PLATE 1: EXPLANATION OF FIGURES (CONTINUED) 



into the region between myotone and neural tube. The anlagen of the somatic 

 motor nerves of the myotomes immediately anterior to the one shown in the 

 section have already made their appearance as protoplasmic outflows from the 

 neural tube, an outflow which is correlated with a movement of the sclerotome 

 cells. 



3 A portion of a cross-section (II 5-3-12) of a Squalus embryo of 7 mm. 

 (Balfour's Stage I) in the cloacal region, showing the relations of neural tube 

 and somite immediately before the appearance of a somatic motor nerve. Nerv- 

 ous, or protoplasmic, connection of neural tube and myotome has already been 

 effected in the metameres just anterior to the one from which this section was 

 taken. The deeper staining properties of the cell (cl.n'bl.) may indicate that 

 it is a neuroblast about to extend a process toward the somite. The migration 

 of the mesenchyma cells from the sclerotome has already begun. This and figures 

 1 and 2 show that before outflows of neuroblastic cells make their appearance 

 there is no protoplasmic connection between the nervous and muscular systems. 

 In the intercellular space between the two, however, may be demonstrated a 

 dilute, plasmoid substance containing a minimal amount of vacuolated coagu- 

 lable substance more resistant to stains than the cellular protoplasm. To show 

 it at all in a drawing greatly exaggerates its visibility. 



4 A cross-section (IL 4^3-11) of a Squalus embryo of 7 mm. (Stage I of 

 Balfour) in the trunk region just anterior to the cloaca, showing a very early 

 stage of protoplasmic connection between neural tube and myotome, established 

 by an amoeboid outflow of a single neuroblast cell of slightly deeper staining 

 properties than those of the surrounding cells. The pseudopodial processes extend 

 in various directions toward the somite and show finer branches which have 

 connections with the vacuoles of the intercellular plasmoid substance. That the 

 processes are genetically related to the neuroblastic cell, however, is evinced by 

 their staining properties. 



6 A portion of a cross-section (IL 4-2-1) of the same embryo as the one shown 

 in figure 4 in the second metamere anterior to it. At least two cells appear to 

 participate in the protoplasmic outflow from the neural tube. Focussing brings 

 out the fact that the amoeboid processes extend antero-posteriorly along the 

 surface of the myotome as well as dorso-ventrally. The neuroblast shows the 

 characteristic deeper staining qualities of the neuroblast cell. 



6 A portion of a cross-section of the same embryo as figures 4 and 5. The 

 section (IL 4-2-14) is through the metamere immediately posterior to that in 

 figure 5 and anterior to that in figure 4. As compared with figure 5, the amoe- 

 boid processes seem further extended and the limiting membrane of the neural 

 tube seems interrupted for a greater extent than in the sections anterior and 

 posterior. The outer boundaries of at least three cells are extended beyond the 

 limiting membrane of the neural tube. 



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