DEVELOPMENT OF THE ELASMOBRANCH LIVER 353 



From a short study of early Torpedo embryos I am inclined to 

 think that in this form the stage in which the liver exists as a 

 pak of lateral diverticula must be very brief if at all present, 

 because embryos of this form separate from the blastoderm at an 

 earlier period than do Acanthias embryos and have a compara- 

 tively small yolk-stalk. The rapid formation and elongation of 

 the fore gut accompanying these changes may involve the hepatic 

 areas before they are differentiated as pouches. As regards Acan- 

 thias and probably other Selachii, it appears to me very probable 

 that investigators have been misled from a study of embryos 

 which have advanced to a considerable extent in the process of 

 development. It is interesting to note that in embryos of other 

 groups of animals possessing large yolk-laden ova the liver forms 

 at a stage when other organs are in quite a primitive condition, 

 and in only slightly teleolecithal ova the time of origin is still 

 younger. At the time when Balfour, Brachet and Laguesse record 

 the appearance of the liver in elasmobranch fishes the embryo is 

 well established, several gill slits are fully formed, the sense organs 

 are completely invaginated, the spinal nerve anlagen are laid 

 down and the limb fundaments are about to. appear. The earliest 

 anlage of the liver is not extensive and can hardly be recognized 

 without a previous study of somewhat later stages. Again after 

 the primitive paired liver diverticula are well formed they are 

 often to a considerable extent obliterated by the falling of the 

 embryo to one or the other side as it becomes top-heavy by separa- 

 tion from the yolk which has supported it up to this time. This 

 flexure causes one or more large transitory folds which tend to 

 render inconspicuous the pouch in the side upon which the embryo 

 comes to lie, and at the same time almost obliterates the opposite 

 pouch by stretching. The histologic characters of these areas 

 remain unchanged however. Figure 13 is a cross section through 

 the liver region of an embryo 4.8 mm. in length, showing these 

 changes. 



Although in this form of selachian at least the anlage of the 

 liver is a paired one, it does not follow that this is the original 

 condition of the structure. It seems probable that it has been 

 brought about purely by the mechanical influence of the large 



