346 THE CHORDATE BLASTULA AND ITS SIGNIFICANCE 



Two Other men contributed much to the layer theory of development, 

 namely, Heinrich Christian Pander (1794-1865) and Karl Ernst von Baer 

 (1792-1876). In 1817, Pander described the trilaminar or triploblast con- 

 dition of the chick blastoderm, and von Baer, in his first volume (1828) and 

 second volume (1837) on comparative embryology of animals, delineated 

 four body layers. The four layers of von Baer's scheme are derived from 

 Pander's three layers by dividing the middle layer into two separate layers 

 of tissue. Von Baer is often referred to as the founder of comparative em- 

 bryology for various reasons, one of which was that he recognized that the 

 layer concept described by Pander held true for many types of embryos, 

 vertebrate and invertebrate. The layer concept of development thus became 

 an accepted embryological principle. 



While Pander and von Baer, especially the latter, formulated the germ- 

 layer concept as a structural fact for vertebrate embryology, to Kowalewski 

 (1846-1901) probably belongs the credit for setting forth the idea, in his 

 paper devoted to the early development of Amphioxus ( 1 867 ) , that a primary, 

 single-layered condition changes gradually into a double-layered condition. 

 The concept of a single-layered condition transforming into a double-layered 

 condition by an invaginative procedure soon became regarded as a funda- 

 mental embryological sequence of development. 



Gradually a series of developmental steps eventually became crystallized 

 from the fact and speculation present during the latter half of the nineteenth 

 century as follows: 



( 1 ) The blastula, typically a single-layered, hollow structure, becomes con- 

 verted into 



(2) the two-layered gastrula by a process of invagination of one wall or 

 delamination of cells from one wall of the blastula; then, 



(3) by an outpouching of a part of the inner layer of the gastrula, or by 

 an ingression of cells from this layer, or from the outside ectoderm, a 

 third layer of cells, the mesoderm, comes to lie between the entoderm 

 and ectoderm; and finally, 



(4) the inner layer of mesoderm eventually develops into a two-layered 

 structure with a coelomic cavity between the layers. 



This developmental progression became accepted as the basic procedure 

 in the development of most Metazoa. 



The original concept of the germ layers maintained that the layers were 

 specific. That is, entodermal tissue came only from entoderm, ectodermal 

 tissue from ectoderm, etc. However, experimental work on the early embryo 

 in which cells are transplanted from one potential layer to another has over- 

 thrown this concept (Oppenheimer, '40). The work on cell lineage and the 

 demonstration of the early presence of the presumptive, organ-forming areas 



