BY W. A. HASWELL AND J. P. HILL. 367 



uniform mass of small cells with only slight indications of division 

 into an outer stratum and a central mass ; but in sections these 

 are found not to be sharply marked off from one another, the 

 entire bud appearing as a mass of nearly uniform cells of small 

 size. In the larger buds granules collect in the central mass, 

 and a distinct hyaline cuticle becomes developed over the entire 

 surface. The first trace of internal differentiation consists in the 

 appearance in the interior, of a group of long and narrow cells 

 (fig. 7), which lie parallel with one another in such a way as to 

 form a circlet ; these are the cells destined to form the hooks of 

 the rostellum. At this stage vacuoles have appeared among the 

 cells, but there is no regular cavity; a definite cavity first appears 

 in the next stage, when the hooks have become developed. 



In the next stage observed the hooks characteristic of the 

 fully-formed Cysticercoid had appeared in the interior of the still 

 solid mass of cells constituting the bud. Up to this point the 

 calcareous corpuscles are not developed, and there is no trace of 

 histological differentiation. 



The next stage found is separated by a somewhat wide hiatus 

 from that last described ; but we have hitherto failed, in the 

 many hundreds of specimens examined, to find any intermediate 

 conditions. The calcareous corpuscles have now become formed 

 in the outer layers of the developing Cysticercoid, and histological 

 differentiation is well advanced. A cavity has appeared in the 

 interior, so situated as to separate a distal, comparatively thin, wall 

 from a proximal thick wall, on the middle of which is an inwardly 

 projecting process — the rostellum — 'with a double circlet of hooks. 

 On each side of this process on the inner surface of the proximal 

 part of the wall are the rudiments of two of the suckers. The 

 rostellum is at this stage entirely unconnected distally with the 

 wall of the cysticercoid : it is capable of being to some extent 

 invaginated v.'ithin the i)OSterior part of the rudimentary head. 



The next stages (PI. xix. fig. 9, and PI. xx. figs. 1-3) show the 

 rostellum more completely developed and now capable of being 

 entirely invaginated within the posterior part of the head, which 



