No. 3.] PODARKE OBSCURA VERRILL. 403 



generation indicated by a coefficient, while the " micromeres " 

 are indicated by small letters, each with a coefficient indicating 

 the generation and a subscript indicating its position in the 

 generation. Thus, A, B, C, D form the 4-cell stage. At 

 their next division from A arises lA and la ; from B, iB and 

 lb, etc. I A then divides into 2 A and 2a, while la divides 

 into lai and ia2. 



We have continued the use of the terms " dexiotropic " and 

 " leiotropic " to indicate the direction of the spiral cleavages, but 

 propose the terms " dextral " and "sinistral" to designate the 

 respective daughter-cells resulting from a spiral cleavage, 

 reserving the terms "right" and "left" to apply to the sides 

 of the bilaterally symmetrical body. Thus, in a dexiotropic 

 cleavage, the upper cell when viewed from the animal pole 

 would lie to the right, and is the "dextral" cell, the other is 

 "sinistral," and vice versa in a leiotropic cleavage. In all 

 cases the cell nearer the vegetative pole has the larger expo- 

 nent, regardless of absolute size. When a cleavage is merid- 

 ional, the sinistral cell receives the smaller exponent. 



This method has one disadvantage, in that symmetrically 

 placed cells which arose by a meridional division do not receive 

 corresponding subscripts. Thus (see PI. XXXVIII, Fig. 27), 

 3C2 and 3d2 divide in a meridional direction and so that their 

 products are symmetrically placed with respect to the median 

 plane of the embryo. After one more division their larger 

 daughter-cells become the larval mesoblast. One of these 

 cells is 3C2.1, and the other 3d2.2, according to the proposed 

 system. I believe, however, the advantages of the system 

 will outweigh any disadvantages resulting from a possible 

 confusion. This nomenclature differs from that adopted in 

 my preliminary paper (No. 29, a), but seems enough of an 

 improvement over that to warrant the change. 



Cleavage. 



The eggs are small, measuring 62.9 /ot in diameter, and are, 

 in the fresh condition, very opaque. As already stated, they 

 are not provided with any albuminous coating, and when laid 



