6 Journal of Comparative Neurology. 



from one and the same law : the abolition of the /miction of the 

 cell ; in other words, when one member of the functional group 

 is destroyed the other suffers also and the result is the same 

 whichever member is first affected. 



This case, too, we may add, has an important bearing 

 upon the problem of the ultimate source of the cranial and 

 spinal ganglia with the peripheral nerves related to them. His 

 in 1890 affirmed that these ganglia arise neither from the epider- 

 mis nor from the nerve-tube, but from a band of ectodermal 

 elements lying between the two (Zwischen Strang'). Baird, on 

 the other hand states that the Zivischen Strang is not nervous 

 at all and that the real proton of the ganglia is derived primarily 

 from the nerve-tube, with a subsequent reenforcement from the 

 skin, so also Balfour, Van Wihje, Froriep, Kupffer. Still more 

 recently Mitrophanow 1 has adduced evidence to show that in 

 Selachians the peripheral nervous system with its ganglia is 

 derived from the central system, and that the nerve-tube is com- 

 pletely closed and separate from the ectoderm before this pro- 

 cess begins. Since the entire central and peripheral nervous 

 system arises from a single proton, he would insist that any 

 segmental arrangement exhibited by the latter, as in the sense- 

 organs and the organs of the lateral line, must be due to pre- 

 existing arrangements of the body to which the nervous out- 

 growths conformed by secondary adaptation. It is hard to 

 reconcile these observations with the presence of well developed 

 spinal ganglia and nerves exhibiting the normal metameric 

 arrangement, in the total absence of both brain and spinal cord. 

 It is highly important that Professor v. Leonowa's suggestive 

 observations be extended and, if possible, that the early embry- 

 onic conditions in such cases be studied in order to determine 

 whether the brain and spinal cord were absent from the first or 

 whether it is a case of atrophy of organs primitively present. 



J See p. xxxiii of this number. 



