86 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
apical quartet, a — d^^ and their neighbors, An especially large 
vacuole is formed immediately at the animal pole. 
Inasmuch as the vacuolation of the animal half of the egg is an impor- 
tant and very prevalent occurrence in the later stages in the cleavage of 
Limax it deserves a detailed description. In surface view these cavi- 
ties are seen to be arranged in general along the line of the cell bounda- 
ries, which they obscure to such an extent that the superficial margins 
of the facets of contact are detected only by careful focusing upon the 
immediate surface of the egg properly illuminated. As soon as the plane 
of the focus is lowered toward the level of the nuclei, the boundaries are 
at once lost and nothing but a clear space can be found. The proto- 
plasm peripheral to the cavity is therefore comparatively thin, and does 
not present the granular structuro of the deeper lying regions. The 
cavities in many cases extend laterally upon either side some distance 
from the superficial line of contact of the two cells, and sometimes, as in 
the cell c? Figure 28, they even lie between the nucleus aud the 
external surface of the cell. In all cases it is possible to detect a sharp 
and definite boundary to these cavities, when the egg is so oriented as 
to bring the margin of the cavity into the proper relation to the optical 
axis of the microscope. These boundaries have the same appearance in 
whole preparations and in sections that cell boundaries have, and indeed 
I believe that they are cell * membranes," and that the cavities are 
strictly intercellular, That part of the facet of contact lying peripherad 
to the cavity is not continuous through the cavity with the part centrad 
(Plate IIL. Figs. 24, 25), but is in direct continuity with the wall of the 
cavity. This seems to me to be indisputable proof that these vacuoles 
aro intercellular structures, just as the lenticular spaces and central 
cavity of the earlier stages of cleavage and the large cavity of the twenty- 
four-cell stage are. The question as to whether these should be called 
the cleavage cavity will be discussed later. 
The appearance of these cavities in section is shown in Figures 24 and 
25 (Plate IIL). The egg here represented is a very small one, only 
99 
80 y in diameter, and is shown in toto in Figure 23. It has just been 
derived from the sixteen-cell stage by the division of the quartets 5.1 and 
5.9. Traces of this division can still be seen in the derived quartets 
6.1, 6.2, and 6.3, 6.4. The sections were cut obliquely to the vertical 
axis, and so directed as to cut longitudinally the remnants of the spindles 
in one of the quadrants of the quartets 6.5 and 6.4. There is a medium- 
sized central cavity, which, owing to the recent division and consequent 
rounded condition of the cells concerned, lies nearer the vegetative pole. 
