Fig. 68. Side view of a ripe ego. x 100. 
Figs. 69—79. Segmentation of the egg and larval 
stages of Arenicola cla paredit. 
Vig. 69. Two-cell stage. he two polar bodies are 
shown. Four and a half howrs after addition of the 
spermatozoa to the sea water containing the eggs. x 210. 
Fig. 70. Four-cell stage, seen from the anterior pole. 
eG: 
Bie (1, Night-cell stage, seen from the anterior 
pole. The first four ectomeres (la, 1b, lc, 1d) have been 
cut off from the lower cells. x 210. 
Fig. 72. Sixteen-cell stage, seen from the anterior 
pole. Mach of the first ectomeres has divided into two and 
the second quartette of ectomeres (2a, 2b, 2¢, 2d) has been 
eut off from the lower cells. Of the latter only one (24) 
is shown in the figure. 2d is the largest cell in the egg 
and forms a conspicuous landmark. x 210. 
The cleavages shown in the three preceding figures 
occupied about two hours 
Fig. 73. The Jarva (blastula) at the end of 24 
hours, seen from the posterior pole. The four quartettes 
have been formed, the first three of which are ectomeres. 
One of the fourth quartette (4d) is the mesoblast cell and 
it has already sunk into the segmentation cavity. The 
three other members of the fourth quartette (4a, 4b, 4e) 
and the four lower cells (44, 46, 4C, 4D) are endoderm 
cells. In this specimen each of these seven primary endo- 
derm cells, except 4c, has again divided, so that there are 
thirteen endoderm cells at the posterior pole of the larva. 
xe LO: 
Big. 74. Larva about six hours later than the pre- 
ceding. Ventral aspect. ‘The pre-oral band of cilia is 
shown just in front of the stomodzal invagination. The 
larva is at this stage constantly rotating in the vitelline 
