Introduction to Animal Morphology. 35 



or carried by ducts, called respectively oviducts and 

 vasa deferentia. All reproductive processes seem only 

 to differ from growth processes in that the results are 

 discontinuous, and they may be divided into two 

 series — ist. Agamogenesis, where no second or male 

 element is required to stimulate the (often internally 

 produced) germ into development. The simplest form 

 of agamogenesis, leaving out of account fission and 

 gemmation in plastides, is the production of the so- 

 called winter eggs in sponges, where we have a con- 

 stant and, as far as we know, continuous asexual pro- 

 pagation not necessarily interrupted by any periodic 

 recurrence of sexual reproduction. Such a form is 

 monogenetic agamogenesis, as the progeny and parents 

 are similar. In another case a primary form or proto- 

 zooid produces, by internal gemmation (without having 

 any developed sexual organs), a secondary form or 

 deuterozooid, and this may in time produce a tritozooid, 

 or it may develop sexual organs and produce ova, 

 which require the male elements to fertilize them. To 

 this process the name metagenesis has been given, and 

 of it there are varieties : thus, when the deuterozooid is 

 sexual, and so differs from the protozooid, the process 

 is metagejiesis zvith successive heterogojiy, or if the pro- 

 tozooid be sexual, and by internal gemmation give 

 rise to a deuterozooid in which sex organs are also 

 developed, the metagenesis is said to be with contempo- 

 raneous heterogony, and it may be isogenetic or diverse^ 

 according as the deuterozooid agrees or disagrees with 

 the protozooid in sex. Where there are successions 

 only of two stages, the name digcnesis is sometimes 

 given in place of metagenesis. 



In other cases the protozooids have generative 



D 2 



