258 PRINCIPLES OF EMBRYOLOGY 



4. The trunk: ectodermal organs 



As might be expected, it is in the main body of the embryo that the 

 three fundamental layers, of ectoderm, mesoderm and endoderm, are 

 most typically developed and most clearly defmed. 



The ectoderm produces only three main organs or organ-systems. The 

 mid-dorsal part forms the central nervous system, v^hich in the trunk is 

 in the shape of a tube with thick walls, a thin floor and roof, and a cavity 

 wliich is high and narrow in transverse section. From this tube the ventral 

 motor roots grow out in a series corresponding to the somites. The dorsal 

 sensory roots, with the accompanying ganglia, although they are even- 

 tually so closely associated with the main central nervous system, have a 

 different origin in that they arise from the neural crest material. This, the 

 second of the three ectodermal systems, also comprises the sympathetic 

 nervous system and contributes to the mesodermal sheaths of the spinal 

 cord; moreover the greater part of the pigmented cells of the body, 

 whether they lie in the skin or in the linings of the gut and other internal 

 organs, arise from the same source (Rawles 1948, 1953) (the most im- 

 portant pigmented tissues with another origin are those of the iris and 

 outer layers of the eyeball). Finally the third system formed from the 

 ectoderm is the outer layer of the skin, the epidermis. The skin as a whole 

 is a composite structure, a major part of its thickness being contributed by 

 the mesodermal layer known as the dermis, which originates from the 

 upper layers of the somites. 



The skins of different classes of vertebrates are by no means simple 

 structures, but contain several sorts of sweat glands, oil glands and so on. 

 Two of these structures are of particular importance, and are worth 

 mentioning very briefly, namely feathers and hairs. Feathers are formed 

 from a so-called feather germ or follicle. This is a slight hillock on the 

 skin, from the tip of which a canal extends down through the thickness 

 of the follicle. At the base of the canal lies a conical papilla, and it is from 

 this that the feather actually grows. The papilla is a double structure, with 

 an epidermal (ectodermal) cap fitting over a dermal (mesodermal) core. 

 Many experiments have been performed on this structure, since it is one 

 of the few organs in a bird which will continue to go through its develop- 

 mental performance late into adult life; when a feather is plucked it will 

 be regrown from its original follicle. Wang (1943) was able to peel the 

 epidermal cap away from the core, and transplant the latter into follicles 

 from which their own papillae had been removed. He showed that the 

 dermal core can induce such a follicle to produce a new epidermal cap, 

 and eventually a feather. Tliis is an example of a 'secondary' organiser 



