22 Dixon—On the Development of the Branches of the Fifth Cranial Nerve in Man. 
distinctly states: ‘‘die primitiven Ketten gleichsam nur die Leitbahnen abgeben, 
lings welcher die Fibrillen in der einen oder anderen Richtung vorwachsen.”* 
It is probable that such cellular tracts also occur in mammalian embryos, but 
here they are not at all sharply defined from the surrounding mesoderm. At all 
events it is certain that when a developing nerve is first recognisable nuclei are far 
more plentiful among its fibres than they are after the nerve is well established. 
The observations recorded in this Paper were commenced at Leipzig in the 
summer of 1893; and, since then, I have attempted to verify the results obtained 
for the human embryo by the study of a number of rat embryos of different 
ages by means of models constructed from microscopic sections. In every 
important particular a complete correspondence was found to exist between the 
human and the rat embryos. The nerves appear in the same order, and the 
connections of the fifth with the other cranial nerves are developed in the same 
manner. 
The five stages in human embryo described in this memoir are Professor His’s 
embryos :-— 
Bi . four weeks old, ; . 69mm. 
uy; . fifth week, . : = (29a mona 
C.R., . beginning of sixth week, 13°6 mm.f 
F.M., . seven weeks old, . a liresygervaney 
Mr., . eighth week. (See figs. 1 to 4, Plate I.) 
In addition to these five stages, models were also made of the cranial nerves in 
embryos Wt. and Ob.; but, as one of these is only a little older and the other a 
little younger than F. M., separate detailed accounts of them are not given. 
Method of Investigation. 
I had not proceeded far with this work before it became evident that the more 
usual method of microscopic reconstruction by the use of wax plates would not 
yield satisfactory results in modelling fine branching nerves. This difficulty was 
overcome by Professor His, who suggested to me that the reconstruction might be 
effected by means of glass plates. The process is a very simple one. A series of 
sections through the head of an embryo, having been drawn enlarged 25 or 50 
diameters, by means of a camera lucida, the drawings are traced on to glass plates 
covered by a transparent varnish, such as is commonly used in coating photogra- 
phic negatives. These glass plates are 25 or 50 times as thick as the sections, 
and it therefore follows that when they are placed one over another, a model 
of the head of the embryo, with its nerves, vessels, brain, &c., 25 or 50 times 
* «Studien zur vergleichenden Entwicklungsgeschichte des Kopfes der Kranioten,” Munchen, 1894. 
Heft ii., p. 75. 
} Measured from cervical to caudal bend. 
