104 



THE EVOLUTION OF SEX. 



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. 



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," consist- 

 ing 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. Occasionally, as the diagram shows, 

 there is a departure from the predominant phase of cell-life. Thus in 

 the threadworm Ascan's, 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 



Fig. 27. — "Spermatic Animalculi" of the Rabbit ami the Dog. 

 — From Buffon, after Leeuwenhoek. 



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 {Polyphemus 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 congruent 

 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 complexities 

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

 important of these, the reader is referred to the Encyclopedia Britannica, 

 article "Reproduction." A few points only need be noticed here. Thus most 

 spermatozoa exhibit not only a head (almost wholly from the nucleus of the 

 mother-cell), and a mobile tail (from the substance of the mother-cell), but a 

 median portion connecting these. The tail is not unfrequently, as in salamander 

 and man, furnished with a very delicate undulating or vibratile band. Com- 

 plexities such as axial filaments, striations, and the like abound. In a few 

 cases, as in the threadworm, the sperm is not left without any nutritive capital, 

 but furnished with this in the form of a cap, which falls off before the essential 



