240 



THE SCIENCE OF LIFE. 



of growth and repair. (Chap. IV, Sec. 13, d.) During 

 embryonic life it is represented by the notochord, a fibro- 

 cellular rod, tapering to either end, but this is replaced 

 by a more highly developed column of cartilage or bone, 

 except in the doubtful Amphioxus. The column and 

 cranium are never absent, although other parts may be 

 wanting, as the ribs in Frogs, limbs in Snakes, etc. The 

 limbs never exceed four, and when present, are always 

 articulated to the internal skeleton, on the ventral side 

 of the body, while the limbs of Invertebrates are devel- 

 oped from an external skeleton, on the neural side. The 



FIG. 139. Muscular Fibers. Magnified 200 diameters. 



muscles moving the limbs are attached to the endoskel- 

 eton and not to the exoskeleton, as in Invertebrates. 

 Muscular tissue is found in all animals, from Radiates to 

 Man. The most complete development of muscles is in 

 the Pentacrinus. (Chap. XII, Sec. 5.) Voluntary mus- 

 cular tissue always has a transversely striated appearance 

 under the microscope, (Fig. 139,) while those fibers not 

 under the control of the will are smooth. 



