VERTEBRATE SKELETON 



necessary here as they are given in every text-book of embryology. 

 More important is the differentiation of the skeletal-forming (sclero- 

 blastic) tissues and their distribution so that the steps in the forma- 

 tion of the skeletal parts may be followed. 



In the early stages of all Vertebrates three germ layers are present 

 - — ectoderm and entoderm, with the third, the mesothelium between 

 them. At first the mesothelium forms pairs of closed sacs, the cavi- 

 ties of these being the ccelom. One face of each sac — the somatic — 

 is turned towards the ectoderm, the other (splanchnic) faces the 

 entoderm. The dorsal part of each coelomic sac becomes divided 

 by transverse incisions into a series of quadrate bodies (myotomes) 



which a little later separate from 

 the lower part of the sac, each now 

 forming a closed vesicle, its cavity 

 being the myocoele. 



In the lower and simpler Verte- 

 brates these three layers constitute 

 the whole of the early embryo. 

 (Amphioxus never has more than a 

 slight differentiation of the fourth 

 (mesenchymatous) layer.) At first 

 these layers are distinct, being sepa- 

 rated by an actual or potential space, 

 the remains of the archicoele of the 

 cleavage stage of the egg. 



The fourth layer, the mesen- 

 chyme, owes its cells to all of the 

 other layers, but the greater part of 

 them — and practically all which are 

 -come from the myotomes. A part 

 of the splanchnic wall of each myotome (fig. 2) buds cells into the 

 space between the rest of the mesotheHum and the axial structures 

 (blood vessels, notochord, and central nervous system), these cells 

 forming the mesenchyme for most of the axial skeleton. At first 

 these cells form groups (sclerotomes), metameric like the myotomes 

 from which they arise. 



A second source of mesenchyme lies in the somatic myotomic 

 walls, the cells of which lose their epithelial character, and, lying 

 just beneath the ectoderm, form the deep layer (corium) of the skin, 



Fig. 2. — Section of Lacerta^embTyo, 

 showing proliferation of mesenchyme 

 from splanchnic side of myotome to form 

 scleroblasts. a, aorta; /, limb-bud; m, 

 muscle-forming part of myotome; w, 

 notochord; sk, sclerotomic part of 

 mesenchyme. 



concerned in skeletal formation- 



