10 WILLIAM F. ALLEN 



certain ganglion cells, possibly sjonpathetic, which are found 

 along the course of the spinal and vagus nerves. The present 

 paper includes a study of the mode of origin of the fourth ven- 

 tricle in several groups, an account of a structure similar to the 

 fourth ventricle found in the caudal part of the spinal cord of an 

 adult Polistotrema, and a study of the development of the 

 central canal, and the causes underlying the flattening and the 

 ventral indenting of the spinal cord in Cyclostomes. 



Material and its preparation 



Since the material for this paper came from such diverse 

 sources, and such different modes of technique were employed, 

 too much time and space would be consumed if a detailed de- 

 scription were given of each method employed. With the 

 exception of the Petromyzon material, which was fixed in Flem- 

 ming, corrosive-acetic, and Perenyi's fluid, my own material 

 was fixed either in Tellyesnicky's or Bouin's fluid. It was 

 sectioned after paraffin or the combined celloidin-paraflfin 

 method of imbedding. The latter method gave by far the bet- 

 ter results, since it appears to have all of the advantages of 

 celloidin in causing almost no shrinkage and its ability to hold 

 yolk granules and blood corpuscles intact; besides allowing 

 the sections to be cut as thin as paraffin alone, and causing no 

 more difficulty in manipulation. For the most part the sections 

 were stained in Heidenhain's iron hsematoxylin and counter- 

 stained in an alcoholic solution of orange G plus a little acid 

 fuchsin. In a few instances, in very young Petromyzon embryos, 

 where all the tissues were filled with yolk, carmine and hsemalum 

 were used to advantage. 



Acknowledgments are due to Prof. T. G. Lee for the gift 

 of a very complete series of Petromyzon embryos, which were 

 obtained from the Connecticut River. Also to Prof. R. E. 

 Scammon for the loan of his very complete serial collection of 

 Squalus embryos, and to Prof. J. B. Johnston for the use of a 

 similar serial collection of Amblystoma embryos. The splendid 

 series of human and pig embryos belonging to the Institute of 



