K0F01D: DEVELOPMENT OF LIMAX. 81 



F- The Cleavage Cavity. 



I. hi Limax. 



After the blastomeres have reached the widely divergent state seen 

 in Plate I. Fig. 14, they begin to flatten against each other, gradually 

 losing their individual spherical contour and assuming a hemispherical 

 shape. This process occupied, in a case recorded, about an hour, and 

 was comparatively more rapid in the latter part. It results in the ap- 

 proximate restoration of the egg to the form of a single sphere. The 

 superficial region of contact of the two cells appears in the living egg as 

 a somewhat irregular line in the now almost obliterated furrow. Very 

 soon after this process is completed there appear along this line len- 

 ticular or irregular spaces, devoid of the granular structure of the proto- 

 plasm, and apparently filled with a clear fluid. Deeper focusing reveals 

 the fact that the centre of the apposed faces of the blastomeres is occu- 

 pied by a slight cavity, wedge-shaped toward the vegetative pole, and 

 broader and rounded toward the animal pole. This cavity gradually 

 increases in size, the minor lenticular spaces increase also, and con- 

 tiguous ones may be seen to coalesce. Finally, as the central cavity 

 increases more and more, and approaches the periphery of the facet of 

 contact, the lenticular spaces themselves disappear, probably contributing 

 their contents to the encroaching central cavity. The latter now pre- 

 sents the form of a broadly lenticular clear space extending from the 

 animal to the vegetative pole of the egg, and 

 symmetrically developed with reference to these 

 poles. The two cells are thus almost com- 

 pletely separated from each other by the fluid 

 filling the cavity, as will be seen in the accom- 

 panying Figure E, giving an optical section 

 in the plane of the equator of a two-cell stage 

 of Limax agrestis, showing a cleavage cavfty. 

 They remain in intimate connection, however, 

 at the peripheral margin, but this margin of union is in some cases 

 reduced to a very thin layer of protoplasm. There is apparently no 

 difference in the extent of the union at the two poles. The growth 

 of the cavity results in an appreciable increase in the volume of the o^, 

 and its contour, as well as that of the cavity itself, is suggestive of the 

 high state of tension existing in the e<:^ as a result of this increase in 

 volume. In extreme cases, as in Plate V. Fig. 34, and in Figure E, the 



VOL. XXVII. — xo. 2. 6 



