SEA-FISHERIES LABORATORY. 325 



Fig. 08. Side view of a ripe egg. x 100. 



Figs. 69 — 79. Segmentation of the egg and larval 

 of Arenicola claparedii. 



Fig. 69. Two-cell stage. The two polar bodies are 

 shown. Four and a half hours after addition of the 

 sjDerniatozoa to the sea water containing the eggs. x 2.10. 



Fig. 70. Four-cell stage, seen from the anterior pole, 

 x 210. 



Fig. 71. Eight-cell stage, seen from the anterior 

 pole. The first four ectomeres (la, 16, lc, Id) have been 

 cut off from the lower cells. x 210. 



Fig. 72. Sixteen-cell stage, seen from the anterior 

 pole. Each of the first ectomeres has divided into two and 

 the second quartette of ectomeres (2a, 26, 2c, 2d) has been 

 cut off from the lower cells. Of the latter only one (2 A) 

 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 larva (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 (4a 7 ) is the mesoblast cell and 

 it has already sunk into the segmentation cavity. The 

 three other members of the fourth quartette (4a, 46, 4c) 

 and the four lower cells (4.A, 4B, 4C, 4Z?) 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, 

 x 210. 



Fig. 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 stoniodseal invagination. The 

 larva is at this stage constantly rotating in t'he vitelline 



