176 



THE GERM- PLASM 



ontogeny, and this presupposes not only a difference in 

 the arrangement of the determinants in the germ-plasm as 

 compared with the meta-germ-plasm (' Nach-Keimplasma'), but 

 also the presence of different determinants for some of the 

 embryonic stages. The case becomes still clearer if we take one 

 particular species of Daphnid {Leptodora hyalind) into con- 

 sideration. In this animal the embryogeny of the winter-eggs 

 only extends as far as to the formation of the primitive crustacean 

 larva, or nauplius, which possesses three pairs of limbs : the 

 summer-eggs, on the other hand, develop at once into the adult 

 form of the animal, in which all the limbs are present. The 

 summer-eggs certainly also pass through the stages from the 

 ripe ovum to the nauplius, but these are abbreviated, and 



Fig. 9. — Nauplius larva of Leptodora hyalina. (After Sars, from Korschelt and 

 Heider's ' Lehrbuch der vergleichenden Entwicklungsgeschichte.') 



though this nauplius also possesses three pairs of limbs, these 

 are only rudimentary, and are useless as swimming organs. 

 There must therefore be two kinds of germ-plasm in Leptodora., 

 one of which still contains all the determinants of the nauplius. 

 while the other only contains a portion of them, and even these 

 have probably undergone some change. The two kinds of 

 germ-plasm must be passed separately along the germ-tracks 

 from one generation to another, so that each must always con- 

 tain the other, which is, so to speak, stored away in it in an 

 inactive condition. It seems to me impossible to explain the 

 facts in any other way, for it is inconceivable that the germ- 

 plasm of the summer-eggs, which has undergone reduction, 

 and possesses comparatively few determinants, should be able to 

 develop the lost determinants ojit of its own snbstajice. 



