KOFOID : DEVELOPMENT OF UMAX. 85 



narrow, nnd tlio iiulividiuil cells are in ojitical section somewhat lo/xnv'C- 

 sliaped. Tho cell a'^- (Kig. 34) presents a curions Lnd-like ])roces8 

 extendin;^ into the central cavity, and the superficial extent of tho cell 

 is somewhat less than tliat of the other memhers of the same (piartet. 

 Tliis process sujij^ests the mesenchyma cells which Staull'acher ('i)3) 

 figures in his Tafel XIV. Figg. 2') a and 25 c^ but in this projection 

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

 a more amu'boid outgrowth of no permanent significance. 



A comparison of tho computed volumes of tho whole c^ii;, of its 

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

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

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

 wiiich 188 represent that of the cavity, 241 that of tho blastula wall, 

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

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

 objects measured, and are therefore only approximately correct ; still 

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

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

 but not so voluminous as tho 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 l)een developed to any great extent at the expense of the volume 

 of the proto])lasm of tho egg. There can be no question that this egg 

 presents tiie condition of a typical "blastula" with a tyj)ical "cleavage 

 cavity" or blastocoel. Indeed, Kabl could not have found for llaockel 

 and his Gastnea 'I'heory a better illustration among mollusks of tho 

 "morula" and "blastula" stages than these two twcnty-four-C(dl stag(\s 

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

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

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

 our usage of the term cleavage cavity by Stauffacher in his i-ecent paper 

 ('9.3), wo shall be compelled, in view of tho fact that the cavity is sooner 

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

 cavity simply " ein heller Raum." 



It is dilFicult to establish any regularity or uniformity in tho sequence 

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

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

 and by no means constant conditions. Tho twenty-fonr-coll stage repre- 

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

 Figure 28, also a twenty-four-coll stage, shows at the animal polo a 

 number of Iacun;e or intercellular vacuoles between tho cells of the 



