OF ARTS AND SCIENCES. 109 



anterior edge of the canal (PI. I. fig. 1, a to $), in the cervical, dorsal, lumbar, 

 anil sacral regions of embryos from 38 to 185 mm. in length. 



In the column marked L is given the length of the embryo in millimeters,* 

 and in C, the computed ratio of measurements in column B to those in 

 column A. 



Those measurements which are marked with a star are from the cervical en- 

 largement, and those marked with a dagger are from the lumbar enlargement. 



EXPLANATION OF PLATES. 

 The following letters have the same signification wherever used: — 



a. Anterior white columns. g. Internuclear connective tissue. 

 B. Burdach'sclie Keilstrange. h. Posterior horns. 



b. Region of modified epithelium near- i. Thickening of cuticula interna. 



ly free from nuclei. I. Lateral columns. 



c. Central canal. n. Nuclei in the white substance. 



d. Anterior commissure. r. Reticulum between the dorsal 



e. Epitlielium of the central canal. edges of GoU's Kcilstrsinge. 



f. Posterior horn fibres. x. Transverse fibres connecting 

 G. GoU'sche Keilstrange. the posterior horns. 



Plates I. and II. 



[Reproduced from ink drawings by Heliotype process. ] 



Outlines of a series of cross sections through the cervical, dorsal, lumbar, and 

 sacral regions of embryos from 43 to 165 mm. in length ; magnified 33 diame- 

 ters. All the sections in the upper row are from tiie cervical region ; those of 

 the second row are from the dorsal ; the third, from the lumbar ; and the fourth, 

 or bottom row, from the sacral region. The corresponding sections from suc- 

 cessive stages are not from exactly corresponding regions, which will explain 

 the difference in size and proportions. 



Figs. 1-4 from an embryo 43 mm. in length. 



Plate III. 



[Reproduced by the Albertype. process from pencil drawings.] 

 Fig. 1. A transverse section through the posterior part of the lumbar enlarge- 

 ment of an embryo 43 mm. in length, magnified 410 diameters, e', cells formerly 

 a part of the epithelium of the central canal. 

 Fig. 2. A transverse section through the cervical region of an embryo 54 mm. 



* Compare page 101. 



