ABBREVIATIONS 



m.l.int., membrana limitans interna ff.sp., ganglion spinale 

 m.l.ext., membrana limitans externa s., mesodermic somite 

 r.ant., radix anterior 



PLATE 1 



EXPLAXTION OP FIGURES 



This plate illustrates, among other things, the behavior of mitochondria in 

 early stages of development. All the specimens figured have been prepared by 

 Meves' modification of the iron hematoxylin method so that the mitochondria 

 appear as black, filamentous or granular structures against a gray background. 



1 Epithelial cells from the ventral half of the neural tube in the region of the 

 tenth somite in an embryo of 12 somites, thirty-four hours incubation (p. 401). 



2 Epithelial cells from near the middle line of the medullary plate of an em- 

 bryo of twenty-four hours incubation at 39°C., in which the primitive groove, fold 

 and pit alone were differentiated. The mitochondria appear as granules and 

 straight or wavy filaments of a black color. They occur in all parts of the cyto- 

 plasm and may readily be distinguished from the black spherical yolk granules 

 (p. 401). 



3 Epithelial and germinative cells from the neural fold in the region of the 

 head process of an embryo of 3 somites, twenty-four hours incubation. The 

 mitochondria are distributed throughout the cytoplasm but show a tendency to 

 become accumulated in the distal parts of the cell. Only a few spherical, jet- 

 black yolk granules may be distinguished at this stage (p. 401). 



4 Radix anterior {r.ant.) with the neighboring portion of the neural tube, 

 which have become slightly separated from each other during the preparation. 

 Taken from opposite the tenth somite on the right side of an embryo of 31 somites, 

 seventy hours incubation. In the radix anterior mitochondria and neurofibrils 

 may be made out together; the former, rods or wavy filaments of different lengths, 

 the latter, threads of finer diameter and of a lighter color. The mitochondria in 

 the cytoplasm of the neuroblast (a) are shorter, curved more closely packed to- 

 gether and do not fuse together to form a network (p. 410). 



5 From the neural tube opposite the tenth somite on the right side of an em- 

 bryo of 26 somites, and fifty-eight hours incubation. The cell outlines are indis- 

 tinguishable. The chromosomes in a dividing germinative cell are shown at (a) 

 and may be differentiated from the filamentous mitochondria by their character- 

 istic arrangement. The mitochondria are somewhat larger than in the preceding 

 figures (p. 401). 



420 



