ON PARASITES. 29 



The following laws are the result. They are positive. Laws of 

 Generation : 



I. All animals spring ultimately from eggs. 



II. These eggs are spontaneously produced in the female. 



III. These eggs are spontaneously discharged. 



IV. Zoosperms are spontaneously produced in the male. 



V. These Zoosperms must come in actual contact whilst living with 

 ovules of the same species, wherehy those ovules are fertilized. 



VI. In the higher animals the spontaneous production of eggs is 

 periodical. 



These laws were established by observations among all classes of 

 vertebrata. 



Abundant observation among invertebrata establishes their applica- 

 bility (with the exception of the sixth) to these animals as well. 



The separation of male and female organs in different individuals is 

 by no means essential. They may exist as well in the same individual. 

 Many of the invertebrata are hermaphrodite. In some cases the con- 

 currence of two individuals, as in the case of snails, seems to be necessary, 

 but in others the male part, through an intromittent organ, brings the 

 zoosperms in relation with the ovules in the female generative passages 

 without any concourse of separate individuals. Fecundation having 

 by whatever process been accomplished, development proceeds, through 

 the primary step of segmentation of the vitellus and disappearance of 

 the germinal vesicle. A blastodermic membrane is formed by the 

 packing of cells against tiie vitelline membrane, and then fusing 

 together more or less completely. This stage accomplished, an 

 organised structure has been formed, myriads of such structures are to 

 be seen floating in water, constituting the simpler infusoria. These 

 have the power of generation by gemmation and fissation, but altern- 

 ately both they and their progeny assume a more complex structure, 

 and in a male and female apparatus of some kind produce zoosperms 

 and ovules. Sooner or later the gemmation and fissation are exhausted, 

 and a recurrence to the ovular generation takes place. 



This principle lies at the foundation of all the complex phenomena 

 that are manifested in the evolution of animal organisms. 



But after the blastodermic membrane has been formed a special con- 

 centration of cells may take place at some part of the membrane, and an 

 organism of higher type appear. 



