DEVELOPMENT OF THE TADPOLE. 207 



In this figure there will be observed six dark patches, 

 marked ga., which are also surrounded by the sheath. These are 

 six of the ten sympathetic ganglia, lying immediately below the 

 spinal cord proper, and not yet apparently connected by any 

 commissure. These ganglia are intimately connected with struc- 

 tures which have been called the supra-renal bodies, which Balfour 

 considers to be actually developed from them. 



My sections of March roth, as drawn in Plate XXI., Figs, i, 

 2, and 3, give most interesting and instructive details of one stage 

 in the growth of the notochord and its sheath. From the peculiar 

 form of the embryo at this stage, these .sections show both 

 transverse and longitudinal strictures from the same animal. In 

 Fig. I we have a transverse section of the notochord {nc), toge- 

 ther with the terminal attachments of the fibrous sheath, both 

 anteriorly and posteriorly. In front, the origin of the fibrous 

 bands is seen to occur at the junction of the epiblast with the 

 brain, the fibrous bands rapidly narrowing down to a single line, 

 so that at these points on either side of the brain the three struc- 

 tures — epiblast, forebrain, and fibrous sheath— are so closely 

 connected and amalgamated, that no distinction can be perceived 

 between them, although they immediately separate from each 

 other and become most distinct structures. Posteriorly, these 

 bands, after closely surrounding the notochord, spread out dis- 

 tinctly, and then immediately commence to lose their fibrous 

 character by resolving themselves into a number of separate 

 roundish cells, arranged in about five rows, which gradually coa- 

 lesce and run between the epiblast and undifferentiated yelk-mass, 

 at first in two rows and ultimately in a single row of cells, forming 

 a line of mesoblast. 



In the same figure as that giving a diagram of the notochord 

 (PI. XIX., Fig. 2) will be found a sketch of the auditory capsules 

 {au.) of the same date, together with indications of nerve-sub- 

 stance proceeding from the brain to them. These auditory 

 capsules on March 9th were distinctly seen to be formed by an 

 invagination of the epiblast. At that time they appeared to be 

 nothing more than a pear-shaped sac, having a very minute 

 opening on the exterior. As seen in Fig. 2, PI. XIX., they con- 

 sisted of a sac containing much detached granular matter, the 



