220 SIDNEY F. HARMER. 



disappears in the region of the permauently uncalcified part of 

 the ovicell; i.e. of the definitive aperture. 



Fig. 14 illustrates a condition of the tube of the ovicell 

 which I have observed in one or two cases. The ectocyst is 

 drawn out into a long narrow tube, which was probably un- 

 calcified and which opens to the exterior. This recalls the 

 condition described in the zooecia of certain Cyclostomata, in 

 which the zooecium is closed by a (calcareous) lid, perforated 

 by a small central aperture.^ I am not prepared to state 

 whether or not this is a normal character of ovicells at any 

 particular stage, nor can I suggest any satisfactory explanation 

 of the meaning of the phenomenon. 



The central mass of cells diflferentiated from the thickened 

 part of the tentacle-sheath in fig. 12 is destined to undergo 

 certain very important modifications. In fig. 16 (more highly 

 magnified than the previous figures, and belonging to the same 

 ovicell from which fig. 6 was drawn) some of the nuclei of the 

 central mass are growing larger. This is especially the case 

 in the neighbourhood of the lumen of the tentacle-sheath, 

 where there is a tendency for the nuclei to group themselves 

 in small numbers. In fig. 9 some of these multinucleated 

 masses of protoplasm are breaking ofi" into the lumen of the 

 tentacle-sheath. 



From this stage onwards the characteristic multinucleated 

 cells, which have been formed from the thickened distal part 

 of the tentacle- sheath, are a normal feature of the ovicell, being 

 found in the vacuoles of the protoplasmic reticulum in which 

 the young larvae lie. One of them is seen in fig. 11, and 

 others are shown, more highly magnified, in fig. 18 (PI. XXIV). 

 In the latter figure the multinucleated cells contain nuclear 

 and other structures which are obviously degenerating ; and 

 they are clearly not unlike the " giant-cells " which are known 

 to occur in certain tissues in Vertebrates. 



The giant-cells make their appearance at just that stage 



1 Cf. Mesentipora meandrina (Busk, No. 8, pi. xvii,fig. 2) ; Reticu- 

 lipora dorsalis (Waters, No, 36, pi. xvii, fig. 4); and other cases referred 

 to by Waters. 



