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XV. On the Chorda Dorsalls. By Martin Barry, M.D., F.R.SS. L. and E. 



Received January 3, — Read January 7, 1841. 



r OR the discovery of the remarkable structure in the embryo of some of the Ver- 

 tebrata denominated the chorda dorsalis, we are indebted to Professor Baer. This 

 naturalist considered it " the axis around which the first parts of the foetus form," 

 and " the true virga mensoria for the whole body and all the chief systems -f-." Rei- 

 chert supposes it to be that embryonic structure which serves as a " support and 

 stay" for parts developed in two halves^. 



In the course of my researches on the mammiferous ovum, an object was noticed 

 which seems to correspond in appearance to the incipient chorda now referred to. 

 I am desirous of drawing attention to this similarity in appearance, but more parti- 

 cularly to point out some important differences between my own observations and 

 those of others, as to the mode of origin of the objects in question, and their relation 

 to surrounding parts. For, should it be found that these objects are the same, my 

 belief is that even the most recent views on the incipient growth of the embryo must 

 undergo a change. 



Before entering upon the comparison, I must ask the reader to place before him 

 my Second Series of Researches in Embryology §, Plates VI. VII. and VIII.; to 

 certain of the figures in which I will now briefly refer. 



The object hb, figs. 113 to 116, is the rudimental embryo of the Rabbit in its 

 earliest stage. It is a finely granular, hollow sphere — the nucleus of a cell. Its 

 centre, the nucleolus, is brilliantly pellucid. 



The changes which this nucleus undergoes are two-fold. The one consists in the 

 formation of a pointed process, as in fig. 121 D, which is sometimes, and perhaps 

 generally, curved ; its concave side being directed towards the centre of the ovum. 

 In this state the rudimental embryo appears to correspond to what has been sup- 

 posed to be the ^^ primitive trace" in the ovum of the Bird. The other change which 

 this nucleus undergoes consists in the origin of fresh substance in its pellucid centre, 

 and the expansion of its peripheral portion into cells. Some of these cells occupy 

 the area pellucida: others, coalescing, form a little sac (fig. 117- bli^'.), which, ex- 

 panding and receiving the yelk into its cavity, assumes the form of a network (fig. 

 132.), and lines the remainder of the ovum (fig. 119. ii^'.). 



t Ueber Entwickelungsgeschichte der Thiere. Beobachtung und Reflexion, 1828. Taf. I. fig. II. III. a. p. 15. 

 X Das Entwickelung&leben, 1840, p. 108. — The chorda dorsalis has received other names; viz. Chorda ver- 

 tebralis, Ruckensaite, Spinalsaite, Wirbelsaite.. 



§ Philosophical Transactions, 1839, Part II. p. 307. 

 MDCCCXLI. 2 D 



