EMBRYOMIC FISSION IN CYCLOSTOMATOUS POLYZOA. 221 



when the vacuolation of the follicle is commencing ; and they 

 appear to be closely connected with the carrying out of this 

 process of vacuolation, although it is clear that the first small 

 vacuoles (cf. fig. 7) make their appearance independently of 

 the giant-cells. 



At later stages each giant-cell is usually seen to lie in a 

 large, sharply-marked vacuole of the protoplasmic reticulum. 

 The cell may be apposed to one wall of the vacuole ; or may, 

 apparently, lie quite freely within it. There is good reason to 

 believe that the vacuoles which are at first occupied by giant- 

 cells are later occupied by the young larva. Each embryo, 

 soon after its formation, comes to lie in a sharply-marked 

 vacuole in the protoplasmic reticulum. Taking into consider- 

 ation the facts (1) that the giant-cells are formed simul- 

 taneously with the appearance of large vacuoles in the follicle, 

 and (2) that they contain fragments of degenerating cells or 

 nuclei, it may be concluded that one of the functions of the 

 giant-cells is to excavate spaces in the follicle in which the 

 larvae can develop. These spaces are probably filled with some 

 albuminoid fluid, at the expense of which the embryos develop 

 — probably by diffusion through their tissues, as they have no 

 recognisable means of absorbing nutriment. 



The function of the giant-cells would thus be closely similar 

 to that of the osteoclasts or myeloplaxes of bone '' which ex- 

 cavate small shallow pits .... in the part which is under- 

 going absorption^' (27, p. 104). Their structure, too, is in 

 accordance with the descriptions of various observers of the 

 multinucleated giant-cells in Vertebrates. 



In the mature ovicell the remains of the distal thickening 

 of the tentacle-sheath are always found as a dense mass of 

 nucleated protoplasm which is attached to the ectocyst, not in 

 the tubular aperture of the ovicell, but invariably at its base, 

 on the side which is further from the back of the ovicell (figs. 

 9 and 16). The valve constantly projects from the back of the 

 ovicell into the proximal part of this mass of cells in the 

 manner shown in figs. 9 and 16. It appears to me probable 

 that the function of the valve is to ofi'er an obstacle to the 



