EMBRYONIC DEVELOPMENT 



207 



from each one, so that the layers may be thought of ais containing the 

 germs of those organs. They are not irrevocably destined to form these 

 organs, for, as we shall see later, their fate may be experimentally altered 

 in a variety of ways. 



From the ectoderm ordinarily arise the epidermis of the skin, reptilian 

 (but not fish) scales, feathers, hair, nails and claws, the nervous system 

 including nerves and their endings, and some glands which discharge at 

 the surface. From the endoderm comes the lining of almost the whole 

 digestive tract and of all the organs which branch off from it, such ai. 

 the lungs, liver, and pancreas, and of the thyroid gland which, though 

 wholly separate in the adult, is an out-pocketing of the digestive tract 



cc- 



en — 



vm 

 Fig. 179. — Mesoderm formation in the amphioxus, in cross section. A, evagination of 

 ridges (dotted) at upper lateral regions of endoderm; B, these ridges pinched off as tubes; 

 C, mesoderm (black) expanded so as almost to surround the digestive tract; c, coelom; dg, 

 digestive tract; dm, dermatome; ec, ectoderm; en, endoderm; m, mesoderm; mc, myocoele; 

 mp, mesodermal pouch; mt, myotome; n, neural plate; nd, notochord; nt, neural tube; soin, 

 somatic layer of mesoderm; spl, splanchnic layer of mesoderm covering the digestive tract; 

 vm, ventral mesentery. (A and B after Hatschek.) 



in the embryo. From the mesoderm are derived muscle, bone, connec 

 tive tissue, blood vessels, and the thick inner layer of the skin. 



The development of the several organs from the ectoderm and endo- 

 derm is in its early stages a bending or folding of these layers, which \6 

 called invagination or evagination according as the sheets of cells are 

 bent into, or out from, some enclosed space. The following account 

 of their origin is limited to the vertebrate animals. 



The Early Embryo. — Several of the chief systems of organs are laid 

 down at a very early time. One of the first changes visible externally is 

 the appearance of two prominent ridges, close together, along the 

 dorsal side of the future embryo. These extend lengthwise and are 

 roughly parallel except at the anterior end where they diverge from 

 one another (Fig. 181). In a cross section of the frog these ridges 

 appear as in Fig. 182, nf. They are the neural folds, the beginning 

 of the central nervous system. Where these folds are near one 

 another, the spinal cord develops; the divergent folds in front form 



