3O6 D. D. WHITNEY. 



to a mature spermatozoon were easily seen, especially in the 

 species of rotifers whose males form only a few spermatozoa. 



Brachionus mulleri, the salt-water rotifer, was the first species 

 examined and because of the relatively small number of sperma- 

 tozoa produced by the males it proved to be the most favorable 

 form in which to see the early stages in the development of the 

 spermatozoa as well as for making accurate counts. The motile 

 spermatozoa were extraordinarily large and measured about one 

 third the total length of the male individual that produced them. 

 Probably on account of the large size of the spermatozoa a male 

 is physically unable to form very many. These motile sperma- 

 tozoa were long and somewhat slender with a pointed anterior 

 end. The posterior two-thirds of the spermatozoon is very 

 motile and has a prominent, undulating, median, and dorsal 

 membrane which aids greatly in the locomotion. The sketch 

 of a mature spermatozoon in Fig. i, E, is a dorsal view and 

 does not show adequately this membrane. These spermatozoa 

 swim freely about in the water for a short time after they are 

 forced out from the body of the male. 



The spindle-shaped spermatozoa are incapable of any loco- 

 motion. They are uniform in shape and never change their 

 form. They are very much smaller than the motile spermatozoa. 

 Fig. 2, D, shows several of these. As the tissues of the males 

 are very transparent both kinds of these spermatozoa may be 

 readily seen inside the testis. The spindle-shaped spermatozoa 

 are usually found collected together near the posterior end of 

 the testis, although sometimes only a few are thus collected 

 together while the remainder are scattered throughout the 

 testis among the motile spermatozoa. 



When the immature males are crushed the spermatids still 

 united in masses separate readily from the general debris. Vari- 

 ous stages in the development of the spermatids and spermatozoa 

 may be seen in accordance with the stage of development of the 

 male. All spermatids or immature spermatozoa in each male 

 are generally found in the same stage of development. 



The relative number of the two kinds of spermatozoa varied 

 widely when counts were made from the mature free swimming 

 males taken from a general culture of rotifers. It is very prob- 



