EPIGENESIS AND THE GERM-LAYER CONCEPT 345 



(3) a notochordal area is present which later gives origin to the primitive 

 axis; 



(4) the future mesodermal tissue is represented by two areas, one on either 

 side of the notochordal area. In Amphioxus, however, this mesodermal 

 area is present as a single area, the ventral crescent, which divides 

 during gastrulation into two areas; 



(5) the entodermal area, which gives origin to the future lining tissue of 

 the gut, occupies a position in the blastula either at or toward the vege- 

 tative pole; 



(6) there is a possibility that another potential area, containing germinal 

 plasm, may be present and integrated with the presumptive entoderm 

 or mesoderm. This eventually may give origin to the primitive germ 

 cells; 



(7) the pre-chordal plate region is associated with the notochordal area 

 in all chordates in which it has been identified and lies at the caudal 

 margin of the latter. In gastrulation it maintains this association. The 

 pre-chordal plate material is an area which gives origin to some of 

 the head mesoderm and possibly also to a portion of the roof of the 

 foregut. It acts potently in the organization of the head region. Ac- 

 cordingly, it may be regarded as a complex of entomesodermal cells, 

 at least in lower vertebrates. 



C. Theory of Epigenesis and the Germ-layer Concept of Development 



As the three classical germ layers take their origin from the blastular state 

 (see Chap. 9), it is well to pause momentarily to survey briefly the germ-layer 

 concept. 



That the embryonic body is derived from definite tissue layers is an old 

 concept in embryology. Casper Friedrich Wolff (1733-94) recognized that 

 the early embryonic condition of the chick blastoderm possessed certain layers 

 of tissue. This fact was set forth in his Theoria Generationis, published in 

 1759, and in De jormatione intestinorum praecipue, published in 1769, de- 

 voted to the description of the intestinal tract and other parts of the chick 

 embryo. In these works Wolff presented the thesis that embryonic develop- 

 ment of both plants and animals occurred by "a host of minute and always 

 visible elements that assimilated food, grew and multiplied, and thus gradually 

 in associated masses" produced the various structures which eventually be- 

 come recognizable as "the heart, blood vessels, limbs, alimentary canal, kid- 

 neys, etc." (The foregoing quotations are from Wheeler, 1898.) These state- 

 ments contain the essence of Wolff's theory of epigenesis. That is, that develop- 

 ment is not a process of unfolding and growth in size of preformed structures; 

 rather, it is an indirect one, in which certain elements increase in number and 

 gradually become molded into the form of layers which later give rise to the 

 organ structures of the organism. 



