — into the spirally coiled mid-gut. As the 

 difterentiation of the mid-gut proceeds the swollen character of its anterior portion containing the main 

 mass of yolk becomes less and less pronounced until at stage 36 the thickest and yolkiest part of the mid- 

 gut is some distance back in the second turn of the spiral. Later (stage 37) the intestine forms in external 

 appearance a straight cylinder the turns of the spirally coiled endodermal tube being now ensheathed in 

 a thick cylindrical wall of connective tissue. By this stage the rectum is slightly dilated and dorsal to it 

 passes forvvards the elongated finger shaped rudiment of the cloacal caecum (see p. 10). 



Liver. 



The liver is in its early stages an outgrowth of the alimentary rudiment anteriorly, becoming 

 distinguishable (about stage 31) by its yolk assuming a fine grained character. An ingrowth of vascular 

 mesoderm cuts off the liver rudiment except for the narrow stalk by which it remains continuous with the 

 gut wall. The liver rudiment rapidly increases in bulk particularly in anteroposterior diameter and it soon 

 loses its at first symmetrical shape and mesial position becoming rotated round so as to lie on the right 

 side of the stomach. 



Pancreas. 



The dorsal pancreas makes its appearance about stage 32 as a rounded yolk-laden projection from 

 the gut wall dorsally and to the left of the middle line and situated about the level of the posterior 

 nephrostome in Protopterus, rather farther back in Lepidosiren. In Lepidosiren the rudiment, even in early 

 stages, forms a hollow diverticulum of the gut wall while in Protopterus it is solid except for a small 

 irregulär closed lumen. A little after the appearance of the dorsal pancreas, a pair of ventral rudiments 

 develop, one on each side of the bile duct opening. These latter meet and fuse dorsal to the bile duct 

 and then the dorsal pancreas fuses with the right ventral so as to produce the single pancreatic complex 

 of the adult. By stage 35 the pancreas is penetrated by a network of blood vessels and is becoming 

 histologically differentiated. The most noteworthy feature in the later development of the pancreas is that 

 at no period does it come to project conspicuously beyond the general outline of the gut wall : it remains 

 throughout life concealed within the mesodermal sheath of the gut. This led to its existence being ignored 

 up to the date of Parker's paper upon the structure of the adult Protopterus. 



Coelomic organs. 



The mesoderm and notochord are represented in early stages (stage 12) by the medium sized 

 blastomeres with medium sized yolk granules which occupy the space between the dorsal wall of the 

 archenteron and the medullary plate, thinning out towards either side and eventually passing laterally 

 without any break into the large blastomeres. This common rudiment of mesoderm and notochord becomes 

 marked off from the endoderm except at its outer edge by a distinct split, while later on (ca. stage 14) 

 another split appears on each side which demarcates the lateral mesoderm rudiment from the median 

 chordal rudiment. Segmentation of the mesoderm on each side begins about stage 17, while the lateral 

 unsegmented mesoderm continues to extend by delamination from the large-yolked cells of the definitive 



