390 Dr. E. H. Henderson on the Develojyment 



a stalk-ca^lom, which is cast off, and a portion remaining 

 ^vith the body called the axial sinus {ax.). 



The alimentary tract is hard to define in many sections. 

 No trace of either mouth or anus is found to exist. All that 

 can be seen of an alimentary system is an extensively folded 

 and coiled gut-wall lying in the interior yolk. In PI. XIII. 

 fig. H I have endeavoured to show its appearance. One is 

 able to follow its general outline fairly well in some sections, 

 but the interior within the outer line of the gut-wall reveals 

 only a disturbed mass of broken-down yolk, portions of gut- 

 wall, and amoebocytes. The gut-cells are oblong or oval in 

 shape with irregular ends towards the lumen, and seem to be 

 engulfing yolk, as particles of yolk can be seen not only 

 among them but also in their substance. In places where 

 the lumen can be seen it often appears full of yolk which is 

 granular, showing that it is being broken up by a process, 

 perhaps, of digestion and absorption. 



The yolk forms by far the largest part of the whole star- 

 fish, at least nine-tenths of the whole bulk is yolk. Stained 

 with eosin this becomes easily differentiated from the other 

 tissues. It is composed chiefly of large globular or irregularly 

 shaped masses (PL XIII. fig. J), which are closely apposed to 

 one another, leaving few chinks intervening. This yolk not 

 only tills up the centre but penetrates everywhere ; in fact, 

 the whole of the tissues would seem to have been built up 

 around it, as, indeed, they really are. It is found among the 

 ectoderm cells, between the ectodermic wall and the peri- 

 toneal wall of the coelomic spaces, and among the cells of the 

 gut-wall_, while with the gut it composes the main mass of 

 the interior. Here it so presses on the coelomic sacs as to 

 almost obliterate them — the two peritoneal walls being 

 brought into close apposition, so that the ccelom is not seen 

 at all as an open space, but only as a layer of peritoneal 

 tissue in the midst of yolk. 



Histology. — The structure of the body-wall differs on the 

 oral and aboral surfaces. On the dorsum the ectoderm 

 consists of a single layer of flattened cells, which, as the 

 ventral surface is approached, gradually merge into long and 

 narrow cells (PI. XIII. fig. H). On the dorsal surface the 

 ectoderm lies in close relation to the peritoneum, while on 

 the ventral surface it is separated by a mass of yolk, among 

 which are found amoebocytes and tissues of the mesenchyme. 

 On the ventral surface beneath the hydroccele the ectoderm 

 cells increase in number, and become long and filamentous, 

 and closely compacted, so that if the section be thick there is 

 nothing to be seen but a mass of nuclei. 



