370 THE NEW INDIVIDUAL Part IV 



are now appropriately located in the floor of the enteron, the future digestive 

 tract (Figs. 19.7, 19.9). As this cavity enlarges, the blastocoel is practically 

 blotted out. 



Mcsoderni and Notochord. The ingrown mid-dorsal cells are the future 

 notochord and mesodermal somites (Figs. 19.8, 19.9, 19.10). They form a 

 temporary roof of the enteron whose sides and floor are made of endoderm. 

 The enteron soon acquires an endodermal roof by the upgrowth and meeting 

 beneath the notochord of the endodermal cells that form its sides. The chorda- 

 mesoderm is continuous on each side with other potential mesodermal cells. 

 These have turned in along the lateral lips of the rim of the blastopore and lie 

 between the outer ectoderm and the inner endoderm. 



Crevices now appear in the mesoderm along the sides of the body; these 

 widen and extend forward and backward, splitting it into two layers, one that 

 unites with ectoderm (somatopleure) and forms the future body wall, and the 

 other that unites with endoderm (splanchnopleure) to be the future wall of 

 the alimentary canal. The crevices between the layers are the beginning of the 

 future coelom which will contain the digestive canal, kidneys and other organs 

 of the body (Fig. 19.10). 



Thus the three principal layers, ectoderm, mesoderm, and endoderm and 

 the notochord are established. Cell division, movement, and differentiation 

 have gone on together. All over the embryo parts are growing and changing 

 partly because of what their inherited genes make them and partly because of 

 their environment, the effects of their neighbor cells. 



Nervous System and Epidermis. While the mesoderm and notochord are 

 being established the nervous system is also taking shape largely under their 

 influence. A broad band of thickened ectoderm that extends forward from the 

 blastopore lies directly over the notochord and its adjoining mesoderm. This 

 is the neural plate, the material of future brain and spinal cord (Fig. 19.8, 

 19.9, 19.10). Along its borders, cells accumulate in ridges, the neural folds 

 which gradually come together and unite to make the neural tube. The neural 

 tube then differentiates; the front part of it becomes brain; the remainder be- 

 comes nerve cord. During the closing of the neural tube some of the cells are 

 left along each side. These are the neural crests from which the dorsal ganglia 

 of the spinal nerves arise. 



Cilia are now abundant on the skin ectoderm and their steady backward 

 beat keeps the embryo slowly turning over and over while it is still within the 

 egg membranes. After they hatch, tadpoles are moved smoothly forward by 

 their cilia. 



Form and Organs of the Tadpole 



The embryo grows rapidly, especially its head and tail. As it lengthens, it 

 loses its stumpy form and looks more and more like a corpulent fish. 



