Hepatic Ontogenesis

It is generally accepted by comparative physiologists that the liver is a unique organ which is found only in vertebrates and which may possess limited homophyly with the midgut glands of invertebrates.A variety of anatomical terms for such organs is currently used by investigators studying invertebrate gastroenterology.The terms include midgut glant, hepatopancreas, hepatic caecum, liver, liver diverticulum, digestive gland, digestive diverticulum, midgut caeca, racemose gland, midgut diverticulum, pyloric caecum and glandular media intestini.The anatomical differences of these invertebrate glands and the multitude of names assigned to them confuse biologists as to their true physiologic nature.There have been 1.12 million species of animals described; 920,000 are insects (1).It is confusing to the systematist that, although the organs of animals differ in anatomical design and complexity, they may all perform fundamentally similar or identical activities.Such analogous devices of highly different species occur despite their different evolutionary origins.This phenomenon of convergence led early scientists to assume that likeness in structures suggests the existence of one ancestral stock as a monophyletic function.It is now well known that such similar organs have arisen polyphyletically by "different evolutionary patterns of adaptive radiation" (1).Early zoological literature commonly referred to digestive glandular outgrowths of the enteron as "liver"; in more recent years, the use of the term "midgut gland" has become popular.Since 1960, the highly conspicuous and more fully studied digestive gland(s) and midgut gland(s) of molluscs and crustaceans have often been referred to as the hepatopancreas ( 2 ) .In 1973, van Wee1 agreed that these glands secrete many digestive enzymes, but concluded that direct evidence for the existence of liver-like functions assigned to this organ is "scant, if not lacking."He strongly recommended retention of the term midgut gland(s) and that the term hepatopancreas is a "misnomer, basically incorrect'' and "ridiculous" (2).Van Weel's plea had limited visibility and acceptance.