258 Basic Structure of Vertebrates 



THE THIRD LAYER, MESODERM 



The greater extent of the ectoderm of the embryo persists as the 

 essential layer, epidermis, of the adult skin. The endoderm gives rise 

 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 embryonic material which comes to be inter- 

 polated between the outer and inner layers of the gastrula. This middle 

 and third layer, the mesoderm, spreads extensively between the pri- 

 mary layers and at first appears to be quite undifferentiated through- 

 out. Later it undergoes local differentiation 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 anteroposterior with 

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

 somewhat flattened. Figure 210D shows a sagittal section of the em- 

 bryo at this stage. Figure 213A shows a section cutting the embryo 

 transversely and within the anterior third of its length. Except for the 

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

 Parts B-G of Fig. 213 show transverse sections at stages successively 

 later than that of Fig. 213A. Several things are happening simulta- 

 neously. 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 neighboring ectoderm. This process involves 

 the middorsal ectoderm continuously 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. 213D-F) it becomes convoluted along 

 three lines extending lengthwise of the embryo. Its median, slightly 



