THE DEVELOPMENT OF ORGANISMS 729 



latter restricted to the maturation of the germ-cells, may think of 

 two ways of dividing a boxful of wooden matches. One might 

 with a penknife split each of the two dozen matches up the middle, 

 and put two dozen halves into a new box. That would correspond 

 to mitotic division. Or one might put a dozen of the matches into 

 a new box, and that would correspond to meiotic division. This is 

 the gist of the contrast, though there are other differences, such as 

 the fact that the double-chromosomes arrange themselves in meiotic 

 division at right angles to the equatorial plane, whereas they lie in 

 the plane of the equator in mitotic division. 



FERTILISATION. — In the seventeenth and eighteenth centuries 

 it was believed by some naturalists, nicknamed "ovists", that the 

 ovum was all-important, only needing the sperm's awakening touch 

 to begin unfolding the miniature model which it was supposed to 

 contain. Others, called "animalculists", were equally certain that 

 the sperm was the essential element, though it required to be fed 

 by the ovum. It was but slowly that it became clear that the two 

 kinds of germ-cells are complementary, but even then many 

 naturalists thought that actual contact was unnecessary, since 

 fertilisation might be effected by an aura seminalis ! Though sperma- 

 tozoa were distinctly seen by Leeuwenhoek and Hamm in 1679, 

 their actual union with ova was not observed till 1843, when Martin 

 Barry detected it in the rabbit. A knowledge of the details of the 

 intimate and orderly union is much more recent. 



Among the many facts now known in regard to fertilisation, the 

 following are most important : 



(i) In its maturation the ovum makes an attempt at independent 

 activity, which succeeds in cases of parthenogenesis. Usually, how- 

 ever, the ovum sinks into static equilibrium — paralysed, according 

 to some cytologists, by its own waste-products. It cannot be acti- 

 vated except by the entrance of a spermatozoon, or by the stimuli 

 utilised in bringing about artificial parthenogenesis. The reduced 

 female pronucleus has often passed to the centre of the ovum, and 

 the nucleus of the spermatozoon has to make a journey through the 

 cytoplasm to find it, attracted by some influence from a distance 

 which, though not understood, is called chemotaxis. In many 

 cases, such as starfishes, Annelid worms, and mammals, the sperma- 

 tozoon seems to be able to enter the ovum at any point; or it may 

 be restricted to a micropyle, or to a particular region with little 

 yolk. In some cases it is met by a hillock of protoplasm which rises 

 from the surface of the egg-cell, indicating that there is more than 

 mere passivity on the ovum's part. There is indeed some visible 

 engulfing of the boring sperm. The whole process is usually over in 

 one minute. 



When entrance has been effected, the delicate vitelline membrane 



