450 WILSON. 
EXPLANATION OF PLATE XVI. 
Lumbricus fetidus. 
Fic. 1. Unsegmented ovum, surrounded by the vitelline membrane, with two 
groups of polar cells. 
Fic. 2. First cleavage (in the plane of a group of polar cells). 
Fics. 3 TO 5. Four-celled stage, showing the segmentation cavity. 
Fics. 6 AND 7. Two succeeding stages, showing the division of the large cell into 
two equal parts, followed by a division of one of the three smaller cells. 
Fics. 8 AND 9. Two views of another specimen in the six-celled stage (the indi- 
vidual cells numbered for identification). 
Fics. 10 AND II. Two views of a seven-celled embryo. 
Fics. 12 AND 14. Three views of an eight-celled stage. 
Fics. 15 TO 19. Various views of a blastula (optical section in Fig. 19), consisting 
of thirteen cells, showing the entire structure. The protoplasmic prominence marked 
x, attached to cell No. 4 (which is probably accidentally caused) serves to orient the 
embryo. 
Fic. 20. Later blastula with a large segmentation cavity but no cleavage-pore. 
Fics. 21 AND 22. Surface-view and optical section of a blastula somewhat younger 
than the last, showing the cleavage-pore. 
Fic. 23. Surface-view of blastula, showing the two primary mesoblasts at the 
surface. 
(All the figures drawn with the camera from preserved specimens. Enlarged 300 
diameters.) 
