I lO 



THE EVOLUTION OF SEX. 



hybrids ; Von Siebold demonstrated their presence in many of the lower 

 animals ; and lastly, in 184.1, KoUiker made one of his many important 

 contributions to biology, in proving that the sperms had a cellular origin 

 in the testes. 



§ 3. Structure of the Sperm. — The sperm, then, is a cell. 

 Though some, such as Kolliker, have inclined to regard it 

 rather as a nucleus, its truly cellular character may be regarded 

 as proven beyond dispute. We have, as in the ovum, to deal 

 with cell-substance and nucleus, with this marked difference, 

 that the cell- substance is generally reduced to a minimum. 



" Spermatic Animalculi " of the Rabbit and the Dog. 

 — From Buffon, after Leeuwenhoek. 



The sperm is almost always, moreover, a cell of a very 

 definite type or phase. It is like one of the highly motile 

 Protozoa, like a flagellate infusorian. Usually it consists of a 

 minute " head," consisting almost entirely of nucleus, and of a 

 long contractile tail, which, working behind like a screw, propels 

 the essential " head" through the water or along the ducts. Oc- 

 casionally, as the diagram shows, there is a departure from the 

 predominant phase of cell-life. Thus in the threadworm 

 Ascaris, the sperm has a blunt pear-shaped form, and exhibits 

 slight amoeboid movements. In some crustaceans and other 

 arthropods, the cell is even more quiescent, and may exhibit 

 curious forms such as that figured for the crayfish. The 

 relatively dormant activity may however wake up, and the 

 sperm exhibit active amoeboid movements. Zacharias has 

 made some interesting experiments, showing the modifiability 

 of sperms under reagents ; thus, in a little crustacean (Foly- 

 p/ie/nus pediculus), he first caused the cylindrical sperm to form 

 amoeboid processes, and afterwards to replace these by what 

 were to all intents and purposes cilia. This is entirely con- 

 gruent with other experiments and observations on the passage 

 of cells from one phase of the cell-cycle to another. 



The progress of microscopic technique has demonstrated many com- 

 plexities in the sperm as well as in the ovum. For a discussion of some of 



