196 DR. MARTIN BARRY ON THE CHORDA DORSALIS. 



This two-fold change is but the commencement of a series of metamorphoses, by 

 which the minute object hh of %. 113, is converted into the objects bh^^ bb"^, and 

 bb"^', collectively, of fig. 122. For it is to be remarked that there is a continual origin 

 of fresh substance in the centre ; by which means previously formed parts are pushed 

 further out. These parts consist of cells arranged in layers (fig. 122.), each cell 

 being the seat of a process essentially the same (fig. 149.). As the cells are pushed into 

 a more external situation, they expand and become pale ; while in the centre the con- 

 tinued origin of new and unexpanded cells, presents the appearance of a row of glo- 

 bules, nearly black (fig. 122.). The larger end (fore-end) of this row of globules is 

 spherical, and, as we have seen, has a minute pellucid centre. It is this row of glo- 

 bules, in the mammiferous ovum, which I think must correspond to Baer's incipient 

 chorda dorsalis of the Bird, described by him in the following passage. 



" The chorda dorsalis," he says, " originally consists of a simple row of dark glo- 

 bules, which towards the fore-end are more closely pressed together, and towards the 

 hinder end more separated.***It becomes thicker and darker, from an increase in the 

 number of its globules. The fore-end is at a very early period developed into a 

 round, much thicker knob ; and hence the whole chorda resembles a very fine needle 

 with a minute head-}-." 



Although the object thus described by Baer, as seen in the ovum of the Bird, pre- 

 sents a sufficiently close resemblance to that which 1 had delineated and have just 

 described in the mammiferous ovum, to warrant the belief that both objects are the 

 same, yet there are some important differences between the two descriptions. — Von 

 Baer speaks of the " fore-end" of his incipient chorda as becoming " at a very early 

 period developed into a round, much thicker knob." With me, on the contrary, the 

 linear portion proceeds from the round and thicker knob. — Again, he makes no men- 

 tion of the remarkable pellucid cavity, contained within this round and thicker knob 

 — a part of prime importance, if it be, as my observations show, the main centre 

 for the origin of new substance. Whether these differences between the objects of 

 our respective descriptions really exist (supposing them to be the same objects), or 

 whether, if the embryo of the Bird were examined in a sufficiently early stage, it 

 would not be found to begin in the way I have described as producing the embryo of 

 the Mammal, future observation must determine. 



Von Baer farther says that his row of dark globules is " surrounded by a pellucid 

 border :" and that " the border is seen from all sides," being therefore " a sheath for 

 the chorda. Originally the chorda and its sheath are one}-." How far these parti- 

 culars accord with my observations, will be seen on reference to figs. 121 to 123. 



Von Baer considered the chorda to arise simultaneously with his "laminae dor- 

 sales ;" and this by " a separation of the [supposed] primitive trace into two lateral 

 halves (the laminae dorsales) and a middle streak (the chorda) J." The separation here 

 mentioned, I can regard as no other than the pushing out of the lateral portions of 

 t L. c, pp. 15, 16. X L. c, p. 15. 



