402 JOHN BEARD, 



ganglion-cell in the myotome of one side, a corresponding one is to 

 be seen in the myotome of the other side. 



From all these facts the conclusion is forced upon one that the 

 transient apparatus of Baja at any and every period of its existence 

 is in a somewhat degenerated or vestigeal form. It is not intended 

 by this statement to imply that the system is disappearing from the 

 ontogeny. The meaning to be attached to it is rather that, as judged 

 by the marks of asymmetry and the absence of more definite form 

 and structure, the apparatus is, perhaps, now no longer what it once was. 



The constancy of its appearance in the development and its simi- 

 larity in many other cases prove that it has not lost all its im- 

 portance , even though the above characters should indicate some 

 diminution in its value. 



It is a natural question, which may need more detailed consideration 

 on another occasion, whether the parts of this apparatus represent the 

 sole transient elements in the development. A full consideration of 

 this problem would be somewhat lengthy. For the moment the reader 

 may be reminded of the "merocytes" of the yolk as an example in 

 point. Other curious, apparently degenerate, cells are encountered in 

 various embryos and in different positions. Two such have been figured 

 in Figs. 69 and 70 of plate 20. The one lay in a section of the spinal 

 cord, the other in the fore-brain, and both occur, like many others* 

 met with, in young embryos. 



The nature of all these elements need not be discussed now. It 

 may suffice to remind the reader that other transient and degenerate 

 elements are to be encountered, and the explanation to be ofl'ered of 

 them, with a full discussion of the nature of the transient nervous 

 apparatus itself, may be deferred. 



Comparison of the transient nervous system with the 



permanent one. 



It would appear to be a piece of superfluous labour to indicate 

 with any detail the enormous morphological differences which separate 

 the transient and the permanent nervous systems from each other. 



These differences are apparent, and of so fundamental a nature 

 as to negative any supposition that the one is merely an earlier 

 edition of the other. The absence of segmental arrangement, and 

 the limitation of the transient system to a restricted area of the 

 embryonic body have been previously commented upon. 



