kofoid: development of limax. 87 



The section shown in Figure 25 cuts the cells of the animal pole ob- 

 liquely, and thus exaggerates their relative size somewhat. In addition 

 to the central cavity, thei'e are a number of smaller cavities between the 

 cells of the animal pole. Their relation to the cell boundaries can in 

 every case be readily determined in the sections. The larger cavity 

 a; of Figure 24 is between two cells whose facet of contact lies parallel 

 to the plane of the section ; the cavity therefore appears to traverse a 

 cell, though in reality it does not. In the figure it is in direct continuity 

 with cavities which are readily seen to be intercellular. These latter 

 cavities appear lenticular in cross section, but they are themselves elon- 

 gated as the cavity x is. There are, in addition to the intercellular 

 cavities just described, two others (vac, Figs. 24 and 25), which seem to 

 be intracellular). They are both near the central cavity, though not 

 in direct contact with the cell membrane in any direction. They are 

 both approximately spherical in form, and neither has the sharp and 

 definite outline separating it from the protoplasm of the cell that the 

 intercellular spaces just described have. Their foi-m, position, and limits 

 thus indicate their intracellular nature. They probably are merely intra- 

 cellular vacuoles. Their position is suggestive of their fate. They lie 

 very near the central cavity, and it may be that their contents ulti- 

 mately find their way into it by osmosis, or less probably by rupture of 

 the " cell wall." 



The fate of the fluid eliminated from the lenticular spaces of the earlier 

 stages, and from the intercellular spaces of the later stages, is a difficult 

 matter to determine. Direct observation gives negative results, for 

 although these cavities in the early stages can be seen to develop and 

 disappear, it is impossible to say what becomes of their contents. I have 

 never seen any evidence of the extrusion of their contents from the q^^, 

 but this might escape observation, for the cavities are small and their 

 contents are transparent, and it is often difficult to determine the point 

 at which the contents even of the large cavity are expelled. 



There are, however, some facts which lend support to the view that 

 these lenticular spaces of the early stages, and perhaps also the inter- 

 cellular spaces of the later stages, contribute directly to the increase of 

 the volume of the central cavity. The nuclear conditions of Figures 

 23-25 (Plate III.) show beyond question that this twenty-four-cell stage 

 is younger than that of Plate V. Fig. 34, where the larger cleavage 

 cavity is shown. It seems reasonable to derive the conditions of the 

 older stage from those of the younger. The main difference between 

 the two stages lies in the increased size of the cleavage cavity and the 



