No. 2.] THE ECHINODERM SPERMATOZOON. 259 



Cases where the conditions lie so simply are by no means 

 common. But investigation will show that it is much more 

 general than has been supposed, and that cases where the 

 spermatogone gives rise to four spermatozoa have been found 

 in widely different groups, in many Vertebrates, Pulmonates, 

 Lepidoptera and other Arthropods, and Ascaris (similar con- 

 ditions also are found in the pollen formation in many 

 plants) ; hence we are led to consider the probability that 

 this condition, which is well exemplified in the case of the 

 Echinoderm, is the simple ancestral condition, from which the 

 great variety of types of spermatogenesis have been derived 

 in adaptation to the various modes of life of the different 

 animals and plants ; the necessity for a greater or a lesser 

 number of spermatozoa being met by an alteration in the 

 number of cell divisions between the spermatogone and the 

 spermatozoon. The great majority of the Echinoderms have 

 continued in this primitive condition : the number of sperma- 

 tozoa being at least four times the number of the eggs, since 

 each spermatogone, the homologue of the unmaturated &g2,, 

 gives rise to four spermatozoa, but further from the fact that 

 the spermatogones being very much smaller than the ova, an 

 enormously greater number can be contained in the testes. 

 This numerical ratio between the ova and the spermatozoa has 

 been sufficient to maintain the race, particularly since the con- 

 ditions governing the existence of the Echinoderms have under- 

 gone remarkably slight changes from the time of their first 

 appearance up to the present day. 



The demonstration of the close similarity between the his- 

 tories of the male and female cells throughout the animal and 

 vegetable kingdoms will render easier the understanding of 

 hermaphroditism and its kindred subjects. 



In closing, I will call attention to the following points : 



1. The size and shape of the spermatozoa differ in the various 

 classes. In the Holothurioidea, Ophiuroidea, and Asteroidea, the 

 head is spherical; in the Crinoidea and Echinoidea it is conical. 



2. A cell membrane completely surrounds the spermatozoon. 

 The tail is in connection with this cell membrane, and not 

 attached directly to the nucleus or to the middle piece. 



