402 PROCEEDINGS OF THE ACADEMY OF [April, 
REFERENCE LETTERS. 
AD. x sucpentie Apical point. QOS evesscssnces (Esophagus. 
An. Cas Anal cell. Op...............Operculum. 
Bl egret Blastopore. Oteeceeeg Otocyst. 
C.G..aesttee Cerebral ganglion. PYB soe Polar Body. 
Dr: Rees Dorsal retractor muscle. P.C..............edal Commissure. 
Ebl..............Enteroblasts. PiGsisentsie: Pedal ganglion. 
HiGi era Large enteric cell. Rt.R.Ft.....Right retractor muscle of 
101s a a Enteron. foot. 
BxXitekdau Large anal excretory cell. Sec.M.x....... Secondary mesoderm. 
Bt tec Foot. SUE eae Edge of shell. , 
Tntpno Intestine. Sh. Gicessccns Shell-gland. 
Li tiene Liver. St.................tomodeum. 
Lt.R.Ft......Left retractor muscle of | Stom........... Stomach 
foot. elie vvatveens Teloblast. 
Mog cet Mouth. WEE fem First row of velar cells. 
Mele ihiaae Muscles of foot. A eis ae Second row of velar cells. 
Nph.L.........Left nephrocyst. WITTE encs Velar lobe. 
Nph.R........ Right nephrocyst. WA ee Retractor muscle of velum. 

Nots.—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 Pl. XIV are reduced 4 from original drawings, the 
remaining figures}. Figs. 7, 36, 39, 46 and 55 have been omitted from Plates. 
EXPLANATION OF PLATES XXI-XXXV. 
Prats 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- 
nuelei. 
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.—Upper pole view, showing spindles which institute third cleavage. 
Piatr XXII, Fig. 10.—Dexiotropic’ turningKof 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, leotropic in direction, by which 
the second quartet is separated from the macromeres. 
Fig. 14.—Lateral view of same egg as fig. 13. 
Fig. 15.—Leotropic division of first quartet, by which the ‘turret cells” 
(trochoblasts), la?, 1b’, 1c”, 1d’, 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. 

