No. 3.] 



PODARKE OBSCURA VERRILL. 



409 



Fig. 2. — Optical section of stage sliown 

 in Fig. II, PI. XXXVI. 



are exactly equal in size ; in other words, there is no large 

 4d, and so far as we can tell from observation, the cell contain- 

 ing the mesoderm might be either of two cells lying one at 



either end of the second cleav- 

 age furrow. The spindles at 

 the upper pole are inclined 

 downward a little from the 

 horizontal (Text-Fig. 2). This, 

 combined with the small size 

 of the rosette cells, causes the 

 latter to lie somewhat below 

 the surface (see PI. XXXVII, 

 Fig. 16, and the Text-Fig. 4). 

 Later, as a result of the pres- 

 sure of the dividing cross cells, 

 they are forced still farther 

 below the surface. (See Text- 

 Fig. 3.) A similar process has been described by Mead for 

 Lepidonotus (No. 22). As a comparison of Fig. 1 2 (PI. XXXVI) 

 with Fig. 27 (PI. XXXVIII) will show, one result of this divi- 

 sion is further to increase the 

 difference in size which has 

 before been noticed between 

 the cells of the upper and 

 those of the lower hemisphere. 

 2. Forty to fifty-six cells, {a) 

 A division of the trochoblasts 

 to form four cells in each 

 quadrant (PI. XXXVI, Fig. 1 2, 

 PI. XXXVII, Fig. 14, id2.i, 

 id2.2, etc.). These do not 

 divide again, but soon develop 

 cilia and function as the pri- 

 mary prototroch (PI. XXXVII, 



Fig. 15, ld2.I.I, ld2.1.2, ld2.2.I, 



id2.2.2, etc.). This band of ciliated cells is at first broken 

 at four points, but is afterwards completed in a manner to be 

 described later. ip) A division of the intermediate girdle 



Fig. 3.— Optical section of stage shown in 

 Fig. IS, PI. XXXVII. ar, apical rosette; 

 /g-, polar globules. 



