VI Proceedings of the Association of American Anatomists 



regions of the body-wall in which the muscle fibers pass from myoseptum 

 to myoseptum, the motor nerves for the muscle fibers are distributed 

 through plexuses lying in the myosepta. Each spinal nerve gives rise to 

 a plexus in the septum through which it sends cutaneous branches to the 

 skin. From the plexus, motor fibers pass to each myotome bordering on 

 the myoseptum. Occasionally a single nerve fiber may be seen dividing 

 and sending one branch to the myotome anterior to the septum and the 

 other to the myotome posterior. As a rule, the muscle fibers are inner- 

 vated at their extremities where they are attached to the septum. The 

 " basket-like " terminations about the tips of the muscle fibers described 

 by Giacomini (Monit. Zool. Ital. IX, 92-95, 105-110) as sensory endings 

 in the musculature of the body-wall and tail of teleosts, elasmobranchii 

 urodelans and the larvae of anurans and urodelans were probably motor 

 and not sensory endings. They correspond to the endings recently de- 

 scribed by Ceccherelli (Arch. Italiano di Anatomia e Embriologia II, 

 p. 80-86, 1903) on certain muscle fibers of the tongue of the frog. Ret- 

 zius (Biol. TJntersuchungen, III, 1892) has pictured the termination of 

 motor fibers near the extremities of muscle fibers in Elasmobranchii and 

 the bilateral distribution of the fibers of the septal plexus in Myxine 

 glutinosa. 



Those morphologists who maintain that each spinal nerve forms with 

 the myotome of its corresponding segment in the vertebrate embryo a 

 neuromuscular union maintained throughout subsequent development 

 should take into consideration the intersegmental position and the bi- 

 meric distribution of the motor as well as of the sensory elements of the 

 spinal nerves in vertebrates in which metameric segmentation is strictly 

 preserved in the body-wall. 



THE MESONEPHROS OF A THREE WEEKS HUMAN EMBRYO. By 

 Susanna Phelps Gage. 



In the specimen under consideration (No. 148 of the Mall collection 

 of Johns Hopkins University), which is of about 19 days' development, 

 the prominent Wolffian ridge shows what is probably the remnant of the 

 pronephros, consisting of a single pronephric funnel with a short duct 

 extending cephalad from it; a series of 16 mesonephric tubules; a 

 condensation of tissue representing the metanephros; and a Wolffian 

 duct which has its beginning at the first mesonephric tubule, separated 

 by a considerable space from the pronephric remnant, and extends to 

 the cloaca. 



The mesonephric tubules are in a stage hitherto undescribed in the 

 higher mammals. The first seven present the typical S-shape with 



