BY J. P. HILL. 77 



The first stage in the development of the scolex is the appear- 

 ance, at what will become the anterior end of the blastocyst, of 

 a narrow involution. It is lined by a thin cuticle continuous 

 with the outer cuticle, while the granular layer below it is likewise 

 continuous with the similar layer below the external cuticle, only 

 here it is somewhat thicker and contains a large number of small 

 deeply staining nuclei. The first stage in the development of the 

 Synhothrium scolex is thus identical with the corresponding stage 

 in the development of the Cysticerci properly so-called. 



Fig. 15 shows a further stage in the development of the invo- 

 lution. It is now considerably larger, and the granular layer 

 surrounding it is thicker and contains far more numerous small 

 nuclei. The internal tissue of the blastocyst now forms a more 

 compact mass around the involution, while the central cells have 

 for the most part disappeared. At the bottom of the involution 

 cavity, which now communicates with the exterior by a narrow 

 opening, is seen a small elevation, pi'obably the first indication of 

 the scolex. 



Fig. 16 is a longitudinal section of an older cyst, about 5-5 mm. 

 in length by 1 mm. in breadth at its anterior end. This anterior 

 or scolex-containing end is broader than the posterior end of the 

 blastocyst, and is now marked ofi" from it by a well-marked con- 

 striction. The involution cavity is now very much larger than in 

 the preceding stage and communicates with the exterior by a 

 well-marked though narrow canal. The involution cavity is lined 

 by a very thin cuticle, and ai'ising from its bottom is a knob- 

 shaped mass, the rudiment of the scolex. That this knob-shaped 

 mass gives rise to the whole of the scolex is evident when we 

 compare the mode of attachment of the fully-formed scolex (Fig. 5) 

 with that of the scolex bud. In both cases they are attached to 

 the centre of a saucer-shaped area of tissue, which thins out at 

 the edges and has the concavity directed towards the aperture 

 of invagination. The Synhothrium scolex does not then arise 

 directly from the invagination sac, but by the differentiation of a 



