PLATE 2 



EXPLANATION OF FIGURES 



All the figures of this plate were drawn with the same lenses, etc., as those of 

 plate 1. In reproduction the magnification has been reduced one-third. The 

 series illustrates stages in the development of the anlagen of spinal somatic 

 motor nerves in Squalus embryos up to the time of migration of medullary cells 

 into the nerve. 



7 A portion of a cross-section (IL 4-2-17) of a Squalus embryo of 7 mm. (Stage 

 I of Balfour) of the same series as those shown in figures 4, 5 and 6, showing 

 the anlage of the somatic motor nerve of the same metamere as that in figure 6 

 but of the opposite side of the body and three sections posterior. The evidence 

 that the protoplasmic connection between the neural tube and somite is a deriv- 

 ative of the neural tube is the same as that shown in the earlier figures. The 

 section also shows a protoplasmic outflow from a cell in the lateral wall of the 

 neural tube. Such outflows are exceptional and do not appear to persist in later 

 stages. 



8 and 9 Portions of two adjacent sections (II .5-2-17 and II 5-2-18) in the 

 cloacal region of a 7 mm. embryo, showing an early stage in the development of 

 a plasmodesm or anlage of a somatic motor nerve. The protoplasmic connection 

 between the neural tube and the myotome consists of two elements, one derived 

 from the neural tube, that is, a portion composed of the processes of neuroblasts 

 (cl.n'bl.) labeled ax. in the figure, and the other consisting of migrant cells from 

 the sclerotome of the somite (scl.). The two elements are readily distinguishable 

 in the sections, since the processes of the neuroblastic cells stain more deeply 

 than the sclerotome cells and the limiting membranes of the cells are shown 

 distinctly. 



10 A portion of a cross-section (IL 4-1-.30) of a 7 mm. embryo in the middle 

 trunk region where the myotome and sclerotome lie somewhat nearer the neural 

 tube than in the caudal region. The protoplasmic bridge between neural tube 

 and myotome consists of the processes of medullary neuroblasts which extend 

 between the myotome and sclerotome along the median surface of the myotome. 

 A comparison with earlier stages shows that the bridge is not a primary one, as 

 thought by Held ('06) and Paton ('07), but secondary. 



11 A portion of a cross-section (II 5-1-13) of a 7 mm. embryo, showing a 

 somatic motor nerve anlage in a stage of development somewhat more advanced 

 than that shown in figure 10. Processes of medullary neuroblasts may be traced 

 for some distance along the median surface of the myotome, between the sclero- 

 tome and the myotome. The more deeply staining properties of the distal por- 

 tions of the axone processes are noteworthy, in connection with the problem of 

 the origin of the neurofibrillae. The stage corresponds essentially with Paton's 

 ('07) figure 2. The difference in the phenomena and resultant difference in inter- 

 pretation may be ascribed largely to the difference in the methods of staining ami 

 of preservation. What Paton regards as a neurofibril arising in the myotome 

 independentlv of the nervous system is actually, as shown in sections of embryos 

 preserved by the vom Rath method, the distal portion of the neuraxon process 

 of a medullary neuroblast. Instead of appearing within the myotome as stated 

 by Paton, the actual position, as seen in figure 11, is between the myotome and 

 sclerotome. The results obtained by Paton's excellent method of staining the 

 neurofibrillae need to be controlled by a comparison with methods which, like 

 that of vom Rath, bring out the cell boundaries. 



12 A portion of a cross-section (IL 3-2-42) of a 7 mm. embryo, showing an 

 early stage in the process of migration of medullary cells into a somatic motor 

 nerve anlage in the middle trunk region. Evidence has been given in a former 

 paper ('03) that these cells are chiefly, if not entirely, concerned in the process 

 of formation of the neurilemma of the somatic motor fibers. As shown in the 

 figure, the breaking up of the sclerotome into loose mesenchyma has already 

 begun. The relation of the fibrillar portion of the nerve anlage to neuroblasts 

 in the neural tube is not shown in the section, a result of the bending of the axones 

 upon their emergence from the neural tube. 



16G 



