GERM LAYERS 



37 



tube opens to the exterior both anteriorly, at the anterior neuropore, and 

 posteriorly, at the posterior neuropore. Eventually the neuropores become 

 closed over, and the tube is then wholly detached from the epidermal 

 layer. The form of the tube is shown in Fig. 27, which represents a dis- 

 sected reconstruction of a chick embryo. In this dissection the epidermal 

 layer, which covers the upper or dorsal surface of the embryo, has been 

 almost all removed. A portion of it which forms a fold under the head 

 and around the anterior neuropore has been left in place, and also a por- 

 tion around the rhomboidal sinus, which may be regarded as an expanded 

 posterior neuropore. By removing the epidermal layer, the medullary 

 tube has been exposed. Anteriorly it shows a succession of expansions 



D 



E 



FIG. 25. SEGMENTATION OF THE OVUM AND FORMATION OF THE GERM LAYERS IN THE RABBIT. (A-E 

 after van Beneden; F-H, after Duval.) 



A-C represent surface views of the two-cell stage, four-cell stage and morula respectively. D-H are verti- 

 cal sections. In D and the inner cell mass is heavily shaded. Ect., ectoderm. Ent., entoderm. 

 Mes., mesoderm. 



which are to form the brain, and also a pair of lateral outpocketings, or 

 optic vesicles, each of which will become the retina of an eye. Posteriorly 

 the tube is slender, and this part becomes the spinal cord. The brain 

 and spinal cord, which are derived directly from the medullary tube, 

 constitute the central nervous system. The peripheral nervous system 

 consists of bundles of nerve fibers which ramify throughout the body, 

 together with masses of nerve cells associated with these fibers. The 

 nerve cells are detached ectodermal cells, arising chiefly from the dorsal 

 part of the medullary groove, and the fibers are protoplasmic outgrowths 

 of these detached cells and of others which remain in the wall of themedul- 



