1902.] CONKLIN — EMBRYOLOGY OF A BRACHIOPOD. 73 



Description of Figures. 



All the figures illustrating this paper were drawn with Camera Lucida at the 

 stage level under a Zeiss Apochromat. Homog. Immers. Obj. 3mm. Comps. Occ. 

 4. In the process of reproduction they have been reduced about one-third. 



Plate I. 



Fig. I. One-cell stage ; polar body in process of being formed; egg elliptical ; 

 ectoplasmic layer surrounds egg and polar body. 



Fig. 2. Two-cell stage; one cell larger than the other; three polar bodies 

 present. 



Fig. 3. Transitional stage between two-cell and four-cell stages ; showing over- 

 lapping of certain cells and « spiral " character of cleavage. 



Fig. 4. Four-cell stage, two cells (at right) larger than the other two; proto- 

 plasmic areas surrounding nuclei shown ; two polar bodies lie in the polar 

 furrow. 



Fig. 5. Eight-cell stage, apical view; three polar bodies at animal pole. 



Fig. 6. Eight-cell stage, side view ; one polar body at animal pole. 



Plate II. 



Fig. 7. Eight-cell stage, apical view; each cell indented at periphery; polar 



furrows at right angles to each other at opposite poles. 

 Fig. 8. Seven-cell stage ; irregular cleavage ; animal pole indicated by three 



polar bodies. 

 Fig. 9. Sixteen-cell-stage, apical view ; two polar bodies at animal pole. 

 Fig. 10. Sixteen-cell stage, optical section, showing blastocoel, ectoplasmic layer 



{^Em') and polar body or yolk spherule between two of the cells. 

 Fig. II. Tiventy-cell stage; some of the cells elongated and probably dividing 



yolk spherules within blastoccel. 

 Fig. 12, About forty-eight-call stage; egg flattened by pressure and the blasto- 



meres partially separated. 



Plate III. 



Fig. 13. Early invagination stage, optical section. 



Fig. 14. Gastrulation completed; blastoccel obliterated; gastroccel partially 



divided into enteron {E) and enteroccel (6V). 

 Fig. 15. Optical section of older embryo, viewed from posterior; enteron still 



further constricted from enteroccel. 

 Fig. 16. Optical section of embryo of same stage as preceding, lateral view, 



showing anterior extension of enteroccel and partition wall growing down 



on anterior side between enteron and enteroccel. 



