EXPLANATION OF THE PLATES. xix 



A transverse section of the spinal cord, to show the groove on 

 its upper surface, and the black matter on the lluor of the 

 groove. 

 Primitive fibres from one of the lateral muscles. 

 The eighteen pieces of one of the divisions of the hyoid bone. 



The anterior piece carries no ray. 

 A few of the splanchno-ribs, to show their relations to the 

 heart, aorta, and branchial vessels, a a The ribs which bifur- 

 cate ; h b the simple ribs ; oca portion of the heart with four 

 branchial arteries ; d d a portion of the aorta with eight 

 branchial veins. 



Fig. 23. A portion of the aorta, to show the mode of connection be- 

 tween the aorta and branchial venous trunks and loops. 



Fig. 24. A portion of the heart, to show the mode in which the 

 arteries leave it. The heart or ventral vessel is somewhat 

 flattened, its upper and under walls meeting at an acute 

 angle on each side. 



Fig. 25. The lower end of one of the lateral muscular bundles, magni- 

 fied to show the position and configuration of one of the 

 generative organs. 



Fig. 26. Portions of four pieces from the hyoid bone, magnified to 

 show the mode of connection, also the cavities, and irregular 

 masses of cells which they contain. 



Fig. 27. One of the generative organs, highly magnified under com- 

 pression, to show the nucleated cells of which it is composed. 



