EXPLANATION OF PLATES. 



All of the following drawings were made from camera outlines and 

 from preserved material. The mitotic figures are somewhat schema- 

 tized ; especially as regards the astral rays, which are actually very 

 indistinct. 



PLATE I. 



Fig. I. 48 cells forming 56. Second division of the trocho- 

 blasts and first division of the intermediate girdle cells. 



Fig. 2. 56 cells from the upper pole. The first divison of the 

 cross cells, pg^ polar globule. 



Fig. 3. 60-64 cells. The cross formed. 



Fig. 4. 74-80 cells. Rudimentary cells budded off in the pos- 

 terior arms of the cross and in the anterior and lateral intergirdle 

 regions. 



Fig. 5. About 80 cells. Further divisions in the cross cells. 



Fig. 6. Three cells have been added to the prototroch from the 

 anterior and lateral intergirdle regions — ap" to cp" . ar Xq dr, ro- 

 sette cells, cr has divided. 



Fig. 7. Latest stage to which the divisions of the cross cells have 

 been followed. The posterior arms have become conjoined. All 

 of the rosette cells have divided. 



Fig. 8. Posterior view of embryo containing 48-52 cells. Divi- 

 sions of trochoblasts and intermediate girdle cells. The fourth 

 quartet has been formed. 



Fig. 9. 56 cells from the left side. The third quartet has divided. 



Fig. 10. 64 cells from the left side. The primary prototroch 

 now forms a complete ring. The second quartet has divided off three 

 rudimentary cells — 2(^2.1 to 2^2. i- 



Fig. II. Posterior view of 64-cell stage showing the divisions in 

 the c and d quadrants. Formation of the small x^^^ cell. 



Fig. 12. Postero-ventral view of an embryo containing 76-80 

 cells. Further divisions in the X group and also in the third quartet. 



(244) 



