PLATE 6 



EXPLANATION OF FIGURES 



12 To the right, graphic reconstruction of the spinal nerves from the region 

 of the caudal heart to the caudal fin of a 20 mm. Polistotrema embryo. To the 

 left, graphic reconstruction of two spinal nerves several segments in front of 

 the caudal heart of the same embryo. Note, 1) In every instance where the motor 

 and sensory rami have appeared they are well-separated. 2) The extreme caudal 

 nerves are in a very early embryonic stage. The last one consists of an irregular 

 mass of neural crest, and there is a gradual increase in the development of the 

 spinal nerves in passing forward, indicating that the order of appearance of the 

 various parts of a spinal nerve is as follows, — ventral motor ramus, dorsal motor 

 ramus, ventral sensory ramus, dorsal sensory ramus. 3) Of the various spinal 

 nerves which will later cross the M. cordis caudalis, the second, third, and fourth 

 are much longer and have penetrated a mass of myoblast destined to form the 

 M. cordis caudalis. X 50. 



13 From a dissection of several of the spinal nerves of an adult Polistotrema 

 immediately in front of the M. cordis caudalis. This dissection shows how a 

 ventral sensory nerve by dividing and each branch receiving a motor ramus 

 from separate nerves may bring about the change of the caudal rami ventrales 

 receiving their motor components frcm the rear instead of from in front. X 2. 



14 Portion of a transverse section through the cephalic end of the M. cordis 

 caudalis belonging to the series reconstructed in figure 1, showing two muscle 

 spindles in transverse section and one in longitudinal section. A portion of the 

 spiral placque is visible in the longitudinal spindle and both transverse spindles 

 show well-developed lamellated connective tissue capsules. X 100. 



ABBREVIATIONS 



A. i2., anal fin ray R.D.S., ramus dorsalis or posterior 

 B.V., blood vessel (sensory) 



C.A., caudal artery R.V., ramus ventralis or anterior 

 Cap., muscle spindle or tendinous R.V.M., ramus ventralis or anterior 



capsule (motor) 



CF., caudal vein R.V.S., ramus ventralis or anterior 

 D.Myo., dorsal border of myotomes (sensory) 



D.R., dorsal root (radix posterior) Sp.Cd., spinal cord 



M.C.C., M. cordis caudalis Sp.G., spinal ganglion 



M.F., muscle fibers Ten., tendon 



Muc.S., mucous or slime sac V..Myo., ventral border of myotomes 



iVc, notochord V.R., ventral root (radix anterior) 



Neu.C, neural crest V.R. (1), cephalic ventral or anterior 

 PL, annular or spiral placques root 



R.D.M., ramus dorsalis or posterior V.R. (2), caudal ventral or anterior 



(motor) root 



PLATE 7 



EXPLANATION OF FIGURES 



15 Graphic reconstruction of two spinal nerves from a region a little behind 

 the anus of a 27 mm. Polistotrema embryo. It will be seen that the motor and 



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