404 TREAD WELL. [Vol. XVII. 



sink rapidly to the bottom of the dish. They are usually more 

 or less irregular when first laid, but rapidly become spherical 

 or nearly so. In some cases there has seemed to be a certain 

 amount of axial differentiation, but these differences are not 

 constant. A thin membrane surrounds the egg, and remains 

 attached to it until the latest stage I have studied. Whether, 

 as is commonly stated to be the case among annelids, it 

 becomes the cuticle of the adult, I cannot say. It is smooth 

 in the living Qgg, but becomes more or less wrinkled under the 

 influence of reagents. Except in Fig. i, I have not attempted 

 to represent it in the plates. 



Soon after laying, the first polar spindle appears, and the 

 eggs remain in this condition until fertilized. A study of the 

 maturation and fertilization stages is reserved for a future 

 paper, and the present account begins with the first cleavage. 



The first cleavage begins, with very slight variations in time, 

 one hour after fertilization. At this time the animal pole is 

 indicated by the position of the polar globules and by a certain 

 amount of protoplasmic differentiation easily seen in stained 

 material. The vegetal pole of the egg stains very slightly with 

 haematoxylin, and has a granular appearance, while the animal 

 half is much more homogeneous in texture and stains more 

 deeply. The first spindle lies slightly nearer the animal than 

 the vegetal pole. (See PI. XXXVI, Fig. i.) 



The first cleavage furrow cuts rapidly through the egg, 

 sinking down more rapidly at the upper than at the lower pole 

 {cf. Wilson, No, 34, d, and Mead, No. 22), and the 2-cell stage 

 results. (See PI. XXXVI, Fig. 2.) Here it will be seen that 

 the first cleavage is exactly equal. The two nuclei are directly 

 opposite one another, there being no indication of the rotation 

 described by Conklin in the 2-celled stage of Crepidula. A len- 

 ticular cleavage cavity is formed, which persists and becomes 

 the large cleavage cavity of the later stages. (See Text-Figs. 

 I and 2.) 



Two to Foiir Cells. — The next three cleavages follow one 

 another at intervals of approximately fifteen minutes, but be- 

 yond this, time records are of little value. The eggs of the 

 same lot are usually in practically the same stage at any one 



