MUSEUM OF COMPARATIVE ZOOLOGY. 147 
Calcareous plate. 
. Posterior rod and posterior arm. 
Smaller segmentation spheres, micromeres. 
Anal rod. 
Stdlate calcareous body. 
Spine. In Pls. I. and IV. the trifid calcareous rod of the pluteus. 
i. Spicules of the two posterior rods after absorption of the body of the plu- 
teus by the growing urchin. 
Transparent layer. 
Anus. 
Vaso-peritoneal vesicle, water-tube. 
Vitellus. 
. Water-tube. 
PLATE L 
Figures drawn with camera lucida, Obj. D. D. eye-piece 2, Zeiss. Reduced one 
third in photography. 
Fig. 
“ 
1. 
2. 
3. 
4. 
6. 
Bods es lifer) 
17. 
Ovum of Ophiopholis aculeata, found free in water after? fertilization. 
p, Superficial cortical layer of the yolk. 
Same, showing a single polar globule (7) at d. 
Egg in the 2-cell stage with polar globules, d and first plane of cleavage, 
1 cl pl. 
The same without polar globules. 
Egg in the 2-cell stage with the first cleavage plane, so turned as to show 
the thickness of the transparent plasmic region following the first 
plane of cleavage. 
Egg in 4-cell stage. 
The same. 
Egg in 8-cell stage. 
The same showing the segmentation cavity, cav. Twelve hours old. 
Blastosphere. The cluster of cells in the middle of the figure are not 
cells in the blastoccelic cavity, but cells of the blastoderm with granu- 
lations. All the blastodermic cells have the same appearance. One 
day old. 
Blastosphere, showing the beginning of an invagination of the archen- 
teron, ach, forming the blastopore. 
The same seen from the side. Ameeboid, mesoblastic cells at a cl. 
A little older larval stage, from the ventral side. A small cluster of cells 
represented with granulations near the right-hand lower corner. 
Older gastrula from ventral side. 
The same from a lateral view. 
An older gastrula from ventral side. Some of the epiblastic cells in the 
lower left-hand side are granulated. This character is not confined to 
- the cells of this region. 
An older gastrula in which the lateral prominences, which later form the 
lateral arms, are shown. The prominence in the medial line above is 
the oral lobe, ol, of later figures. 
