Wader.] 402 [December 20, 



throughout the spinal column." It is evident that these are fair objec- 

 tions, and I call upon others to aid in their removal. 



But in my opinion, the most conclusive evidenee of a meketropic 

 homology between cephalic and caudal regions, lies in the fact that 

 in the very earliest stages of the vertebrate embryo, no difference 

 whatever can be detected between them ; the primitive disk is circu- 

 lar, and homogeneous, and, in the turtle at least (200, 2, 536), the 

 very first step toward the formation of organs is the depression of the 

 surface at two points upon opposite sides, which mark the position of 

 the future head and tail ; the primitive furrow appears later ; so that 

 if development is given the importance which most now allow it, we 

 can say that the two ends of the body are set off against each other, 

 as homologous and antagonistic parts, even before the right and left 

 sides are separated by the primitive furrow ; moreover, at this time, 

 the head and tail are nearer each other than the right and left borders 

 of the embryonic disk, and the subsequent elongation and narrowing 

 is adaptive and not of morphical importance. 



Prof. Wyman's fifth kind of evidence embraces the facts of resem- 

 blance between the organs at the oral and the anal outlets of the 

 alimentary canal, which was first alluded to by Oken; but it is pro- 

 bable that all determinations of the softer parts must wait until those 

 of the bones are satisfactorily made out. When, however, they are 

 taken up, it ought to be ascertained whether the reversed relative 

 position of the urinary to the intestinal orifices in Teleostei as compared 

 with other vertebrates, affects the homology of the parts, or whether it 

 may be regarded as comparable to the differences in the connection 

 of the pneumatic duct of Lepidosteus and Erythrinus, (Ow. 63, 1,494), 

 as compared with Lepidosiren and the true air-breathing vertebrates. 



THE * ' NATURE OF LIMBS." 



Can we now demonstrate the second proposition, that the armi 

 are appendages of the cephalic region of the trunk, and the skelea 

 of the caudal region, and thus find reason for regarding them as 

 similarly related ? 



The " Nature of Limbs " has been very differently interpreted, and 

 the minor problems involved in the general question are many and 

 complex. 



1. What is the normal number of membra ? 



1 See Hunter's Anat. Memoirs, edited by Owen, vol. 1, p. 193. 



