xxili UNICELLULAR AND MULTICELLULAR ANIMALS 261 



of two nuclear bodies, one contributed by the male gamete 

 or sperm, the other by the female gamete or ovum. It 

 follows from this that the essential nuclear matter or chro- 

 matin of the oosperm is derived in equal proportions from 

 each of the two parents. 



Moreover, as both male and female pronuclei contain only 

 half the number of chromosomes found in the ordinary cells 

 of the species, the union of the pronuclei results in the 

 restoration of the normal number to the oosperm. 



In some cases the astrospheres of the sperm and 

 ovum as well as their nuclei appear to unite with one 

 another, but more usually the egg-centrosome degenerates 

 and disappears, the centrosome of the oosperm — and conse- 

 quently of all the cells of the fully-formed animal — being 

 derived from the centrosome of the sperm, i.e. from the 

 male parent. 



Fertilization being thus effected, the process of segmenta- 

 tion or division of the oosperm takes place as described in 

 the preceding lesson (p. 246). 



In concluding the present lesson, we shall consider briefly 

 a point which has probably already struck the reader. 

 Among the plant-forms which have come under our notice 

 there has been a very complete series of gradations from 

 the simple cell, through the non-cellular filament, linear 

 aggregate, and superficial aggregate, to the solid aggregate, 

 whilst among the animals already discussed there has so 

 far been no attempt to fill up the very considerable gap 

 between unicellular and multicellular forms. In Amoeba, 

 Vorticella, &c., the entire animal is a single cell, while 

 our next animal type. Hydra, is not only a solid aggregate, 

 but has its cells arranged in two definite layers enclosing 

 a digestive cavity. Moreover, in unicellular organisms repro 



