ZOOLOGY A.ND BOTANY, MICROSCOPY, ETC. 871 



Dimorphism of the Spermatozoa in Paludina.* — It has been 

 known for some time that the spermatozoa of this mollusc are of 

 two kinds ; the fact was originally discovered by von Siebold, and 

 commented upon by subsequent writers. The subject has been 

 recently studied by Mas von Brunn, who contributes an elaborate 

 paper upon the structure, function, and development of the two 

 kinds of spermatozoa. 



1. Structure. — The two forms are at once distinguishable by their 

 size ; the " hair-like " spermatozoa are 88 fx in length, while the 

 " worm-like " forms are from 180-190 /a in length; the latter also 

 are considerably thicker ; the hair-like spermatozoa consist of a 

 slender body narrowing at the " tail " into a delicate thread, and a 

 head twisted in a corkscrew fashion for six turns ; the " worm-like " 

 spermatozoon is nearly uniformly cylindrical, and terminates in a 

 bunch of fine cilia ; the whole spermatozoon is traversed by an axial 

 thread which commences at the base of the slightly thickened head, 

 and terminates in the posterior bundle of cilia ; a more minute ex- 

 amination shows that the central thread is in reality composed of a 

 bundle of fine fibres, each one of which corresponds to a terminal 

 cilium ; the whole is enveloped by a protoplasmic sheath which forms 

 the rest of the spermatozoon. 



2. Development. — At first the testis cells which are to produce the 

 two kinds of spermatozoa are indistinguishable, but later two kinds 

 of cells are recognizable ; one set divide again, and become the 

 proper seminal cells which are to produce the " hair-like " spermatozoa ; 

 the latter are large cells which become directly modiiied into the 

 " worm-like " spermatozoa. The nuclei of the cells that are to 

 produce the latter break up into a number of small round bodies 

 which eventually disappear, with the exception of a single one of 

 conspicuous size which remains ; at the same time a bundle of fine 

 threads springs from the surface of the cell, close to which is the 

 remnant of the nucleus ; the bundles of threads indeed appear to take 

 their origin from it, but this is not absolutely certain ; this bunch 

 of threads is undoubtedly the bunch of cilia already mentioned as 

 attached to the " tail " of the spermatozoon ; the nucleus to which 

 they are attached becomes the head of this spermatozoon ; the cell 

 gradually elongates and forms the body of the spermatozoon. The 

 formation of the " hair-like " spermatozoa is as follows : — In the ripe 

 seminal cells the nucleus assumes the well-known spindle form and 

 divides, division of tho cell accompanying nuclear division ; the 

 first recognizable sign of the metamorphosis into spermatozoa is that 

 tho nucleus becomes homogeneous, and shows no nucleolus ; in the 

 next stage a fine thread is seen projecting from the cell, and close to 

 the point where it is connected with the latter are several highly 

 refracting bodies ; tho nucleus sends out a prolongation towards this 

 thread, which includes the small round bodies, and which eventually 

 becomes the middle portion of the developed spermatozoon, while the 

 nucleus itself becomes its head. 



• Arch. f. Mikr. Anat., xxiii, (1884) i-p. 413-99 (2 pis.)- 



