1871.] 401 [Wilder. 



both these differences are explicable like the preceding, and that they 

 would not exist in the Amphioxus. 



" Fourth. The first traces of vertebral segments are to be found in 

 three or four pairs of plates, which appear on either side of the primi- 

 tive axis, midway between the two ends ; the ossification of the bodies 

 of the vertebrae takes place in the same order, beginning in the mid- 

 dle and extending in either direction." 



Upon this point Dr. Cleland has written me as follows : " Remak 

 is quite explicit in the statement that the primordial vertebrae are 

 developed from before backward; it is quite true that the first three or 

 four which appear, are placed about the middle of the embryo, but 

 that is because the cephalic part of the embryo forms so large a por- 

 tion of the whole." May 7th, 1868. Upon this I cannot give an 

 opinion, because it is not yet determined where the middle point 

 of the vertebrate embryo lies ; indeed, it seems to shift position from 

 stage to stage of growth. Agassiz's statements respecting the turtle 

 agree with Wyman's, but if we exclude the head from the length, 

 then the first vertebrae appear to be formed in the neck ; and I have 

 observed that in a large adult skate, Raia ocellata, the segmentation 

 of the vertebral column appears at some distance behind the occiput, 

 and increases gradually toward the middle of the length. 



Bischoff's figures, especially in the paper on Kaninchen-Eies (figs. 

 53 and 54), indicate that the primitive vertebrae begin at some dis- 

 tance behind the cerebral vesicles; but Huxley (78, 11) states that 

 the protovertebrae commence at the anterior part of the cervical re- 

 gion and gradually increase backward. The matter can only be 

 decided by observations made with the present doubt in view. 



But for this as for the previous questions, I believe we must look to 

 Amphioxus. 



I quote further from Cleland's letter. " A strong point against 

 primitive antero-posterior symmetry, is found in the construction of 

 the vertebrae ; the body of each vertebra, according to Eemak, is 

 originally formed, the anterior half from one primordial vertebra, the 

 posterior from another ; you have these two parts seen in the shape of 

 two cones placed apex to apex, and if there were primitive sym- 

 metry, surely, when the arch and ribs are in connection with the 

 anterior cone in the anterior vertebrae, they ought to spring from the 

 posterior cone in the posterior vertebrae ; but it is not so ; they always, 

 I believe in all vertebrates, come from the anterior half; . . . .the 

 nerves also, lie behind the arches and ribs of the permanent vertebrae, 



PROCEEDINGS B. S. N. H.—VOL. XIV. 2 AUGUST 6, 1872. 



