V ASCARIS, TRICHINA 197 



extrudes the sex chromosome bodily in one or other of the two meiotic 

 divisions. 



Care must be taken not to attribute necessarily any active sex- 

 determining role to the sex chromosomes : the present state of our know- 

 ledge does not justify any such conclusion. All that we are strictly 

 justified in saying is that the extrusion of the sex-chromosome in one 

 type and its non-extrusion in the other provides us with a kind of label 

 indicative of a deep-seated sex-producing difference between the two 

 sets of microgametes. 



Whether the existence of two different sex-producing types of micro- 

 gametes is to be regarded as the normal cause of the determination of 

 the sex of the sexually-produced individual throughout the animal 

 kingdom is a question to which a definite answer must wait for future 

 research. Probability appears on the whole to point to this answer 

 being in the affirmative. 



(9) Lastly we have seen how the " germ track " is continuous from 

 generation to generation — the bodies of the successive individuals in 

 a chain of descent being as it were " side-tracked " from it. Here we 

 are in touch with one of the most impressive ideas in biological science 

 namely that the living substance of each creature existing on the earth 

 to-day is continuous right back through the eternal past with the living 

 substance which first came into existence in the dawn of the evolution 

 of living things. 



Various nematode worms are liable to occur as parasites of man and 

 we will now survey the life-histories of a few of the more important of 

 these. 



Trichina 



T. spiralis, the female of which measures about 3-4 mm. in length 

 and the male about 1-5 mm., is apparently primarily a parasite of the 

 rat, although it is liable to spread to many other animals including man. 

 The worms reach maturity in the small intestine. After the act of 

 fertilization the male dies while the female grows rapidly to its full length, 

 bores into the intestinal wall and settles down there in the connective 

 tissue. At about the sixth day the birth of young commences : it con- 

 tinues during several weeks, a single female producing many thousands 

 of young larvae. 



The larvae, which measure about -i mm. in length, pass along the lymph 

 spaces and a large proportion of them eventually reach the blood-stream 

 and are by it distributed through the body. These larvae are small 



