The determinatiou of sex in animal development. 743 



these are the ones, which really develop (for in some instances the 

 one, or the other, or both, may do this without fertilisation), that the 

 determination of sex for the next generation rests with the female 

 Metazoan individual, or, rather, with the germ-cells, of which it is 

 the bearer. 



Summing up, the problems of sex thus become those relating to 

 unicellular organisms, of which the individuals are of two kinds. For 

 the continuance of these more than two categories of gametes are 

 needful. Prior to conjugation each kind of individual originally "pro- 

 duced", that is to say, broke up into the forerunners of two cate- 

 gories of gametes, and these together made up four distinct kinds, 

 so discriminated and selected, that their pairings gave rise to but 

 two sorts of individuals (zygotes). Conceivably, the formation of the 

 forerunners may originally have been a mere undoing of the previous 

 union : now, whatever it may have been in its origin, it is something 

 more than this (for a simple separation of the parts originally bound 

 together would but result in two sorts of gametes), and in fact it is 

 such, that four kinds arise instead of but two. 



Owing to the constant ditferentiation either of two forms of 

 gametes, or of the forerunners of such, by every Metazoan individual, 

 and because two categories of gametes arise in every hermaphrodite 

 individual, one may define a Metazoan as an animal form, in 

 w^hich either two kinds of gametes, or the forerunners 

 of such (males), are constantly differentiated. 



Nature would appear to work her reproductive mill in the Meta- 

 zoa by means of three sorts of functional gametes : spermatozoa, male- 

 eggs, and female-eggs. With one of these she can dispense in herm- 

 aphroditism : with two on occasion, and, perhaps, sometimes entirely, 

 in parthenogenesis. A fourth sort, giving two kinds of functional 

 elements in the male, only appears to obtain as an abortive, futile, 

 or useless sport. Indeed, to all appearance it would be a needless 

 luxury, leading to difficulty, and even to disaster; and the mill can 

 go on, merrily grinding away ^), without it. 



VI. Strasburger's Researches on Dioecious Plants. 



With the close of this portion of the present manuscript it has 

 been possible to study Strasburger's recent record of detailed and 



1) "The world rolls round for ever like a mill; 



It grinds out death and life and good and ill." 



James Thomson (B. V.), '-The City of Dreadful Night." 



