DEVELOPMENT OF JACANA 303 



epidermic layer, which invests a central core of compact 

 fibrous tissue, this in turn being supported by a long pro- 

 jection developed at the radial side of the first metacarpal." 

 The first metacarpal, which in most birds has a projection 

 on its radial surface, has become elongated to form a base 

 for the great spur. It is first noticeable as a very slight en- 

 largement of the bone in the newly hatched chick. As the 

 chick becomes older, the projection continues to grow, but 

 the epidermis does not commence to harden until the bird 

 is at least half grown. Up to that time it is only a bony 

 knob covered with ordinary soft skin. From now on, how- 

 ever, it begins to take the form of a spur, but does not become 

 a sharpened point until the bird reaches full maturity. 



Forbes failed to mention the second spur, if spur it may 

 be called. It is a small blunt protuberance situated just 

 below the large spur and consists of an "external, translu- 

 cent, yellow epidermic layer." It is supported by a small 

 bony ridge on the ulnare which extends up across the inner 

 side of the wrist. In the young bird the ulnare possesses a 

 ridge on its inner posterior surface, caused by an enlarge- 

 ment of the third metacarpal on which the ulnare rests. Al- 

 though late to ossify, the ridge finally becomes a base for 

 the secondary spur. It is doubtless used to strengthen the 

 larger spur as a means of defense. 



SOME ONTOGENETIC A r ARIATIONS 



In Figure 100, the body length of an adult is taken as a 

 constant and the bodies of chicks and embryos of various 

 stages are proportionally raised to that size. Thus, if the 

 body of an embryo were as large as an adult, the wing and 

 leg would be as long as in the blocked figure. By this means, 

 an idea may be obtained of the relationship, throughout dif- 

 ferent stages of development, of the several limbs to the 

 body. 



The embryo shows a fairly well defined balance between 

 the leg, wing and body such as one would ordinarily expect 



