THE ONTOGENESIS OF VERTEBRATES 49 



Among multicellular organisms, the gametes are produced 

 in definite organs, the gonads, or germ-glands, which pro- 

 duce but a single sort of gamete, either macro-gametes (ova) 

 or micro-gametes (spermatozoa). Those glands are termed 

 respectively ovaries and testes, and may occur in the same or 

 in different individuals. In the latter case the individuals 

 are said to be of separate sexes and are termed male and fe- 

 male, the former secreting the micro-, the latter the macro- 

 gametes. Individuals possessing both sorts of gonads are 

 termed hermaphroditic or bisexual, but owing to the fact 

 that usually the two sorts of organs are functionally active 

 at different times, the organisms are seldom functionally bi- 

 sexual, but alternately male and female. Such hermaphroditic 

 forms are frequent among invertebrates, and occur regularly 

 in certain classes, but in vertebrates they are found only 

 among the Cyclostomes (Myxinoids-), although in all cases the 

 curious homology between the parts in the two sexes (Cf. 

 Chap. VIII) suggests that the phenomenon may have been 

 widespread or even universal among the ancestors of modern 

 vertebrates. 



In most aquatic animals the gametes are liberated in the 

 water and conjugation takes place without any act on the 

 part of the parents, through the motor action of the micro- 

 gametes themselves, exactly as in Protozoa; in terrestrial 

 forms, however, since the gametes need a liquid medium, this 

 latter is supplied by glands, and the seminal fluid of the male, 

 in which the micro-gametes swim actively, is conveyed to the 

 female by some form of copulation. 



Since the superficial phenomena are so obvious that they are 

 universally recognized without technical study, while the es- 

 sential details require for their detection the care and patience 

 of an experienced microscopist, and since especially the 

 parallel phenomena occurring among the Protozoa have re- 

 mained unknown until within comparatively recent years, it 

 may be easily comprehended that the terms in common use 

 relative to these phenomena fail to express the underlying bio- 

 logical principles and are not of universal applicability. Thus 



