186 



THE GERM LAYER DERIVATIVES 



Ectoderm 



neura 



epithelia 



Neural crest 



l^er\/e cord 



Neurocoel 

 \^ Sclerotome 

 Myotome 

 Dermatome 

 Mesomere 



Early organogeny. The 5 mm. frog tadpole at mid-body level. Photograph of 



cross section. 



ately co-mingle with the afferent fibers, already described. The ventral 

 nerve roots or rami are entirely efferent, the dorsal nerve roots are 



entirely afferent, but the nerve trunks contain, within a common 

 sheath, both the afferent and efferent fiber trunks. Many of these spinal 

 nerve trunks fork to send both types of fibers dorsally to the sense 

 organs and ventro-laterally to the muscles and to the limbs. 



The tadpole may have a total of 40 or more pairs of spinal nerves, 

 but only the anterior 10 pairs remain after the degeneration of the tail 

 at metamorphosis. The first pair (hypoglossal) emerges from between 

 the first and the second vertebrae, to innervate the tongue and some 

 of the muscles of the hyoid arch. The second pair (brachial) emerges 

 from between the second and the third vertebrae and are very 

 large. They have connections with the first and third spinal nerves and 

 thus form the large brachial plexus, innervating the forelimbs and the 

 muscles of the back. The third pair has the aforementioned connection 

 with the brachial but it then supplies the external oblique and trans- 

 verse muscles and the skin. The fourth, fifth, and sixth spinal nerves 

 supply the skin and muscles of the abdominal wall. The seventh, 

 eighth, and ninth spinal nerves together form the lumbosacral or 

 sciatic plexus which innervates the posterior abdominal region and 



