206 DEVELOPMENT OF THE TADPOLE. 



sideration of the history of this structure, these would appear not 

 to be true cells, but rather to represent vacuoles which have 

 become filled with gelatinous substance. As we proceed towards 

 the tail, we find other layers intercalated between these, so that a 

 longitudinal section shows from three to five or more of such 

 layers irregularly disposed. As shown in the diagram, the noto- 

 chord penetrates into the head beyond the auditory capsules, 

 which are seen on either side of it, and terminates almost close to 

 the mid-brain in the region of the pituitary body, with which, 

 however, it has no connection. The cellular portion is not a 

 straight rod, but is composed of a number of slight constrictions 

 and expansions, caused apparently by the pressure at intervals of 

 the fibrous sheath with which it is surrounded. 



This sheath does not entirely envelope the central portion, for 

 it is attached, as shown in the diagram, to the first constriction 

 immediately behind the auditory capsules. In front of this the 

 cellular portion is surrounded only by the delicate second layer, 

 spoken of before. This layer, which envelopes the whole noto- 

 chord, is very similar to the membrane mentioned in the last 

 article as surrounding the pro-renal ducts and all other organs at 

 this period. It consists of a number of delicate, nucleated, 

 spindle-shaped cells, broad in the centre, and terminating in 

 points where they are attached to each other. 



The external sheath consists of a number of parallel fibrous 

 bands, having much the appearance of muscular tissue. This 

 sheath, with the notochord proper within it, can be seen in the 

 majority of the sections already described. It contains not only 

 the gelatinous rod and its covering proper, but also the great 

 double nerve-cord, which ultimately forms the spinal cord. As 

 the anterior portion of the sheath certainly extends under that 

 portion of the spinal cord known as the medulla oblongata, whilst 

 it does not surround it, there would appear to be an opening on 

 the dorsal side of the sheath, whence the nerve-cord penetrates it 

 just at the point where the medulla oblongata narrows down into 

 the medulla spinalis. An external view of the sheath shows it to 

 be composed of a number of irregular, transparent ridges, con- 

 nected by depressions, so as to form what almost looks like a 

 series of waves (PI. XIX., Fig. 4). 



