1012 REPOKT OF COMMISSIONER OF FISH AND FISHERIES. [32] 



longer rays or actiuophores of the pectoral shows how close this par- 

 allelism is. 



The relation of the epaxial and hypaxial arches, iu the course of their 

 development, to the chorda, and the similar close relation of the paraxial 

 processes, the ribs, to a spongy a.nd reticular mesoblastic tract which 

 invests the chorda and brain, and also extends upwards and downwards 

 between the lateral musculature of the body in the middle line, and 

 even shows itself in the vicinity of the pectoral at the time of its out- 

 growth, demonstrate very clearly that the belief in the doctrine that 

 the wiiole skeleton is derived from an interstitial mesoblastic tract which 

 is developed as a continuum over the chorda, the cerebrospinal axis and 

 also invests the muscular system, is securely founded upon observed 

 fact. 



Evidence of the occurrence of the i)rocess of concrescence, as observed 

 in the limbs of higher forms, also exists, as may be gathered from the 

 longitudinal section through the basal portion of the hind limb of a 

 chick of the sixth day. The segments s h I, which afford the nerve 

 sui)ply and muscles for the hind limb, are evidently concrescing, as 

 shown in Fig. 1, PL X. The sternal ends of the thoracic segments s t 

 also exhibit such a tendency and seem crowded forwards. Other evi- 

 dence on this point may be gathered from a paper by Miss Alice John- 

 son.* 



Investigations such as these ijromise great results when applied to 

 human and mammalian development generally. If the outgrowth of 

 the limbs can be traced by means of microtomy through a sufficient 

 number of stages, remembering also that more or less twisting or rota- 

 tion of the limb-rudiment occurs during its outgrowth, we may be able 

 to trace the nerves, blood-vessels, and muscles to their proper embry- 

 onic segments, formerly denominated protovertebrai, but now generally 

 spoken of as somites by the more recent and more exact embryological 

 writers. By the use of this method we may probably yet be able to say 

 that the subclavian artery represents one of the intersegmental vessels 

 of a young fish ; that the complex muscular system of the limbs of higher 

 forms is derived, as in the fishes, from the myotomes. 



If also it is true that the proximal skeletal elements of the limbs of 

 the lower forms are derived from a series of chondrificatious belonging 

 to successive metameric segments, then, inasmuch as it is probable that 

 the higher forms have descended from the lower, the ])roximal elements 

 of their limbs, humerus and femur, have probably also been derived from 

 an originally compound structure made up of a series of metameric ele- 

 ments. This idea is to some extent countenanced by the existence of 

 a sej)arate series of proximal and distal epi[>hysial centers of ossifica- 

 tion ill the humerus of man. The epiphyses of the bones in higher 



* On the development of the pelvic girdle and hind limb in the chick. Quar. Journ. 

 Mic. Sci., and in Studies from the Morph. Laboratory Univ. of Cambridge, 11, 1884, 

 pp. 13-25, Pis. IV, V. 



