1890.] On Rapid Variations of a Difference of Potential; 89 



is really different in important respects from the enclosure of a space 

 by downgrowth of large folds, though ultimately no doubt the two 

 contrasted modes of formation come to the same thing so far as the 

 more obvious morphological relations are concerned. The mode of 

 formation which really occurs in Ainphioxus is readily harmonised 

 with the existence of the post-atrioporal extension of the atrium 

 which gradually tapers to a fine ceecal canal. It also gives us an 

 essentially different view of the region called " epipleur " by Lan- 

 kester, and generally so designated, from that which Rolph's theory 

 necessitated. That portion of the epipleur into which the myotomes 

 of the body-wall extend is seen now to be no downgrowth, no exten- 

 sion or fold. It is the original unchanged body-wall which bounds 

 the sides of the animal's body in front of the atriopore, just as much 

 as it does behind. The only new growth in the atrial region which 

 takes part in the limitation of the surface is the sub-atrial growth 

 formed by the two little horizontal folds which floor in the atrium 

 when it is a mere canal. These in the adult are represented by the 

 limited region of longitudinally pleated ventral wall between the two 

 metaplenra. 



Lastly, the formation of the atrium as a narrow groove which 

 closes, sinks into, and expands within the body of Amphioxus, is 

 much more readily comparable to what is known of the formation of 

 the atrial chamber in the Ascidians than is the Kowalewsky-Rolph 

 scheme. In the Ascidian a pair of in-pnshings are formed, each with 

 a circular orifice of invagination ; they expand within the body, fuse 

 with one another to form one cavity, and one of the circular orifices 

 disappears. In Amphioxus we have a single in-pushing with a longi- 

 tudinal orifice of invagination, which closes as the invagination forms, 

 excepting at ifcs hindermost border, and then expands to a greatly 

 increased volume. 



The comparison of the so-called epipleura of Amphioxus with the 

 opercula of Fishes has only a remote morphological basis, and prob- 

 ably no genetic relationship exists between these two structures. 



IV. " On a Method of determining the Value of Rapid Varia- 

 tions of a Difference of Potential b y means of the Capillary 

 Electrometer." By GEORGE J. BURCH, B.A. Communi- 

 cated by Professor BURDON SANDERSON, F.R.S. Received 

 April 25, 1890. 



In 1882 a paper by Professor Burdon Sanderson* appeared in the 

 ' Biologisches Centralblatt,' in which an account was given of the 



* Burdon Sanderson, " Die elektrischen Erscheinungen am Dionaeablatt," ' Bio- 

 logisches Centralblatt," 15 Oct., 1882. 



