REPRODUCTION 



69 



The epimere undergoes three kinds of diflferentiation. Its heavier 

 inner wall is mainly converted into striated body-muscle, not only the 

 dorsal but the ventral muscle. The 

 myotome material grows ventral- 

 wards, pushing its way between the 

 ectoderm and the somatic meso- 

 derm, until it reaches the mid- 

 ventral plane (compare Figs. 60 and 

 61). The medial region of the 

 epimere gives rise to loosely aggre- 

 gated cellular masses (mesen- 

 ch5mae) surrounding the notochord 

 and neural tube (Figs. 58, 60). This 

 material produces such supporting 

 structures — connective tissue, carti- 

 lage and bone — as may later be 

 developed around these two axial 

 organs. The thin outer wall of the 

 epimere breaks up to form loose 

 cellular masses, mesenchyme, which 

 give rise to the dermis, the deeper 

 fibrous and vascular layer of the 

 skin. 



The terms myotome, sclerotome 

 and dermatome are applied respec- 

 tively to the muscle-forming, skeleton-forming and dermis-forming regions 

 of the epimere. (Fig. 58) 



The mesomeres give rise to the tubular structures of the kidneys. 

 The process begins in the more anterior mesomeres and progresses pos- 

 teriorly. Certain differences in mode of development and in eventual 

 structure compel the distinction between an earlier and more anterior 

 system of tubules, the pronephros (Figs. 59, 62), and a later more posterior 

 and more extensive system, the mesonephros. In anamnia the meso- 

 nephros becomes the adult kidney and the pronephros disappears except 

 that in a few fishes it is the definitive and only kidney. In amniotes, 

 following development of a pronephros and a mesonephros, the tubule- 

 forming process continues backward, but with some modifications, to 

 form a third kidney, the metanephros, which becomes the adult kidney. 

 The tubular epididjnnis, associated with the testis of the adult amniote, 

 is a part of the embryonic mesonephros which otherwise disappears 

 except for certain vestiges which are apparently of little functional 

 importance. 



Fig. 61.— 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.) 



