30 H. V. NEAL 



of these problems warrants a reinvestigation of those stages in 

 vertebrate embryos which may contribute to their elucidation. 

 Taking up these problems in order we may first ask whether 

 or not protoplasmic connections between myotome and tube are 

 primary or secondary. 



1. Are muscle and nerve connected with each other ab initio? 

 The Hensen view of the primary connection of nerve and mus- 

 cle is not supported by the evidence presented by Squalus em- 

 bryos as shown in the drawings represented in figures 1, 2 and 

 3, which demonstrate the relations of tube and somite in stages 

 previous to the appearance of the anlagen of somatic motor 

 nerves. Figure 1 may seem the most significant of the three 

 since it is taken from a series in which no somatic motor nerve 

 has made its appearance anywhere in the embryo. The section 

 is typical of the trunk region of embryos of this stage and the 

 finer histological features are in all essential respects like those 

 presented by standard methods of preservation and staining. 

 That the space between tube and somite is normal and not an 

 artifact is evinced by the fact that it appears in the living em- 

 bryo. In sections this region appears to be filled by a vacuolated 

 non-staining or slightly staining plasma. The most significant 

 fact presented in the section is the absence of protoplasmic 

 bridges between tube and somite. Before the first anlagen 

 of the motor roots make their appearance in later stages, it is 

 impossible to find protoplasmic connections between the two 

 organs. The kind of fixation or the stain used makes no diff^erence 

 in the phenomena, provided the space is not obliterated through 

 shrinkage. Reagents, which like formalin cause considerable 

 plasmolysis and vacuolization, and the use of which is likely 

 to be followed by considerable shrinkage, may exaggerate the 

 vacuoles and cause their confluence so that the granular mate- 

 rial, which in Flemming or vom Rath preparations is quite evenly 

 distributed, as shown in the drawing, becomes aggregated in 

 denser strands and may simulate protoplasmic strands extend- 

 ing between tube and myotome. However it does not stain 

 like protoplasm, even when aggregated in the manner suggested, 



