MUSEUM OF COMPAEATIVE ZOOLOGY. 147 



pi. Calcareous plate. 



pr. Posterior rod and posterior arm. 



r. Smaller segmentatiou spheres, micromeres. 



rd. Anal rod. 



s. Stellate calcareous Dody. 



sp. Spine. In Pis. I. and IV. the trifid calcareous rod of the pluijus. 



spi. Spicules of the two posterior rods after absorption of the body of the pla- 

 tens by the growing urchin. 



t. Transparent layer. 



V. Anus. 



vp. Vaso-peritoneal vesicle, water-tube. 



vt. Vitellus. 



wt. Water-tube. 



PLATE I. 



Figures drawn with camera lucida, Obj. D. D. eyepiece 2, Zeiss. Reduced one 

 third in photography. 

 Fig. 1. Ovum of Ophiopholis aculeata, found free in water after? fertilization. 



p, Superficial cortical layer of the yolk. 

 " 2. Same, showing a single polar globule (?) at d. 

 " 3. Egg in the 2-cell stage with polar globules, d and first plane of cleavage, 



1 dpi. 

 " 4. The same without polar globules. 

 " 5. Egg in the 2-cell stage with the first cleavage plane, so turned as to show 



tlie thickness of the transparent plasmic region following the firt 



plane of cleavage. 

 " 6. Egg in 4-cell stage. 

 " 7. The same. 

 " 8. Egg in 8-cell stage. 



" 9. The same showing the segmentation cavity, cav. Twelve hours old. 

 " 10. Blastosphere. The cluster of cells in the middle of the figure are not 



cells in the blastocoelic cavity, but cells of the blastoderm with granu- 

 lations. All the blastodermic cells have the same appearance. One 



day old. 

 " 11. Blastosphere, showing the beginning of an invagination of the archen- 



teron, ach, forming the blastopore. 

 " 12. The same seen from the side. Amoeboid, mesoblastic cells at a cl. 

 " 13. A little older larval stage, from the ventral side. A small cluster of cells 



represented with granulations near the right-hand lower corner. 

 " 14. Older gastrula from ventral side. 

 " 15. The same from a lateral view. 

 " 16. 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. 

 " 17. 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. 



