Embryonic Development} Differentiation of Tissues - 283 



Origin of the Skeleton and Musculature. 



Except for certain parts of the skull — which 

 arise from the ectoderm overlying the de- 

 veloping brain — the skeleton arises from 

 mesoderm. Initially the skeletal material is 

 cartilage; but this cartilage tissue (p. 289) 

 is almost completely replaced by bone dur- 

 ing later embryonic development — except in 

 the lower cartilaginous fishes (p. 669). How- 

 ever, the form and pattern of the skeletal 

 parts remain essentially the same during the 

 period when the cartilage is resorbed and 

 replaced by bone. 



In vertebrate embryos (at the stage shown 

 in Fig. 15-1 ID, E) one can identify a dorsal 

 mass of mesoderm, which lies on either side 

 of the notochord and neural tube. As devel- 

 opment proceeds, this dorsal mesoderm be- 

 comes subdivided by a number of transverse 

 creases into a series of blocklike segments, 

 called somites. The mesodermal somites (Fig. 

 15-1 IE) give rise to the skeletal and mus- 

 cular structures in the several segments of the 

 body. Each vertebra is blocked off by the 

 mesoderm surrounding the notochord and 

 neural tube, as this tissue differentiates into 

 cartilage. The cartilage grows dorsally around 

 the neural tube, forming a neural arch (Fig. 

 15-1 IE); and ventrally, the cartilage first 

 surrounds (Fig. 15-1 IE), and then invades 

 the notochord. Gradually all the notochordal 

 tissue is resorbed, leaving the segments of 

 cartilage, which become the centra of the 

 vertebrae. Accordingly, the vertebral column 

 is made up of a series of segments, or ver- 

 tebrae, linked together end to end. Collec- 

 tively the vertebrae form a flexible skeleton 

 — the vertebral column — that lies embedded 

 in the dorsal body wall. The neural arch of 

 each vertebra surrounds one portion of the 

 spinal cord, so that the column as a whole 

 provides a protective tunnel, which houses 

 the spinal cord throughout its length. This 

 protective housing becomes even more effec- 

 tive when the cartilage is later replaced by 

 bone (in all except the lower vertebrates). 



Virtually all of the muscle in each body 

 segment arises from the lateral part of the 



mesodermal somites. Some of these mesoderm 

 cells remain in situ, giving rise to the massive 

 muscles of the back, but others migrate ven- 

 trally — through the mesentery into the gut 

 wall — and into the lateral and ventral parts 

 of the body wall. In the gut wall this meso- 

 derm differentiates into visceral (nonstriated) 

 muscle (p. 286); but in the body wall it gives 

 rise to skeletal (striated) muscle (p. 286). 

 Most of the connective tissues (p. 288) also 

 arise from cells that migrate out from the 

 mesodermal somites. 



Further Development of the Digestive Tract 

 and Its Outgrowths. The mouth, in verte- 

 brates, arises as a shallow invagination, the 

 oral pit, which pushes inward and joins the 

 archenteron near the anterior extremity of 

 the embryo (Fig. 15-1 IF); the anus arises 

 similarly at the posterior end of the enteron, 

 just ventral to the blastopore. Thus part of 

 the lining of both the mouth and the anus 

 is derived from ectoderm, and both these 

 passages display a line of junction between 

 the ectodermal and enclodermal tissues. 

 Among invertebrates, the blastopore persists, 

 becoming the mouth, as in the earthworm 

 (Fig. 15-9). 



The gill slits originate as a series of out- 

 pocketings (Fig. 15-1 IF), which push laterally 

 from the endoderm to join a corresponding 

 set of inpocketings from the surface ecto- 

 derm. In higher vertebrates the penetration 

 of the gill passages is usually incomplete — 

 although one pair of clefts forms a passage 

 that persists and gives rise to the Eustachian 

 tubes (p. 425) and middle ear chambers of 

 the adult. 



The lungs, liver, pancreas, and urinary 

 bladder all originate as tubular ontgroivtlis 

 from the developing enteron. Accordingly all 

 these structures possess an internal lining 

 derived from endoderm. However, visceral 

 mesoderm always envelops these outgrowths 

 at the time when they originate from the 

 enteron, and this mesoderm gives rise to the 

 muscle, connective tissue, blood and lymph 

 vessels, and the external epithelia of all the 

 organs that grow out from the gut. In fact, 



