198 DR. MARTIN BARRY ON THE CHORDA DORSALIS. 



shows a "growing from" the sheath, as it is called, of the chorda, — "at the expense 

 of its inclosed nucleus," — or as the chorda is "found to decline," even in these the 

 last stages of this structure. But if we examine it at periods anterior to these, I 

 think we shall find still stronger grounds for believing, that the chorda dorsalis of 

 authors, in an early state, corresponds to the pin-like object in the raammiferous 

 ovum which I have been comparing with it. 



Reichert seems to have been led to form the opinion above-mentioned, that the 

 chorda serves as a "support and stay" for parts developed in two halves, by the fol- 

 lowing observations ; namely, that the chorda becomes visible as a single structure at 

 the same time as the foundations, in two halves, of the central nervous system ; and 

 that the central nervous system on the one hand, and his membrana intermedia on 

 the other, are so intimately connected by means of the chorda, that it is not possible 

 to separate them. He even states, that the chorda passes into the foundation of the 

 embryo. 



Fartlier, Reichert says, " There are developed on the chorda dorsalis the original 

 halves of the central nervous system, with the higher organs of sense, separating as 

 these do from the central nervous system-f-." He states that, with a union of the 

 two halves of the central nervous system — such union taking place first at the fore- 

 end — there is observed a decline in the corresponding part of the chorda. On the 

 subject of the membrana intermedia, the same author remarks, that it is found be- 

 tween the central nervous system and the mucous membrane ; and that it (the mem- 

 brana intermedia) "is the common original foundation of all structures, systems, 

 and organs, which are the means of operation for the two central organs of animal 

 life. Hence from it [the membrana intermedia] there are developed the vertebral 

 system, the dermal system, the circulating system, and finally, all the structures 

 which support the mucous membrane, and which," says he, " I comprehend under the 

 name of system of the intestinal membrane;}:." 



It would thus seem, according to Reichert, that there is little in the embryo which 

 is not developed out of either the central nervous system, or the membrana inter- 

 media. But these are the very parts which the same observer found so intimately 

 connected by means of the chorda, that it was not possible to separate them. And it 

 appears to be these same parts called by Reichert " the foundation of the embryo," 

 into which he says " tjie chorda passes'' 



Taking then the observations of Baer, Rathke, and Reichert, in connection with 

 my own, I venture to believe, that it is not enough to say, with Baer, that the chorda 

 dorsalis is the axis around which the first parts of the foetus form ; nor, with Rei- 



t L. c, pp. 58, 59. 



J L. c, p. 107. "Da die femeren Doppel-gebilde," says Reichert, "sammtlich von der membrana inter- 

 media ausgehen, so tritt die Wirbelsaite [chorda dorsalis] in die innigste Beziehung zu der Letzteren." 



