40 CONK LIN. [Vol. XIII, 



I. The Primary Cleavages. 

 I. The First Cleavage. Figs. 2-6. 



I cannot state exactly the length of time which intervenes 

 between fertilization and the first cleavage, nor between the 

 latter and the following cleavages. However, not less than four 

 hours elapse after the entrance of the sperm into the ovum 

 before the first cleavage begins, and the interval is probably 

 longer. I have frequently found Crepidulas in the process of 

 egg-laying, and after carrying the newly laid eggs several miles 

 to the laboratory and there fixing and staining them, have found 

 on examination that the male and female pronuclei were still 

 far apart. 



No " segmentation nucleus " is formed, i.e., the male and 

 female pronuclei do not fuse before the appearance of the kary- 

 okinetic spindle which introduces the first cleavage. In fact, 

 the male and female chromatin loops remain separate until the 

 equatorial plate stage of the first spindle. About the stage 

 shown in Fig. 2, however, the chromatin loops form a con- 

 tinuous plate ; though the part of the plate lying beneath the 

 polar bodies (the upper side in the figure) probably came from 

 the female pronucleus, while the other portion (the lower half) 

 came from the male pronucleus. The axis of the spindle lies 

 in the long diameter of the protoplasmic area, or rather the 

 protoplasmic area continually enlarges its diameter in the 

 direction of the axis of the spindle from the time the spindle 

 first appears until- the first cleavage is completed. The radia- 

 tions of the archoplasmic bodies at the poles of the spindle are 

 plainly visible in all the surface views, and a large central cor- 

 puscle or centrosome can be seen in the most favorable prepa- 

 rations. After the chromatin is distributed equally to the two 

 poles of the spindle the division of the cell body begins. A 

 furrow first appears at the formative pole, and gradually ex- 

 tends until it forms a constriction all around the ovum, but 

 deeper at the formative pole than elsewhere. Figs. 3 and 4. 

 The cell body then divides into two equal portions AB and CD, 

 Figs. 5 and 6. These blastomeres are at first nearly spherical, 



