DR. MARTIN BARRY ON THE CHORDA DOR8ALI8. 197 



the so-called " primitive trace," by the formation of fresh substance in the interior 

 (compare with my fig. 122.). If so, the formation of Baer's incipient chorda is not 

 simultaneous with, but subsequent to, that of his " laminse dorsales." (The " laminae 

 dorsaies*' of Baer correspond to Reichert's " central nervous system.**) 



Von Baer describes the chorda as " the axis, around which the first parts of the 

 foetus form-f-." The dark pin-like object we have been considering in the mammi- 

 ferous ovum, presents merely one of the many layers of incipient cells into which a 

 nucleus becomes resolved. 



Notwithstanding these differences, however, facts mentioned by authors regarding 

 the chorda dorsalis at later periods I think afford evidence of the identity of the two 

 objects in question, as will be seen by what follows. 



Rathke states that in osseous fishes the chorda dorsalis is inclosed in a membra- 

 nous sheath ; and that from this sheath there grow in pairs a great number of minute 

 filaments, which are the incipient crura of the arches of the vertebrae. The founda- 

 tions of the bodies of the vertebrae appear, from the description given by this author, 

 to consist of minute tables or traces, which proceed out of the vertebral arches ; and 

 he adds, that " the thickening and ossification of the vertebrae take place at the ex- 

 pense of the inclosed nucleus of the chorda J." 



Reichert, in his researches on the development of the Batrachian Reptile and the 

 Bird, found the chorda to decline more and more as the vertebral system advanced 

 in its formation §. "The chorda," says this author, "is reduced in proportion as 

 ossification proceeds, until for the most part only its remains are to be found between 

 the vertebrae ||." 



It is known that the chorda dorsalis in a comparatively advanced state is composed 

 of cells. For this discovery we are indebted to Professor J. Muller. Schwann has 

 since found the characteristic nucleus in its cells^. "If we closely examine," says 

 this observer, " the outer rind of the chorda in Pelohates fuscus — covered as this rind 

 is with scattered grains — we find these grains closely to resemble the nuclei of cells ; 

 only that they are about half the size. They are besides oval, and furnished with a 

 nucleolus. This rind is not distinctly separated from the proper tissue of the chorda 

 dorsalis : and as the cells of the latter rapidly diminish in size towards the rind, I 

 believe these grains of the rind to be the cytoblasts of flattened down cells, which 

 form the rind-}-^." 



I think there is a great deal in what has now been quoted from these authors, that 



t L. c, p. 15. 



X Buedach's Physiologic als Erfahrungswissenschaft, 1837, Band II. pp. 279-281. 



§ L. c, p. 68. II L. c, p. 71. 



% Mikroskopische Untersuchungen iiber die Uebereinstimmungen in der Struktur und dem Wachsthum der 

 Thiere und Pflanzen, 1838, 1839, Tab. I. fig. 4. 



ft L. c, p. 12. It is deserving of notice that Schwann conjectures the chorda dorsalis to contain no ves- 

 sels. 



2 d2 



