8UBKINQD0M METAZOA. 49 



maturation of the ovum and seems to indicate tliat the polar 

 globules are to be regarded as abortive ova. 



The conversion of the spermatids into spermatozoa is 

 simply a differentiation of structures already present. In 

 the air-breathing Mollusca, for instance, the spermatids consist 

 of a mass of cytoplasm containing a nucleus, in close proxim- 

 ity to which may be found the centrosome, while an irregular 

 mass of filaments represents the remains of the spindle-fila- 

 mients. In the differentiation which follows (Fig. 22, D, E, 

 and F) the nucleus elongates and its chromatin-filaments 

 fuse to form a homogeneous mass ; the cytoplasm likewise 

 elongates, and in it appears an axial filament which later will 

 form the tail-filament. The origin of this filament is doubt- 

 ful, some authors maintaining that it is a differentiation of 

 the cytoplasm, while others believe it to be a prolongation of 

 the nuclear substance ; but, however that may be, the spiral 

 fringe which surrounds the axial filament is certainly the 

 remains of the cytoplasm of the spermatid. The remains of 

 the spindle-filaments disappear, while the centrosome prob- 

 ably persists as a structure lying behind the head and termed 

 the " Mittelstiick." 



In some cases, as the insect Pyrrhocoris and the crustacean Diaptomus, 

 the doubling of the chromosomes previous to division into spermatocytes 

 does not take place. In Pyrrhocoris twenty-four chromosomes are typi- 

 cally present and twelve of these pass into each of the spermatocytes, and 

 in the division of these to form the spermatids each of the twelve chromo- 

 somes divides so that each spermatid possesses half the typical number. 

 In Diaptomus the same result is brought about somewhat differently. 

 The spermatogones possess eight chromosomes which assume a dumbbell 

 shape and divide transversely, so that each spermatocyte has the typical 

 number of chromosomes ; the spermatocytes divide without passing 

 through a resting stage, and each spermatid thus contains four chromo- 

 somes, i.e. half the typical number. 



Fertilization of the Ovum. — So soon as the formation of the 

 polar globules has been completed, the nucleus of the ovum 

 migrates towards the centre of the protoplasm and is the 

 female pronucleus (Fig- 23, fp) of conjugation. The penetra- 

 tion of the spermatozoon may occur at any portion of the sur- 

 face of the ovum and may take place before, during (Fig. 21, 

 B, sp), or after the formation of the polar globules, a single 



