DIFFERENTIATION OF SOMITES 



503 



The outer portion of the myotome gives rise to the muscles of the 

 neck and trunk, the muscles of the appendages arising independently of 

 the myotomes. 



>v e . P . 



77 



Fig. 299. 



The development of the mesonephros. A, B. Transverse sections through the 

 mesonephric tubules of the duck embryo with forty-five pairs of somites. After 

 Schreiner. C. Transverse section through the middle of the mesonephros of a 

 chick of ninety-six hours. From Lillie (Development of the Chick). Ao., Dorsal 

 aorta ; B., rudiment of Bowman's capsule ; c., collecting duct ; Cod., crelom ; Col. T., 

 collecting tubule ; d., dorsal outgrowth of the Wolffian duct ; Glom., glomerulus ; 

 germ. Ep., germinal epithelium ; M's't., mesentery ; .., nephrogenous tissue ; r., 

 rudiment of conducting portion of primary tubule ; T. 1, 2, 3, primary, secondary, 

 and tertiary mesonephric tubules ; V.c.p., posterior cardinal vein ; W.D., Wolffian 

 duct. D, Cross section through the head kidney in the region of the gonads of a 4 

 day chick embryo, a, germinal epithelium showing the primary germ-cells c and o; 

 a, portion of the peritoneal epithelium which forms the Mullerian duct ; E, the 

 tissue immediately surrounding the germ cells which forms the stroma of the 

 gonads later ; L, Somatopleure ; m, mesentery ; WK, Pronephros ; y, Wolffian duct ; 

 z, Mullerian duct. (After Waldeyer and O. Hertwig.) 



THE EXCRETORY SYSTEM 



It is on the third day that the intermediate cell mass the mesomere 

 (Fig. 268, mm) lying between the somite proper and the point where 

 the mesoderm splits into somatic and splanchnic layers, becomes very" 

 prominent, being covered with sharply defined epithelial cells (Fig. 

 299). 



It is of great importance for one's future study of embryology as 

 well as for the study of comparative anatomy that the development of 

 the excretory system be thoroughly understood. 



It is this intermediate cell mass or mesomere, now called the nephro- 

 tome, which is to develop into both urinary and reproductive systems. 



