in SYNGAMY 71 



metamorphosed into the acrosome which forms the anterior tip of the 

 ripe spermatozoon ; it appears to act as a spear-head by which the 

 spermatozoon perforates and penetrates into the egg. 



In the spermatid shown in Fig. 30, A, a fine thread, is already growing 

 out from the distal centrosome (i.e. the centrosome farthest from the 

 nucleus). This is the beginning of the axial filament of the tail. Later, 

 the two centrosomes move down towards the nucleus, coming into contact 

 with one another at the same time, so that the base of the tail filament 

 is now continued past the distal into the proximal centrosome. This 

 comes into close contact with the surface of the nucleus and undergoes 

 certain metamorphoses not shown in these figures. The distal centrosome 

 parts company with the proximal one, grows into a ring surrounding 

 the tail filament, and travels down it for a certain distance. The portion 

 of the mature spermatozoon in which the centrosomes are lodged is called 

 the middle piece. 



The chondriosomes, which in the guinea-pig, as in other mammals, 

 are scattered through the spermatid instead of being collected into a 

 " Nebenkern," become concentrated round the tail filament between the 

 two centrosomes, and there form a granular sheath for the proximal 

 part of the filament, the granules being often specially dense along a 

 spiral band round the middle piece (Fig. 30, D). 



By far the greater part of the spermatid cytoplasm is not used up in 

 the process of forming the spermatozoon, and is cast off about the stage 

 shown in Fig. 30, C. 



A few representative types of other spermatozoa are shown in Fig. 31. 



B. SYNGAMY 



With certain exceptions which will be described later on in this chapter, 

 neither the male nor the female gamete is of itself in a position to start 

 development. Before this can occur, syngamy, or fusion of a male and 

 female gamete, must take place. The zygote thus formed is capable of 

 development into a new individual of the species, and, in the great majority 

 of cases, this development starts immediately after the zygote has been 

 constituted. Owing to the fact that the zygote differs superficially but 

 little from the ovum, except in its power of development, the process 

 of syngamy in the Metazoa is generally known as the fertilization of the 

 ovum. 



The details of the process of syngamy vary but little, the essential 

 features being the penetration of the motile spermatozoon or micro- 

 gamete into the immotile egg or macrogamete, and the fusion of the two 

 gametic nuclei to form the zygote, or cleavage, nucleus ; this proceeds to 

 mitosis under the influence of the centrosome introduced by the male 



