GENERAL FEATURES OF CHORDATE DEVELOPMENT 



37 



From the median dorsal wall of the entoderm a solid rod of cells is elevated 

 and separated off. This is the notochord or primitive skeleton. 



These processes are illustrated in Figure 6, also in P and H, page 56; N, 

 page 45; K, page n; and particularly well in W, plates between pages 62 and 

 63. Study also the models of the development of Amphioxus. 



2. In vertebrates. In vertebrates (with the exception of cyclostomes and 

 possibly urodeles) the mesoderm does not arise as pouches from the entoderm. 

 Instead it grows out as a solid sheet of cells from the median dorsal region and 



FIG. 6. Formation of the neural tube, notochord, mesoderm, and coelom in Amphioxus. A-D, 

 cross-sections; E, frontal section. A, differentiation of the medullary plate a, the notochordal plate d, 

 the neural folds b, and the mesodermal pouches e. B, the neural folds have closed across above the 

 medullary plate; the mesodermal pouches are farther evaginated. C, the medullary and notochordal 

 plates are beginning to close; the mesodermal pouches/ are completely separated from the entoderm. 



D, the neural tube.;' and the notochord c are completed; the mesodermal pouches are increasing in size. 



E, frontal section to show the mesodermal pouches / originating from the entoderm segmentally. a, 

 medullary plate; b, neural fold; c, notochord; d, notochordal plate; e, mesoderm; /, mesodermal 

 pouches; g, ectoderm; h, entoderm or archenteron; i, coelom. In all figures the mesoderm is stippled. 

 (From Parker and HaswelTs Textbook of Zoology, after Hatschek, courtesy of the Macmillan Company.) 



around the blastopore and gradually spreads laterally and ventrally between 

 ectoderm and entoderm. Its origin cannot be definitely ascribed to either ecto- 

 derm or entoderm, since at the blastopore these two germ layers are continuous. 

 As the mesodermal sheets spread, a central split appears in them, dividing them 

 into somatic and splanchnic walls ; the split itself is the body cavity or coelom. The 

 coelom of vertebrates thus consists from the first of a single pair of cavities 



