spermatozoa must of necessity be noticed, in speaking of the mode in which the fertilization of the ovum is effected. It will suffice, therefore, if we now examine briefly the structure and mode of formation of these bodies, as far as microscopical examination has yet gone.
The sperm-cell appears to undergo a process of development closely analogous to that of the germ-cell, which we shall have presently to trace; and since no material variation is observable in the for-ination of the spermatozoa in the different classes of vertebrates, the description of any one may serve for that of the rest. Hitherto, in considering this subject, the term 'sperm-cell' has been employed as a general expression, and as the equivalent of spermatozoon; but in speaking now of the development of these bodies, a greater strictness inust be observed. Taking the higher vertebrata as a type, the testes are found to contain in considerable quantity free cells, the so-called parent cells. Within each of these again are found several smaller cells, the daughter cells or vesicles of development-each daughter cell containing a separate spermatozoon. These, by their growth, burst through the delicate cell wall that confines
