The crude older technique of cutting open the animals and removing the gonads is wasteful of material, and often yields gametes so contaminated with body fluids that they are non-fertilizable. For species which fertilize their eggs internally, of course, such methods do not apply, and there are certain other forms for which removal of the excised gonads to dishes of sea water is apparently the only practicable method for obtaining eggs and sperm. However, it is, in general, important to avoid excessive concentrations of sperm. For eggs that elevate fertilization membranes or extrude marked quantities of jelly as a part of the fertilization-reaction, the eggs should be even more widely spread. After embryos have used up the stored yolk, oil, and other nutritive materials originally present in the egg, they must be supplied with appropriate food if development is to continue.
Diatoms are the chief food for marine invertebrate larvae such as trochophores, bipinnaria, etc.
There are several laboratories which maintain one or more strains of Nitzschia and other diatoms in pure and in mixed cultures, and it is often possible to obtain inocula from such cultures. Certain marine protozoa are useful for feeding some types of larva and older stages will often thrive on a diet of mixed plankton. Fundulus hatchlings can be fed Nereis trochophores. This is especially useful for sperm and for very slender structures, such as sperm filaments. The polarization microscope gives superb visualization of living spindle fibers and of other structures with oriented molecules, such as the spicules of echinoderm larvae.