250 GENERAL ZOOLOGY 



chord between the ectoderm and endoderm. On either side 

 of the notochord the mesoderm becomes segmented to form a 

 row of boxlike structures called somites; but lateral to each 

 series of somites, the mesoderm extends as a sheet of cells between 

 the ectoderm and endoderm. Later this sheet splits into two 

 layers, one applied to the endoderm and the other to the ecto- 

 derm, the space between the two forming the coelom or body 

 cavity. The somites later are transformed into muscle and 

 connective tissue. 



The ectoderm of the frog produces the epidermis of the skin, 

 cutaneous glands, the entire nervous system, the lens of the eye, 

 the lining of the oral and cloacal cavities, the enamel of the 

 teeth, and some muscle. In other vertebrates the ectoderm also 

 produces mammary glands, sweat glands, hair, horns, hoofs, 

 nails, scales (of reptiles and birds), and feathers. The meso- 

 derm, in addition to producing most of the muscle tissue, forms 

 the corium of the skin, connective tissue, blood and lymph 

 vessels, most of the organs of the excretory and reproductive sys- 

 tems, peritoneum, and skeleton, including the dentine of the 

 teeth. The endoderm forms the inner layer of the alimentary 

 canal, the epithelium of the liver, the pancreas, thyroid and 

 thymus glands, and the respiratory tract. It also forms the 

 lining of the Eustachian tube and middle ear. 



Neural Tube. — At the end of gastrulation, the ectoderm over 

 the entire surface is divided into an outer and inner layer of 

 cells. As the blastopore is closing, a groove, the neural groove, 

 is formed in the ectoderm, extending forward for about 90 degrees 

 from the blastopore. This groove is flanked on either side by 

 low ridges in the ectoderm called lateral neural folds, which are 

 connected anteriorly by a curved transverse fold, the whole form- 

 ing a continuous low wall enclosing the medullary plate in front 

 and at the sides. The inner layer of the ectoderm of the medul- 

 lary plate is several cell layers in thickness, and a similar thicken- 

 ing of the inner ectodermal layer is present in the outer slope of 

 each of the lateral neural folds. These lateral thickenings of the 

 inner layer of ectoderm, known as neural crests, lie one on either 

 side of the medullary plate, to which each is connected along its 

 inner edge (Fig. 154). The neural folds increase in height, grow 

 toward each other, and fuse along their edges, thus converting 

 the medullary plate into a neural tube. In the formation of the 



