378 AN INTRODUCTION TO ZOOLOGY 



will be seen that, in spite of the difference in size between the two 

 germ cells, the total inheritance of the new organism is derived 

 equally from both male and female parent. This too has to be 

 borne in mind when considering the phenomena of heredity. The 

 sperm, in addition, introduced some stimulus, perhaps closely bound 

 up with the centrosome, that is required to initiate development. 

 It can be shown that this stimulus is nothing to do with the actual 

 presence of the chromosomes themselves by experiments known as 

 " artificial fertilisation/' The eggs of some marine animals can 

 be treated by the addition of certain soluble metallic salts to the 

 sea-water in which they are contained, and in this way it has been 

 found to be possible to supply the stimulus necessary to cause some 

 eggs to segment. In certain cases, even, it has been found possible 

 to produce advanced embryos in this way. But it is necessary to 

 supply a stimulus, apparently a mechanical or chemical one, that is 

 not ordinarily present in the environment of the egg in order to 

 obtain this result, and under normal conditions this stimulus is 

 introduced by the sperm. 



As has been noted previously, certain animals ordinarily produce 

 eggs that undergo development without fertilisation.. This is 

 termed parthogenesis, and so the experiments just described are, 

 on the whole, more accurately termed artificial parthenogenesis. 

 When parthenogenesis occurs normally it is brought about by an 

 entirely different means. It has been observed that during the 

 production of the second polar body such eggs behave differently 

 from those of other animals, and either the second polar body is not 

 formed at all or, if it is, then, before its cytoplasm separates from the 

 parent mass, its nucleus returns and reunites with that of the ovum. 

 So that in these cases the second polar body, as it were, takes on the 

 function of the sperm with the twofold result that the diploid number 

 of chromosomes is restored and segmentation commences. 



Amphioxus. 



In order to get a clear idea of the main features of the 

 embryology of the CHORDATA it is necessary that they should be 

 considered briefly in several types, and we take first of all Amphioxus 

 lanceolatus, where certain of them are presented in a simple con- 

 dition. Amphioxus itself is a small somewhat fish-shaped form, 

 growing to a length of about two inches. It is found at certain spots 

 on the British coast, but more commonly in the Mediterranean, and 

 allied species are found in the shallow seas of many parts of the world. 

 It is almost transparent, and, although spending most of its time 

 partly embedded in the sand, with only its mouth and anterior end 

 protruding, it can swim and burrow into the sand very rapidly. 



