502 DEVELOPMENT AND GROWTH OF T^NIA SOLIUM. 



in the fine twigs. The thickness of the wall is still but slight (0'07 

 mm.) : it exhibits below the cuticle two layers, of which the outer has 

 a finely granular tougli character, and is resolved into countless small 

 cells (O007 to 0*01 mm.), while the inner consists of larger drop-like 

 vesicles, which probably have something to do with the secretion 

 of the lymph. The vessels run outside of both these sheaths. 

 They consist of wide and narrow tubes, which partially anastomose 

 with one another in a network, and are so arranged that the finer 

 processes mostly lie towards the exterior. Besides the vessels one 

 finds in the cellular layer numerous strongly refracting molecular 

 granules, and delicate muscular fibres running in various directions. 



The head-rudiment was preceptible even in the smallest bladder- 

 worms. But it was at first of comparatively small size, and so delicate 

 that the least pressure sufficed to destroy it. The point of attachment 

 could also be distinguished at that early stage, since it was marked by 

 small calcareous corpuscles round about the former papilla, and also 

 by the radiate grouping of the muscular fibres running from it. Very 

 soon the head-rudiment appears at the middle point of the bladder. 

 Accompanying the radial fibres which run from it, there are others 

 which embrace the place of insertion, and not unfrequently mark the 

 region round about with wavy wrinkles. Similarly the adjacent 

 vessels grow in course of time into considerable stems, which run 

 towards the head into which they pass, or, what comes to the same 

 thing, run from the head into the bladder. 



In the smallest bladder-worms the head -rudiment was, as we 

 have said, of but small size. It appeared as a plump rounded ap- 

 pendage of 0*12 mm., which was seated on the wall of the bladder, 

 and hung into the cavity in the direction of the smallest radius, cor- 

 responding, that is, to the equatorial axis of the 

 oval vesicle. It is penetrated internally by a wide 

 blind canal, which runs down the axis and opens 

 on the external surface of the bladder, so that the 

 cuticle of the latter is continued through the 

 opening into the head-papilla. The wall proper, 

 like the vascular layer of the bladder- wall, consists 

 of s">all nucleated cells, 

 with rudiment of the With increase of size the head-papilla soon 



receptacle. (x25.) j oges ^ swo u en f orm J t g rows an d becomes 



by elongation a more club shaped structure, whose internal cavity is 

 enlarged like a bottle at the lower blind end (Fig. 279) : when the 

 rudiment is about 0*2 mm. long (in bladder-worms whose longest 

 diameter measures about 1*5 mm.), then histological differentiation 

 begins. Not only can one distinguish on the outer surface a thin 



