INTRODUCTION 17 



them will be given in the discussion of the groups in which each 

 type belongs. It should be mentioned, however, that these 

 larval stages are frequently considered as having great phylo- 

 genetic significance. Groups having fundamentally similar 

 larvae are usually considered as having developed from a common 

 ancestral form, for there are many evidences supporting the law 

 of biogenesis which states that ontogeny is a brief recapitulation 

 of phylogeny. 



Parthenogenesis is sexual development without fertilization. 

 The eggs of many kinds of animals undergo a modified type of 

 maturation and are then capable of development without fertili- 

 zation. It is not uncommon for parthenogenesis and true sexual 

 reproduction to alternate in the life cycle of the same species. 

 The parthenogenetic habit has become thoroughly established in 

 some species of animals. Frequently, in such instances, males are 

 extremely rare and there are some species which reproduce 

 parthenogenetically in which males have never been observed. 



Paedogenesis and Polyembryony. — The gonads of some indi- 

 viduals become functional before the body reaches adult form. 

 In some of the dipterous insects (flies of the genus Miastor, for 

 example), the larvae become precociously mature and as maggots 

 produce mature eggs. These eggs undergo parthenogenetic 

 development within the body of the larva and a new genera- 

 tion of larvae is produced within the body of each. This 

 type of precocious parthenogenetic development is termed 

 paedogenesis. 



It has been demonstrated that in some instances a single 

 fertilized egg may give rise to more than one individual. This 

 condition, which has been termed polyembryony, occurs in both 

 invertebrates and vertebrates and is especially characteristic of 

 some insects. This power of development of an entire individual 

 from a portion of an embryo calls to mind the fact that experi- 

 mentally the blastomeres of many of the marine invertebrates 

 may be isolated and each blastomere thus separated forms a 

 complete individual. 



Breeding Habits. — Regarding breeding habits, numerous 

 different conditions exist. In the Protozoa and among many 

 lower Metazoa, isolated gametes are set free into the surrounding 

 medium and fertilization occurs entirely apart from the bodies 

 of the parent individuals. Motility, at least of the male gametes, 

 and chemical emanations from the female gamete bring the two 



