92 THE EVOLUTION OF SEX. 



In the case of a well-known fly, Chironomus, Prof. Balbiani, 

 unprejudiced by any theory of heredity, observed the following 

 facts : — Before the segmentation of the ^gg had at all advanced, 

 before what embryologists call the blastoderm was more than 

 incipient, two cells were observed to be set apart externally. 

 (These had nothing whatever to do with the polar globules 

 seen in most ova at maturation.) The development proceeded 

 apace, but the isolated cells took no share ; they may be pre- 

 sumed to have retained intact the characters which they 

 received when first divided off from the ovum. Kx a certain 

 stage, however, the insulated cells sank inwards, took up an 

 internal position, became the rudiments of the reproductive 

 organs. Here then, at an early stage, before differentiation is 

 marked, the reproductive cells are set apart. They must 

 therefore preserve much of the character of the parent ovum, 

 and hand on the tradition intact by continuous cell-division to 

 the next generation. 



In other words, in the preceding case, at a very early stage 

 in the embryo, the future reproductive cells are distinguishable 

 and separable from the body-forming cells. The latter develop 

 in manifold variety, into skin and nerve, muscle and blood, gut 

 and gland ; they differentiate, and lose almost all protoplasmic 

 likeness to the mother ovum. But the reproductive cells are 

 set apart ; they take no share in the differentiation, but remain 

 virtually unchanged, and continue unaltered the protoplasmic 

 tradition of the original ovum. After a while they, or their 

 division-products rather, will be hberated as reproductive cells. 

 These in a sense will be continuous with the parental germ. 

 Their protoplasm will be more or less identical. The original 

 ovum has certain characteristics, abc) it divides, and all its 

 cells must at first more or less share these characteristics ; the 

 body-cells lose them, the insulated reproductive cells must 

 retain them. The ovum of the next generation has thus also 

 the characteristics a b c, and must therefore produce an 

 organism essentially like the parent. 



An early isolation of the reproductive cells, though never 

 so striking as in Chiro7iomus^ has been observed in many 

 cases, — e.g.^ in other insects, in the aberrant worm-t}^pe 

 Sagiffa^ in leeches, in thread-worms or nematodes, in some 

 Polyzoa, in some small crustaceans known as Cladocera, in the 

 water-flea Moi?ia, and in some spiders {PhaIa?tgidcB),B.r\d. probably 

 in other cases. As the series is ascended, the reproductive 



