EXISTING ORGANISMS— EMBRYOLOGY 



59 



tissue appears near this cartilaso, bone spicules form in it, and a 

 definite bony structure makes its appearance. This bone envelops 



3ftS!'-0CCfPiTrtL, j J 



Fig. 37. — Diagram of the bones of the mammalian skull. Cartilage bones 

 dotted, membrane bones lined. 2-12, nerve exits. (From Kingsley\s 

 Comparative Anatomy of Vertebrates, with the permission of P. Blakiston's 

 Son & Co.) 



Meckel's cartilage (Fig. 39), and finally replaces it, but for a time 

 a relationship exists between the two similar to that in the bony 

 fishes. In the appendicular skeleton a similar change takes place 

 during embryonic development, 

 but most of the bones are pre- 

 formed in cartilage. The clav- 



^otochordal sheath with 

 ^, invading cartilage 



icle in the pectoral girdle of 

 man is added as a membrane 

 bone. In the pelvic girdle the 

 most striking changes arc due to 

 the increased stresses incidental 

 to terrestrial life ; one of them is 

 the connection of this girdle 

 with the spinal column by the 

 two dorsal bones, the ilia. 



The Circulatory System. Fishes. 



Extent of one vertebra 

 Fig. 38. — Diagram of a longitudinal 

 section through a developing verte- 

 I)ral column to show the invasion of 

 the notochord by the cartilages from 

 which the centra of the vertebrae 

 develop. (From Woodruff, after 

 Walter.) 



Although any organic sys- 

 tem in the body shows similar evidences of relationship, the circu- 

 latory system surpasses all others in its completeness. Returning 

 to the fishes again for the primitive type, we find a heart consist- 



