402 PROCEEDINGS OF THE ACADEMY OF [April, 



Reference Letters. 



Ap Apical point, 



An.C Anal cell. 



Bl Blastopore. 



Oes (Esophagus. 



Op Operculum. 



Ot Otocyst. 



C.G Cerebral ganglion. [ P.B Polar Body. 



Dr.R Dorsal retractor muscle. P.C Pedal Commissure. 



Ebl Enteroblasts. P.G Pedal ganglion. 



E.C Large enteric cell. Rt.R.Ft Right retractor muscle of 



En Enteron. foot. 



Ex Large anal excretory cell. Sec.M Secondary mesoderm. 



Ft Foot. ! Sh.E Edge of shell. 



Int Intestine. ! Sh.G Shell-gland. 



L Liver. I St Stomodaeum 



Lt.R.Ft Left retractor muscle of 



foot. 



Mo Mouth. 



M.F Muscles of foot. 



Nph.L Left nephrocyst. 



Nph.R Right nephrocyst. 



Stom Stomach. 



Tel Teloblast. 



V* First row of velar cells. 



V^ Second row of velar cells. 



Vl.L Velar lobe. 



VI. R Retractor muscle of velum. 



Note. — In the drawings the trochoblasts are represented with stippled nuclei ; 

 upper pole views show the ectoblastic cross in heavy outline. Plates I-XI, 

 XIII, and Figs. 101-104 of PI. XIV are reduced ^ from original drawings, the 

 remaining figures i. Figs. 7, 36, 39, 46 and 55 have been omitted from Plates. 



Explanation of Plates XXI-XXXV. 



Plate XXI, Fig. 1. — Section of egg of Fiona marina, showing first maturation 

 spindle. 



Fig. 2. — Section. First polar body being given off. Sperm nucleus with 

 astral rays below. 



Fig, 3. — Section. Rise of second polar body. Enlargement of sperm 

 nucleus and astral rays. 



Fig. 4. — Lateral view of entire egg. Approach of male and female pro- 

 nuclei. 



Fig. 5. — First cleavage ; figure seen from side. 



Fig. 6. — Completion of first cleavage, as seen from above. The two polar 

 bodies lie between the nuclei. 



Fig. 8. — Completion of second cleavage, seen from upper pole. A polar 

 furrow is present at the vegetative but not at the animal pole. 



Fig. 9. — L^pper pole xlQVf, showing spindles which institute third cleavage. 



Plate XXII, Fig. 10. — Dexiotropic turning of spindles of the first quartet, 



with constriction and rounding out of these cells. Lateral view. 

 Fig. 11. — Same egg as fig. 10, seen from above. 

 Fig. 12. — Lateral view of slightly older egg than Fig. 10, showing compact 



grouping of blastomeres after division. 

 Fig. 13. — Completion of fourth cleavage, Iteotropic in direction, by which 



tlie second quartet is separated from the macromeres. 

 Fig. 14. — Lateral view of same egg as fig. 13. 

 Fig. 15. — Lfeotropic division of first quartet, by whicli the "turret cells" 



(trochoblasts), la-, Ib^, Ic^, Id", arise. In following figures the turret 



cells and their derivatives are indicated by stippled nuclei. 

 Fig. 16. — Lateral view of same egg as fig. 15. 

 Fig. 17. — First cleavage of the cells of the second quartet (dexiotropic). 



The macromeres are about to give off the third quartet by dexiotropic 



cleavage. 



