ISGT.] 249 [Wynum. 



equal functions will have equal growth and development, while tlie 

 revei-se will be the case in those having the opposite conditions. A 

 close approach to fore and hind symmetry of limbs is found in certain 

 swimming animals, as the OrnithorJn/nclius and Ichthyosaurus, while 

 extremt' asymmetry is found in birds, in which aerial locomotion be- 

 loni^s to the arms, and land or a(|uatic locomotion to the legs, or as 

 in the k mgaroos and other jumping animals, where the hind liinljs 

 j)redom;nate so largely over the fore ones, and in the apes and three- 

 toed sloths, where the reverse is the case. Similar extreme differences 

 are still better indicated by the animals whose tracks are left in the 

 Connecticut River sandstones, as ibr example, in Anonioepus and 

 analogous forms. 



The facts brought to light by the study of embryos, offer additional 

 evidenci! in support of the view that the fore and hind portions of the 

 body are in idea symmetrical. As already stated, this is the more 

 noticeable the nearer the embryo is to the earliest stage of its develop- 

 ment ; and it is to this that attention should be carefully turned, for 

 as the embryo becomes more specialized, and its organs take on those 

 forms Avhich adapt the individual to its future conditions of life, the 

 differences rapidly increase. 



In the general development of the ver- 

 tebrate ambryo, iha firM fact which strikes 

 us, as it Increases in size, is, that this in- 

 crease Is not from a growth from before 

 backward, but from a central, and, as It 



were, a neutral point, both backwards and 



forwards, so that the two ends are made 

 to recede from the centre in opposite di- 

 rections, just as do the radicle and plum- 

 ule of a plant from the point where these 

 are continuous. By this process the head 1 



and tail both become free, Avhile the cen- j 



tral part of the body remains attached to ! 



the yelk. .S'eronrf/^, the primitive groove of p. ^ 



the nervous axis in Its earliest stage (Fig. 



2) is nearly symmetrically enlarged at either end, so as to form two 

 opposite dilatations : one the precursor of the future cerebral vesicles, 

 and the other of the rhomboidal sinus, which last has only a temporarv 

 existence In the mammals, but Is permanent In the birds.* Thirdly, 



* lu some adult fishes the spinal marrow ends in a ganglionic enlargement, form- 

 ing a kind of caudal brain. We have found such a ganglion quite conspicuous in 

 the American io;)/(/M.s', and Quatrefiigos describes one in .-im^j/i/oci/s, wliicli is all 

 the more striking, since it is only a diminutive repetition of the fore end of the 

 axis. Tlie distribution of the last pair of nerves, as seen in liis admirable ligures, 

 is also symmetrical with that of the first. 



