506 INVERTEBEATA CHAP. 



of the usual size (say eight inches in diameter), it is sufficient to acid 

 half a dozen drops of this emulsion to ensure fertilization of every 

 egg, if the mixture be thoroughly stirred up. 



When once the eggs have fallen to the bottom, the supernatant 

 fluid, which contains the superfluous spermatozoa, must be decanted 

 off. Fresh sea-water is then added, and the eggs stirred up in it, 

 and the decantation repeated. In this way all the unused 

 spermatozoa are removed ; if this is not done they die and befoul 

 the water and impede the development of the eggs. Whether the 

 fertilization has been successful or not can be determined by 

 examining a sample of the eggs under a microscope half an hour 

 afterwards. It will be then seen that all the eggs which have been 

 ready to receive a spermatozoon have formed fertilization membranes, 

 between which and the egg there is an accumulation of fluid. 



In eighteen to twenty-four hours the larvae will have burst these 

 membranes, and will be found swimming at the surface of the water. 

 They can then be decanted off into vessels filled with sterilized sea- 

 water (prepared as described under Asteroidea), and supplied with food 

 in the form of a pure culture of the diatom Nitschia, when they will 

 metamorphose into young sea-urchins in a period varying from six 

 weeks to two months. Too many larvae must not be reared in one 

 jar. In practice it is found that twenty or thirty is the largest 

 number which will thrive in a half-gallon jar. Direct sunlight is 

 to be avoided, and the larvae seem to do better when contained in 

 a jar made of green bottle-glass. 



The methods of preservation, both for whole mounts and sections, 

 are the same as those prescribed for the larvae of Asterias and 

 Ophiothrix. 



ECHINUS ESCULENTUS 



The egg divides into two and then four equal cells, and the 

 8-cell stage is reached by the division of the four cells into two 

 tiers of four each. These cells touch each other laterally ; eventually 

 they are separated from one another by a space, the blastocoele. As 

 development goes on the blastocoele enlarges. 



Then the 16 -cell stage follows. This is reached by the 

 members of one tier dividing, each into a very small cell below, 

 termed the micromere, and a larger one above, the macromere. 

 Each of the members of the other tier divides by radial cleavage 

 into two equal cells, and so a circle of eight cells of intermediate 

 size is formed. These are termed mesomeres. Driesch's experi- 

 ments (1900) have demonstrated what Boveri showed to be the 

 case by direct observation in the egg of Strongylocentrotus iividus 

 (1901), viz. that the micromeres are situated at the vegetative pole 

 of the egg, not at the animal pole as had been previously imagined. 



In the next cleavage each micromere buds off a smallest 

 micromere, and the other cells divide into daughters of equal size. 

 In subsequent cleavages each of the smaller micromeres divides 



