S6 BULLETIN: MUSEUM OF COMPAEATIVE ZOOLOGY. 



apical quartet, a^* — cZ^* and their neiglibors. 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 pi'oto- 

 plasm peripheral to the cavity is therefore comparatively thin, and does 

 not present the granular structure of the deeper h'ing 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 and 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 III. 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 

 are 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 ai'e. 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 

 2.5 (Plate III.). The egg here represented is a very small one, only 

 80 /A 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 6.1 and 

 5.2. 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 6.3 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. 



