52 THE AMERICAN MUSEUM JOURNAL 



APPENDIX. 



TECHNICAL DESCRIPTION OF THE MODELS.' 



I. A. The ovum of Crepidula at the time of fertilization. 



1. B. The fertilized ovum showing pronuclei lying in the cytoplasm 

 at the animal pole. On either side of them the centrospheres. At 

 the vegetative poles is seen the yolk-stalk. Jour. Morph., Vol. XIII, 

 1897, fig- I- 



2. First cleavage — appearance of first cleavage furrow. Jour. 

 Morph., Vol. XIII, figs. 3, 4. 



3. Completion of first cleavage furrow. Nuclei and asters oppo- 

 site each other in the two blastomeres. Between the blastomeres 

 are the polar bodies. Jour. Morph., Vol. XIII, fig. 5. 



4. Resting stage after first cleavage. Flattening of blastomeres 

 against each other. Dexiotropic turning of nuclei, asters and pro- 

 toplasmic areas. Jour. Morph., Vol. XIII, fig. 7. 



5. Beginning of second cleavage. Laeotropic turning of spindles 

 and protoplasmic areas. The centrospheres of preceding cleavage lie 

 near the cleavage furrow. Jour. Morph., Vol. XIII, fig. 7. 



6. Second cleavage. Beginning of second cleavage furrow. 

 Laeotropic rotation of spindles. Polar furrow being formed. Jour. 

 Morph., Vol. XIII, fig. 9. 



7. Completion of second cleavage. Asters nearly in position of 

 poles of preceding spindles. Polar furrow well formed. Jour. 

 Morph., Vol. XIII, fig. 10. 



8. Third cleavage. Spindles almost radial, but showing slight 

 dexiotropic rotation. Jour. Morph., Vol. XIII, fig. 12. 



9. Third cleavage. Completion of first quartet. Position of 

 asters shows that division was dexiotropic. Jour. Morph., Vol. XIII, 



fig- 13- 



10. Fourth cleavage. Laeotropic. First quartet has rotated into 

 furrows between macromeres. Jour. Morph., Vol. XIII, fig. 14. 



1 1 . Fourth cleavage complete. Fifth cleavage, laeotropic division 

 of first quartet of micromeres and formation of "turret cells" (trocho- 

 blasts). Jour. Morph., Vol. XIII, fig. 16. 



12. Fifth cleavage complete. Sixth cleavage dexiotropic. For- 

 mation of third and last quartet of ectomeres. Sixteen cells. Jour. 

 Morph., Vol. XIII, fig. 17. 



• The models correspond to the figures in "The Development of Crepidula," 

 by Dr. E. G. Conklin, Jour. Morph., Vol. XIII, 1897, and " Karyokinesis and 

 Cytokinesis," Jour. Acad. Nat. Sci., 2d Ser., Vol. XII, Phila., 1902. 



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