Emmel, C/ionia Tympcmi in Microtiis. 413 



over and in front of the tympanic cavity, so that, in view of the 

 confusion of ideas as noted above, the question now is whether 

 the pre-tympanic position is primary and maintained throughout 

 the embryonic h'fe, or whether it is a position secondarily ac- 

 quired in the development of the tympanum. Of course, the 

 further questions of homology cannot be settled till this point 

 in mammalian embryology is determined. It was for the purpose 

 of contributing something to the solution of this question that 

 the following study in the embryology of Microtiis was under- 

 taken. 



The embryos used in this study were killed and preserved in 

 formalin. To insure a correct conception of the relations of the 

 nerves to all parts of the head a model of a 2.3 mm. embryo, 

 magnified 50 diameters, was made by the Bokn method. Re- 

 productions of two older embryos, also, were made by Kast- 

 CHENKo's method of graphic projection. My observations were 

 made from the same series of serial sections as were employed 

 for the model and projections, and from several other series of 

 slightly different ages and cut in different planes. 



First Embryo. 



The youngest of the embryos was used for the construc- 

 tion of the model. The model and the sections from which it 

 was made demonstrate clearly all the structures which are of 

 importance for this study : the brain and all its flexures, the 

 roots and ganglia of the fifth, seventh and eighth nerve, the 

 mandibular and hyoid arches, and the posterior visceral arches 

 as they are modified to form the sinus cervicalis. At this 

 period the visceral arches are united only by a membrane com- 

 posed of the two layers of epithelial cells, and the nerve trunks 

 can be traced only a short distance into the mesenchyme of the 

 arches. 



Second Embryo. 



From this embryo, more advanced than the first, two 

 graphic projections were made, one of the exterior of the head and 

 the other of the brain, fifth and seventh nerves and pharyngeal 

 cavity. The mandibular and hyoid arches are still conspicuous 



