96 RTOHARD KVANS. 



become modified and form a part of the intermediate layer of 

 the protective coat of the gemmule ; and fourthly, the tropho- 

 cytes, whose function is to supply both the columnar and the 

 yolk cells with food material, and which, like the columnar 

 cells, pass back to the sponge. 



(2) The yolk-cells and the columnar cells draw their food 

 material in solution from the trophocytes; the yolk-cells 

 storing it up as a reserve in the yolk bodies; the columnar 

 cells nsing it in such a way as to enable them to secrete the 

 inner chitinous layer, to grow and pass out between the 

 outer ends of the amphidiscs, their inner ends being modified 

 to form the greater part of the ground substance of the pro- 

 tective coat of the gemmule, and finally to secrete the outer 

 chitinous layer ; processes which mean that there is an enor- 

 mous amount of metabolism going on. 



(3) The amphidiscs are developed in cells, the scleroblasts, 

 which carry them through strands of the sponge tissue to 

 their ultimate position in the protective coat of the gennnule. 



IV. Critical Review op Previous Accounts. 



On perusal of the historical section of this jiaper it will be 

 seen that the views which have lieen expressed as to the first 

 appearance of the gennnule are numerous and conflicting. 

 The only thing certain is that a group or aggregation of cells 

 is formed. How it is formed and whence it is derived no one 

 seems to know, though every one has a theorj^ to put forward. 

 Again, it is equally uncertain whether the gemmule is formed 

 from the group which first appears, or whether this group in 

 order to build up the gemmule structure acquires recruits 

 from among the sponge cells and tissues. 



Probably the first question that should be discussed is 

 whether the group of cells above mentioned is the product of 

 cell migration to one spot, or of cell division either of a single 

 cell or of a gTOup of cells. 



In his first attempt to explain the origin and structure of 

 the gemmule in the year 1849 (2), Carter expressed himself 



