Ill 



THE ALIMENTAKY CANAL 



183 



oesophagus, stomach, intestine and its subdivisions, cloaca. In 

 correlation with the digestive and assimilative function of the 

 intestinal endoderm this serves during early stages as the favourite 

 storehouse of food -yolk, and the concentration of yolk in the 

 abapical portion of the unsegmented egg is to be looked on as a 

 foreshadowing of the fact that this portion of the egg will later 

 become the endoderm. 



In the holoblastic Vertebrates the mass of heavily yolked endo- 

 derm cells becomes, as it were, modelled into a tubular shape by the 



Fig. 104. — Illustrating the modelling of this yolk in Ichthytrphis. (After Sarasins, 1S89.) 



A and B illustrate the same stage, B representing a view from the dorsal side. The small-celled 

 epithelial portion of the gut-wall is seen passing down the centre of Fig. B. C, D, and E represent 

 later stages drawn from the ventral side ; F (7 cm. embryo) ventro-lateral view from the right side. 



reciprocal activity of endoderm and splanchnic mesoderm ; the 

 rudiment so formed undergoing active growth in length and 

 differentiation of structure while the yolk is being assimilated. 



In the two most archaic groups of holoblastic gnathostomes, the 

 Crossopterygians and the Lung-fishes, a feature of special interest is 

 the development of the spiral valve. In Lepidosiren, as is indicated 

 by Tigs. 105 and 106, this takes its origin by the solid mass of 

 yolk-laden endoderm becoming modelled into a right-handed spiral 

 coil — the deep incision which separates successive turns of the 

 spiral being filled up by ingrowing mesenchyme belonging to the 

 splanchnic mesoderm. There can be little doubt that this is a 

 secondarily modified mode of development, but nevertheless it is 

 probable that the spiral coiling of the endodermal rudiment is to be 



