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Explanation op Plates XLIII-XLVIII. 



The figures have, with few exceptions, been drawn with the camera lucida at 

 the table level under Zeiss homo. imm. yV, oc. 2. With whatever lenses used 

 however, all the figures were drawn to the same scale of magnification. 



Reference Letters. 



bl.. Blastopore. mes., Mesoderm. 



br.. Brain. pr., Proctoda?um. 



br. com., Commissure of brain. pro.. Proboscis. 



gl.. Gland st., Stomod^um. 



int.l.. Lumen of intestine. sto.l.. Lumen of stomach. 



m., Mouth. v.p.. Ectoderm of ventral plate. 



Plate XLIII, Fig. 1. — Unsegmented o\aim, after the expulsion of the second 

 polar body. The female pronucleus lies at the animal pole of the 

 egg, surrounded on its lower side by a large aster. The sperm nu- 

 cleus, with its accompanying aster, is seen to the right of and below 

 the centre of the ovum. 



Fig. 2. — Unsegmented ovum after union of the two pronuclei. 



Fig. 3.— Early anaphase of first cleavage spindle. The cell-body is begin- 

 ning to elongate. 



Fig. 4. — Two cells. Prophases of spindles for second cleavage. 



Fig. 5.— Two cells. A-B in metaphase, C-D in anaphase of division. 



Figs. 6 and 7. — Four cells. Spindle fibres still connecting A and B. Ani- 

 mal pole. 



Fig. 8. — Seven to eight cells. Animal pole. Ic and Id formed and in pro- 



