578 PROCEEDINGS OF THE AMERICAN ACADEMY. 



myomeres, as is illustrated in Figure G, and a logical enumeration of 

 the nerves could not allow the association of the same myomere with 

 different nerves in different individuals. Nerve III Ijas a definite 

 character and territory of distribution; its number should not depend 

 upon the number of roots possessed by nerve II, nor on their being 

 variously designated as nerves III, IV or V. It is not an unknown 

 condition for a dorsal nerve in the more posterior parts of the body to 

 have two distinct roots making their exit through the same myoseptum. 

 The possible cranial character of the nerves anterior to the first 

 myomere in the adult can scarcely authorize the enumeration of each 

 separate root in the region of the place of exit of nerve II as a distinct 

 nerve root; because the lack of symmetry on the two sides of the body, 

 and the irregularity of the occurrence of these roots, would leave no 

 basis for rational enumeration. The fact is that nerve II supplies a 

 fairly definite territory, consisting usually of a large part of the ros- 

 trum and an adjoining portion of the dorsal fin. 



I have not observed the small muscle segments which Dogiel 

 figures (his Fig. 60, 66, 8b) anterior to the origin of nerve II. The 

 anterior boundary of the first muscle segment of the adult is often 

 difficult to distinguish, and in my own observations strong artificial 

 light was frec^uently employed to make this clear. Careful camera 

 drawings were made of the dorsal portions of the anterior myomeres, 

 as shown in Figures 6, 11 and 12. In specimens of both species the 

 anterior projection of each muscle segment forms a much more acute 

 angle than Dogiel's figures indicate. In this connection Hatschek's 

 ('92) views regarding the first myomere may be mentioned. He states 

 that in the larvae of x\mphioxus the muscle fibrillae of the rostral 

 process are well formed even out to the tip of the rostrum ; but in the 

 fully developed animal they become rudimentary, only a remnant 

 persisting. In transverse sections of young Amphioxus (not larvae) 

 treated with Mallory's differential stain, I have observed the cut ends 

 of muscle fibers, lying in front of the anterior border of the first adult 

 myomere. These fibers were short and few in number, but clearly 

 differentiated by the stain from the surrounding connective tissue. 



Nerve II was studied to a considerable extent in Branchiostoma 

 caribaeum, and its distribution in the rostrum of this species is shown 

 in Figures 1-5. As this species has been little illustrated, it is thought 

 best to reproduce the conditions rather fully, for such figures form an 

 interesting basis of comparison with Branchiostoma lanceolatum. No 

 very essential differences, however, occur between the two species. 

 Variations in the number and arrangement of the roots of nerve II 



