kofoid: development of limax. 85 



narrow, and the individual cells arc in optical section somewh 

 shaped. The cell a 6 - 2 (Fig. 34) presents a curious bud-like pi 

 extending into the central cavity, and the superficial extent of the cell 

 is somewhat less than that of the other members of the same quartet. 

 This process suggests the mesenchyma cells which Stauffacher ('03) 

 figures in his Tafel XIV. Figg. 25a and 2."> c, but in this projection 

 there is not the least trace of any nuclear structure, and it is probably 

 a mere amoeboid outgrowth of no permanent significance. 



A comparison of the computed volumes of the whole egg, of its 

 cavity, and of the protoplasmic portion, with the volume of another 

 egg (Plate IV. Fig. 27) of the same stage but having no cavity, brings 

 out the following results. The whole egg has 429 units of volume, of 

 which 188 represent that of the cavity, 241 that of the blastula wall, 

 while the twenty-four-cell stage of average size (Fig. 27) has a volume 

 of only 12G units. These figures assume the perfect sphericity of the 

 objects measured, and are therefore only approximately correct; still 

 they show that the first egg, though a large one, is within the limits 

 of variation in size, and that the cavity is larger than the average egg, 

 but not so voluminous as the substance of the egg which contains it. 

 It is also suggested, in view of the large size of the egg, that the cavity 

 has not been developed to any great extent at the expense of the volume 

 of the protoplasm of the egg. There can be no question that this egg 

 presents the condition of a typical "blastula" with a typical "cleavage 

 cavity " or blastocoel. Indeed, Rabl could not have found for Haeckel 

 and his Gastrsea Theory a better illustration among mollusks of the 

 " morula " and " blastula " stages than these two twenty-four-cell stages 

 (Plate IV. Fig. 27 and Plate V. Fig. 34); for the first contains no 

 cavity whatever, and the latter has its cells arranged in a single layer 

 about a cavity. On the other hand, if we accept the limitation set upon 

 our usage of the term cleavage cavity by Stauffacher in his recent paper 

 ('93), we shall be compelled, in view of the fact that the cavity is sooner 

 or later entirely eliminated, to call this beautiful example of a cleavage 

 cavity simply " ein heller Ran m." 



It is difficult to establish any regularity or uniformity in the sequence 

 of the phases of the cavity in these later stages of cleavage. When we 

 examine other eggs in the twenty-four-cell stage we meet with different 

 and by no means constant conditions. The twenty-four-cell stage repre- 

 sented in Plate IV. Fig. 31, shows no trace whatever of a cavity ; while 

 Figure 28. also a twenty-four-cell stage, shows at the animal pole a 

 number of lacunae or intercellular vacuoles between the cells of the 



