DR. MARTIN BARRY ON THE CHORDA DORSALIS. 199 



CHERT, to denominate it a support and stay for surrounding parts : but that it is the 

 continually renewed central portion of the nucleus of a cell, it being out of this 

 nucleus that the embryo arises : — in other words, that growth at the earliest periods 

 consists, not in external additions, but in the continual origin of new substance in the 

 centre, by which means previously formed substance is pushed farther out. The nu- 

 cleus of every cell, also, of which the embryo is composed, seems to be the seat of a 

 like process ; that is, a subordinate point for the origin of new substance. 



The origin of the embryo from the nucleus of a cell, may assist to solve a question 

 on which, I believe, physiologists are not agreed. "The primitive trace," says Va- 

 lentin, " as well as the***chorda dorsalis, has been made use of by Baer and Bur- 

 DACH against Serres, Bourdon, and others, to show that the first rudiments of the 

 parts are not two halves, but a whole, which subsequently splits into two oppositely 

 situated halves. Such positions, however," Valentin adds, "are altogether more 

 adapted for metaphysical acumen, and cannot, and never will, be settled by expe- 

 rience and observation ; since by this we do not learn the act of arising itself, — it 

 shows no more than an arisen turn in the formation.***Such problems must remain 

 far from the province of the observing part of anatomy and physiology-f-." 



I am here compelled to express a different opinion from that of Professor Valentin. 

 The subject in question seems to me very properly to belong to the province of ob- 

 servation. But then it is essential that observation should be directed to a period 

 earlier than that with which physiologists have usually begun. By this means it 

 seems possible actually to observe, that if the nucleus of a cell is a single object, the 

 first rudiments of the embryo are not two halves. 



Unless the condition just mentioned be fulfilled, namely, an investigation of the 

 earliest periods, it is in vain that we attempt to learn what it is of which the rudi- 

 ments of the embryo are composed. It appears to have been because of the non-ful- 

 filment of this condition, that physiologists supposed their ^'•primitive trace" to arise 

 in the substance of a membrane. And to the same cause seems referable the opinion 

 recently advanced by Reichert, that the first traces of the new being are derived 

 from cells of the yelk;):. 



t Handbuch der Entwickelungsgeschichte des Menschen mit vergleichender Riicksicht der Entwickelung der 

 Saugethiere und Vogel, 1835, p. 156. 



X Reichbbt very properly denies that the embryo arises in the substance of a membrane ; and it is gratify- 

 ing to me to find an observation previously published by myself confirmed by this talented investigator. Dr. 

 Reichert, however, does not seem to be avrare that he had been anticipated in this discovery, as he will per- 

 ceive on reference to the "Proceedings" of the Royal Society, April 18, 1839, published at that time, and 

 copied into several periodicals, among which maybe mentioned Von Froriep's " Notizen," No. 228, July 1839, 

 p. 116. 



