REPRODUCTION 95 



tion of the supporting structures — connective tissue, cartilage and bone — 



Fig. 67. — Diagrammatic transverse section of the body of a vertebrate embryo at an 

 advanced stage. The muscle-forming myotome is beginning to extend into the ventral 

 body-wall of the embryo, c, coelotn; g, genital ridge; m, muscle derived from myotome; 

 mc, myocoele; p, peritoneum; pd, pronephric duct; so, somatic layer of somite; v, advanc- 

 ing ventral border of myotome; the finely dotted areas are occupied by mesenchyme. 

 (From Kingsley.) 



which are eventually developed around these two axial organs. The 

 outer or somatic wall of the somite 

 breaks up to form loose cellular 

 masses which spread out under- 

 neath the ectoderm. As the ecto- 

 derm differentiates into the 

 definitive epidermis of the skin, the 

 underlying mesodermal material 

 forms the dermis (or coritim), the 

 deeper fibrous and vascular layer 

 of the skin. As the muscle-form- 

 ing material of the somite is called 

 the myotome, so the medial 

 skeleton-forming region, or the 

 cell-mass derived from it, is con- 

 veniently called the sclerotome, and 

 the somatic dermis-forming layer 

 is called the dermatome (Figs. 63 

 and 67). 



The mesomere in lower verte- 

 brates undergoes a segmentation 

 corresponding to that of the 

 epimeres (Fig. 63), but in amniotes 

 segmentation of the mesomeres is obscured or lacking. In anamnia the 



Fig. 68. — Diagrammatic transverse 

 section of the body of a vertebrate. 

 av, aorta; c, coelom; e, ectoderm; ep, 

 epaxial (dorsal) muscle; g, gonad; ha, 

 hemal rib; hp, hypaxial (ventral) muscle; 

 i, intestine; mes, mesentery; n, nephrid- 

 ium; o, omentum; r, rib; p, somatopleure; 

 sp, splanchnopleure; v, centrum of verte- 

 bra and, above it, neural arch containing 

 spinal cord. (From Kingsley.) 



