CONJUGATION 163 



the centrifugal mode of growth the mycelium is always 

 thicker at the centre than towards the circumference, since 

 it is the older or more central portions of the hyphae which 

 have had most time to branch and become interlaced with 

 one another. 



Under certain circumstances a peculiar process of con- 

 jugation occurs in Mucor. Two adjacent hyphae send out 

 short branches (Fig. 37, i 1 ) which come into contact with 

 one another by their free somewhat swollen ends (i 2 ). In 

 each a septum appears so as to shut off a separate terminal 

 cell (i 3 , gam) from the rest of the hypha. The opposed 

 walls of the two cells then become absorbed (i 4 ) and their 

 contents mingle, forming a single mass of protoplasm 

 (i 5 , zyg), the cell-wall of which becomes greatly thickened 

 and divided into two layers, an inner delicate and trans- 

 parent, and an outer dark in colour, of considerable thick- 

 ness, and frequently ornamented with spines. 



Obviously the swollen terminal cells (gam) of the short 

 lateral hyphae are gametes or conjugating bodies, and the 

 large spore-like structure (zyg) resulting from their union is 

 a zygote. The striking feature of the process is that the 

 gametes are non-motile, save in so far as their growth 

 towards one another is a mode of motion. In Heteromita 

 both gametes are active and free-swimming (p. 41) : in 

 Vorticella one is free-swimming, the other fixed but still 

 capable of active movement (p. 130) ; here both conjugating 

 bodies only exhibit the slow movement in one direction due 

 to growth. 



There are equally important differences in the result of 

 the process in the three cases. In Heteromita the proto- 

 plasm of the zygote breaks up almost immediately into 

 spores ; in Vorticella the zygote is active and the result of 



M 2 



