Evidence from Metazoan Germ-Cells 3 



offspring 1 . 



Especially important for us is it to notice that to a great 

 extent the diversity of structure among spermatozoa is in 

 the locomotor organ, the tail; that is, the organ chiefly 

 concerned with the unique life of the sperm as such, and 

 very little if at all concerned directly with fertilization and 

 hence with hereditary marks of offspring. This fact de- 

 serves attentive consideration. "In its more usual form the 

 animal spermatozoon resembles a minute, elongated tad- 

 pole, which swims very actively about by the vibration of a 

 long, slender tail." 2 In some respects comparison of the 

 spermatozoon of the type here indicated with an Appen- 

 dicularian, a minute Tunicate which possesses a tail through- 

 out its life, is more instructive. Any one who has had op- 

 portunity to observe both sperm cell and appendicularian 

 when alive and active will not have failed to remark the 

 general resemblance, not only as to form but as to kind 

 of movement in the two cases. Is the development of the 

 Appendicularian's tail a manifestation of heredity? Surely 

 no one would think of giving any but an affirmative answer. 

 How, then, deny that the development of the spermatozoon's 

 tail is also a manifestation of heredity? I cannot see that 

 it would be less inconsistent to affirm that the wriggling ap- 

 pendicularian is alive but that the wriggling spermatozoon 

 is not, than to affirm that the ontogeny of the first is guided 

 by heredity while that of the second is not. To bring the 

 point onto somewhat more familiar ground, let us revert 

 to Wilson's comparison of the spermatozoon to the tadpole 

 stage in the life of the frog. Our contention is that the tail 

 of the frog's spermatozoon is as indisputably modeled by 

 heredity as is the tail of the frog's tadpole, and consequently 

 that we are bound to search for the physical basis of hered- 

 ity in the former as well as in the latter. Our inquiry is, 

 then, What observations have we as to the substances con- 

 cerned in producing the spermatozoon tail? 



