As might be anticipated from the mode in which alternations of generations have become established, incomplete approximations to it are not uncommon. Such approximations are especially found in the Arthropoda, where alternations of sexual and parthenogenetic generations frequently take place, in which the individuals of different generations are similarly organised (Psychidæ, Apus, &c.). Another approximation is afforded by the parthenogenetic winter eggs of Leptodora amongst the Phyllopods, which give rise to Nauplius larvæ, while the young hatched from the summer eggs do not pass through a metamorphosis. Numerous transitional cases are also found amongst the forms in which there is an alternation of sexual and gemmiparous generations. The whole of the cases to which allusion has been made in this section may be conveniently classed under the term alternations of generations, but the cases of alternation of two sexual generations, and of sexual and parthenogenetic generations, are classified by Leuckart, Claus, etc. as cases of heterogeny, which they oppose to the other form of alternation of generations. If special terms are to be adopted for the two kinds of alternation of generations, it would be perhaps convenient to classify the cases of alternations of sexual and gemmiparous generations under the term metagenesis, and to employ the term heterogamy for the cases of alternation of sexual and parthenogenetic generations. The term Nurse (_German_ Amme), employed for the asexual generations in metagenesis, may advantageously be dropped altogether. CHAPTER I. THE OVUM AND SPERMATOZOON. THE OVUM. The complete developmental history of any being constitutes a cycle. It is therefore permissible in treating of this history to begin at any point. As a matter of convenience the ovum appears to be the most suitable point of departure. The question as to the germinal layer from which it is ultimately derived is dealt with in a subsequent part of the work; the present chapter deals with its origin and growth. _General History of the Ovum._ Every young ovum (fig. 1) has the character of a simple cell. It is formed of a mass of naked protoplasm (_a_), containing in its interior a nucleus (_b_), within which there is a nucleolus (_c_). The nucleus and nucleolus are usually known as the germinal vesicle and germinal spot. [FIG. 1. DIAGRAM OF THE OVUM. (From Gegenbaur.) _a._ Granular protoplasm. _b._ Nucleus (germinal vesicle). _c._ Nucleolus (germinal spot).]