50 CHORDATE ANATOMY 



directly to the lining epithelium of the adult digestive tube. But in the 

 adult animal a great complex of structures — muscle, skeleton, central 

 nervous organs, lungs, liver, and the reproductive, excretory and cir- 

 culatory organs, making up the greater part of the bulk and weight of the 

 animal — intervenes between the epidermis and the endodermal digestive 

 epithelium. Some of these intermediate organs take origin directly and 

 independently from the primary ectoderm or endoderm. For example, 

 before the close of the gastrula stage the central nervous organs begin 

 to differentiate from the dorsal ectoderm. Later, lungs, liver and pancreas 

 arise as separate localized outgrowths from the endoderm of the early 

 digestive tube. Others of the intermediate organs have an indirect 

 relation to the primary layers of the gastrula. The close of the gastrula 

 stage is marked by the formation of a layer, or system of layers, of embry- 

 onic material which comes to be interpolated between the outer and inner 

 layers of the gastrula. This middle and third layer, the mesoderm, 

 spreads extensively between the primary layers and at first appears to 

 be quite undifferentiated throughout. Later it undergoes local differentia- 

 tion to form muscle, skeleton, kidneys, circulatory organs and various 

 other structures. 



In Amphioxus. At the close of the gastrula stage the Amphioxus 

 embryo is approximately ovoidal, the long axis antero-posterior with the 

 blastopore at its posterior end. The dorsal surface of the embryo is 

 somewhat flattened. Figure 38Z) shows a sagittal section of the embryo 

 at this stage. Figure 42.4 shows a section cutting the embryo trans- 

 versely and within the anterior third of its length. Except for the dorsal 

 flattening, the configuration of layers is as simple as possible. Figures 

 B-G show transverse sections at stages successively later than that of 

 Fig. 42/I. Several things are happening simultaneously. A broad band 

 of dorsal ectoderm {NP), slightly thicker than the adjacent regions of the 

 layer, becomes separated, along its right and left edges, from the neighbor- 

 ing ectoderm. This process involves the mid-dorsal ectoderm con- 

 tinuously from the blastopore almost to the anterior end of the embryo. 

 The median ectoderm thus delimited from the lateral ectoderm is the 

 material of the prospective central nervous organ, the neural tube. In this 

 initial stage it is called the neural (or medullary) plate. 



The dorsal endoderm is at first flattened in conformity with the 

 neural ectoderm but later (Fig. 42D-F) it becomes convoluted along 

 three lines extending lengthwise of the embryo. Its median slightly 

 thicker region becomes sharply folded upward. On either side of this 

 median fold a longitudinal groove appears on the inner surface of the endo- 

 derm. Then the endoderm in the region of each of these grooves assumes 

 the form of a fold extending outward dorso-laterally. Thus arise three 

 folds, one median and a lateral pair, all convex outward, and extending 



