SPERMATOGENESIS OF ASCARIS 443 
Boveri, however, believes that the plutei so produced are of the 
pure Echinus type. This conclusion shows clearly that the ma- 
ternal plastochondria are not bearers of structural characteristics, 
and it cannot be supposed that these bodies in the spermatozoon 
possess hereditary qualities not to be found in those in the egg. 
b. Heredity in hybrids. The experiments of Loeb, Herbst, 
Baltzer, Tennent and others in crossing echinoderms, fishes, and 
so forth, show that heterogeneous hybrids are almost constantly 
maternal in structure. This should never be the case if the 
plastochondria function in inheritance of structure. But the 
larvae produced in these experiments show clearly, when studied 
cytologically, what it is that determines their maternal type of 
structure. Baltzer, Tennent and others have seen cross-ferti- 
lized eggs of echinoderms actually eliminate from themselves cer- 
tain bodies brought into them by the spermatozoon. But these 
bodies are not plastochondria; they are chromosomes. Loeb 
finds that fish hybrids of this kind are always maternal, because 
synapsis is never successfully accomplished. Here again the chro- 
matin alone is concerned. 
As is well known, it is entirely possible, on the other hand, to 
produce ‘intermediate hybrids.’ But the study of cross-fertilized 
echinoderm eggs has shown that it is not the fusion of paternal — 
and maternal plastochondria that produces these, but the reten- 
tion of paternal chromatin by the fertilized egg. This always 
occurs, apparently, in forms sufficiently closely related. 
c. Normal fertilization in Nereis. No clearer proof of the 
secondary importance of cytoplasmic structures in fertilization 
and inheritance could be desired, however, than that furnished by 
the observations of the normal fertilization of the egg of Nereis, 
made recently by F. R. Lillie. One of the enigmas in the study 
of the cytology of the sex cells has been for many years the nature 
and function of the middle-piece. The fact that it alone accom- 
panies the ‘head’ or nucleus of the spermatozoon into the egg in 
fertilization has been generally accepted as proof that it plays 
some essential part in that process. Indeed, it was the discoy- 
ery that the middle-piece and its accessory structures are com- 
