PHYSIOLOGY OF THE INVERTEBRATA. 403 



that access from without, and the occasional influence of a 

 distinct individual, can be shown to be physically impossible." 

 Darwin concludes, from a large number of observations 

 and facts that " an occasional intercross between distinct 

 individuals is a very general, if not universal, law of nature." 



The male element or spermatozoon varies in form and 

 size in different animals, but consists of a head and filiform 

 appendage or appendages."^ The spermatozoa move by 

 vibrations in a fluid called the semen, where they exist in 

 large numbers. 



The female element or ovum is a nucleated cell developed 

 in the ovary. In all animals the ovum is nearly identical. 

 It consists of a vitelline membrane, a protoplasmic contents 

 or vitellus, a germinal vesicle (nucleus), and a germinal spot 

 (nucleolus). 



As already stated it is the union of these two elements 

 which give rise to offspring. Fecundation is brought about 

 by various methods in the animal kingdom. But as far as 

 the Invertehrata are concerned, these methods will be described 

 more in detail later in this chapter. Suffice it to say that in 

 the majority of the unisexual Iiivcrtcbrata copulation or the 

 union of the sexes takes place. In animals higher in the 

 zoological scale — for instance in fishes, the male discharges 

 the semen over the spawn or ova of the female, for there is 

 no act of copulation. In many of the AmjjJtihia and liCptilia, 

 the male clings to the back of the female, and then discharges 

 the seminal fluid or semen over the ova as they pass through 

 the uro-genital aperture. 



In the Aves and Mammalia, and also in many of the 

 Invertehrata, the semen is introduced by the penis into the 

 genital organs of the female. 



By any one of the above acts the ovum becomes fertilised : 



a series of changes occur which result in a more or less 



complete segmentation. If this segmentation or division is 



complete — i.e.^ involves the whole vitellus — it is called 



* In Adacus there are many appendages. 



