86 



Development of Occipital Nerves in Human Embryos 



in its application consisted in making enlarged drawings of the sections, 

 usually fifty diameters, with a projection apparatus or camera lucida, 

 upon separate sheets of transparent paper. Paraffined wrapping paper 

 is serviceable and inexpensive, better than this, being stronger and more 

 transparent, is the " process " paper, used in Germany as " butter-brodt 

 papier" and in this country in packing tobacco. When the drawings 

 were completed, the sheets were piled so that adjacent sections were 

 accurately fitted over each other. A vertical line for reconstruction was 

 then established by marking upon' each sheet two lines perpendicular to 

 each other, forming a series of crosses which exactly superimposed 

 throughout the entire pile. The individual sections were then plotted 

 off on mm. paper by fitting the crosses to a chosen perpendicular line, the 

 distance between the sections being determined by the thickness of the 

 sections and the enlargement of the drawings in the usual way. Many 

 of these reconstructions are diagrammatically shown as text figures 



TABLE OF EMBRYOS STUDIED. 



(Figs. 1-12), in which fibre masses are represented by lines and ganglion 

 cell masses by dots. The same enlargement is used in all of these so that 

 the actual increase in size may be readily seen by comparing them. 



In order to reproduce the third dimension a clay model was made of 

 a 4.3 mm. embryo, and a wax plate reconstruction of a 14.0 mm. embryo. 

 Drawings of these are reproduced in Plates I and II. 



Following the suggestion of Dr. Bardeen much assistance was obtained 

 in studying these structures by making dissections of pig embryos for 

 comparison. It was possible in this way to control the various stages 

 from 8.0 mm. upward. In making such a comparison it was found, 

 however, that the ratio between the length of the pig and the development 

 of its nervous system does not exactly correspond to that of the human. 

 The development of the nervous system of the latter is somewhat more 



