HABITS, ETC., OF CEREBRATULUS LACTEUS. 147 



really egg-shaped or ellipsoidal masses lying side by side, 

 with the polar bodies still attached to the first cleavage 

 groove (fig. 53). When they flatten together an open space 

 is often left in the centre as in the Echinus egg (fig. 52). 



If the surface of the blastomeres bordering this opening 

 be examined under a high power considerable filose activity 

 is revealed. One spin-thread starts from a papilla on the 

 side of a blastomere, and runs diagonally across to an ad- 

 jacent blastomere. Several other fine threads can be seen 

 radiating from the summit of the papilla, but they could not 

 be followed across (fig. 52). Another thread crosses from 

 one blastomere to the one opposite it, and makes a sharp 

 bend or elbow halfway across, very similar to the bends 

 figured for the star-fish blastula and the Echinus four-cell 

 stage in the paper just referred to (5). The third thread 

 visible at this level proceeded out a little way from the re- 

 maining blastomere, and then turned abruptly and ran down 

 diagonally to an adjacent blastomere. 



This central open space is subsequently obliterated by the 

 flattening of the blastomeres. The polar bodies continue 

 their filose activity, remaining attached to one side of the 

 first furrow. In fig. 43 both bodies are seen to be bipolar, 

 but the two poles of the first body are unlike. 



One consists of a large but short and blunt process from 

 whose tip and sides fine threads are given off, while the other 

 is made up of a simple bunch of radiating threads, each of 

 which starts from the body itself. Another blunt process 

 projects toward the second polar body, and forms part of the 

 connecting band between the two. Short fine threads also 

 radiate from this process. 



The second segmentation occupies not more than eight or 

 ten minutes, and is followed by a rest of five minutes. 



The third segmentation is vertical, and divides the egg into 

 eight equal blastomeres. Subsequent segmentations take 

 place more slowly than the first two, and the blastula stage is 

 reached in about fifteen hours. During the second and third 

 segmentations the egg elongates again, each time at right 



