28 C. M. CHILD. 



6. Asexual and Sexual Reproduction in Relation to the Life Cycle. 



In general, where the same individual reproduces both asexually 

 and sexually, the asexual method of reproduction is characteristic 

 of the earlier, the sexual method of the later stages of the life 

 cycle. In other words, when the organism is still relatively 

 young, parts of the body which become physiologically isolated 

 in consequence of increase in size or decreasing metabolism in 

 dominant parts or other conditions (see p. 8) are capable of 

 reacting to the stimulus of isolation by dedifferentiation and 

 rejuvenescence, which is followed by a new developmental cycle. 



But as the organism grows older its parts become more highly 

 differentiated and undergo regulation less readily or not at all 

 when isolated. At this stage of the life cycle the only form of 

 reproduction which is still possible is reproduction by means of 

 gametes. These parts of the organism are more highly differ- 

 entiated than the asexual reproductive parts of earlier stages and, 

 except in cases of parthenogenesis, require fertilization. In short, 

 asexual reproduction is characteristic of the younger, less highly 

 differentiated organism and sexual reproduction of the older, 

 more highly differentiated. These facts constitute further strong 

 evidence in support of the view that the gametes are really highly 

 differentiated parts of the organism instead of undifferentiated 

 germ plasm. 



One of the most striking examples of the relation of asexual 

 and sexual reproduction to the life cycle is found in certain 

 medusae belonging to the family Margelidas. Reproduction in 

 these forms has been described by Chun ('95) and later by Braem 

 ('08). These medusae reproduce asexually during the younger 

 stages and sexually when older. Reproduction takes place by 

 means of bud-like outgrowths which arise in a more or less 

 definite sequence and arrangement on the manubrium, their 

 formation beginning near the base and proceeding toward the tip. 

 All the buds formed in the earlier stages of the life cycle are 

 asexual and give rise to new medusae. As the medusa grows 

 older, however, the outgrowths continue to arise on the manu- 

 brium in the same order as before and in their earlier stages 

 resemble the asexual buds of the younger animal, but they give 

 rise to gonads instead of producing new medusae asexually. 



