REPRODUCTION 7 I 



embryos thus far — that is, through the gastrula stage— are essentially 

 alike. They are all chordates. They merely carry different quantities 

 of food. Herein lies the reason why so much emphasis is laid on the study 

 of the development of Amphioxus. There is reason to believe that the 

 Amphioxus embryo with its nearly minimum encumbrance of yolk reveals, 

 at least in the early stages of development, the primitive and basic proc- 

 esses which in other chordates become more or less obscured by yolk. 



The Third Layer, Mesoderm 



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

 tial 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, Hver, and the reproductive, excretory and circulatory organs, 

 making up the greater part of the bulk and weight of the animal — inter- 

 venes 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 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 differentiation to form muscle, 

 skeleton, kidneys, circulatory organs and various other structures. 



Following cleavage the embryo became a one-layered blastula. This 

 form was succeeded by the two-layered gastrula. The next important 

 event is the development of a mesoderm whereby the embryo attains a 

 three-layered stage. The earlier period of development is concerned with 

 laying out the building materials, the embryonic or "germ" layers. In 

 the later and longer period these layers are shaped into organs. The 

 formation of the central nervous organs and the notochord may begin, 

 however, before the mesoderm is fully established. 



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 some- 

 what flattened. Figure 42D shows a sagittal section of the embryo at this 

 stage. Figure 46A shows a section cutting the embryo transversely and 



