GERM LAYERS 41 



In later stages each somite gives rise to a stream of cells which spread 

 around the medullary tube, notochord and aorta. After these cells have 

 been given off, the somite appears as a plate-like structure (Fig. 30), 

 known as the dermo-myotome. The principal derivative of the dermo- 

 myotome is the voluntary musculature of the body. In producing the 

 various voluntary or skeletal muscles, certain cells of the dermo-myo- 

 tome become transformed into muscle fibers. These are at 'first arranged 

 in segmental masses, but the masses become subdivided into groups 

 representing the individual muscles. The groups become separated 

 from one another and shift to their final positions. Subsequently they 



FIG. 29. 



FIG. 29. TRANSVERSE SECTION OF A RABBIT EMBRYO MEASURING 4.4 MM. (9! DAYS). X6o. 

 FIG. 30. TRANSVERSE SECTION OF A RABBIT EMBRYO MEASURING 5 MM. (n DAYS). X4O. 

 Ect., ectoderm; Ent., entoderm; Int., intestine; Med. tube, medullary tube; Msnch., mesenchyma; 

 Msth., mesodermal epithelium; Nch., notochord; Som., somatopleure; Som. mes., somatic meso- 

 derm; Spl., splanchnopleure; Spl. mes., splanchnic mesoderm; W. d., Wolffian duct. 



acquire their connections with the bones, which develop later than the 

 muscles. The remainder of the dermo-myotome breaks up into cells 

 which are contributed to the deep portion of the skin. 



Connecting the somites with the lateral somatic and splanchnic 

 layers of the mesoderm, there is a narrow neck of cells (as seen in cross 

 section, Fig. 29) which is known as the intermediate cell mass, or 

 nephrotome. The nephrotomes at first are not segmentally divided, but 

 form the floor of a longitudinal groove in the mesoderm, lateral to the 

 somites (Fig. 27). The nephrotomes give rise dorsally to a longitudinal 

 cord of cells, which later becomes a tube, and is known as the Wolffian 

 duct (Figs. 27, 29, and 30). It lies in the groove above the nephrotomes. 

 This duct grows posteriorly and acquires an opening into the entodermal 

 bladder. The nephrotomes then become separated from the somites 

 and from the lateral layers of the mesoderm, and their cells become ar- 

 ranged so as to form coiled tubes, which empty into the Wolfiian duct. 

 In this way the mesoderm gives rise to the renal system, which consists 

 essentially of coiled mesodermal tubes, receiving urinary products from 



