Wilder.] 400 [December 20, 



2. The armus and skelos are appendages of the cephalic and caudal 

 regions respectively. 



Whence it follows that the armus is comparable with the skelos as 

 the two armi or the two skelea are comparable with each other. 



CEPHALICO-CAUDAL HOMOLOGY. 



The evidence in favor of the first proposition is admirably stated 

 by Wyman, 55,249 ; but I think we must eliminate what he regards 

 as " the most striking facts bearing upon the idea of fore-and-hind 

 symmetry," the antagonistic attitude which the membra assume dur- 

 ing the third stage of development, (55,252.); since the syntropists 

 would s-ay that this attitude is only secondary and adaptive with the 

 mammalia, and does not even occur at all with lower vertebrates. 



As to the trunk, we quote Wyman's statements as follows : 



" First. The embryo increases in size, not by a growth from before 

 backward, but from a central, and, as it were, neutral point, both 

 backward and forward, so that the two ends are made to recede 

 from each other in opposite directions." 



To this it may be objected, that with the turtle (Agassiz, 200, 

 2, 539 and 543), and probably with most vertebrates, the cephalic fold 

 is first formed; and retains throughout a prominence by which it is dis- 

 tinguishable from the caudal fold ; but on the other hand we may say, 

 that the turtle is from the beginning a cephalized organism, and all its 

 development must have reference to the after existence of a promi- 

 nent head, so that this priority in appearance of a cephalic over a 

 caudal part is purely telica} and no bar to a morphical<;omparison. 

 I am inclined to doubt whether this objection could arise with Am- 

 phioxus. 



" Second. The primitive groove of the nervous axis in its earliest 

 stage is nearly symmetrically enlarged at either end, fo as to form two 

 apposite dilatations ; one the precursor of the future cerebral vesicles, 

 the other of the rhomboidal sinus." 



" Third. When the spinal groove closes up, it does so, as Reichert 

 has shown, by the union of its lips, first in the middle portion, and 

 then gradually in a symmetrical manner towards either end." 



To the above it will perhaps be answered that with turtles, (200, 

 544, and 546), the primitive furrow appears first nearer the cephalic 

 fold, and its closure also begins in that region ; but it is probable that 



