No. 504] 



NOTES AND LITERATURE 



813 



ferentiation into egg and sperm not now externally visible. 

 Further development may proceed along different paths. Dif- 

 ferentiation may arise between individuals, + and — gametes 

 being then localized in separate members, and the gametes of 

 these same plants may be mutually incapable of union so that 

 only gametes of different plants can copulate. These are here 

 + and — individuals, but the gametes may give no external 

 evidence of a difference. This is the case in Dasycladus accord- 

 ing to Berthold and in Ulothrix according to Dodel. Or there 

 may arise differences between the gametes, the one becoming, by 

 the surrender of motility and the assumption of nutritive func- 

 tions, an egg cell; the other, remaining small and retaining 

 motility, becoming a spermatozoon. Thus hidden differences 

 between -f- and — germ-cells become conspicuous and there 

 appear ' 'female" gametes and ''male" gametes. When the 

 specialized gametes are combined in the same plant it becomes 

 hermaphrodite or monoecious; when separted, dioecious forms 

 appear. 



Correns lays emphasis on several points: (1) That the differ- 

 entiation of gametes into egg and sperm has nothing to do with 

 the union of the two germ-cells for the production of a new 

 individual ; or, in other words, that the externally visible differ- 

 ences between egg and sperm need have no connection with the 

 process of fertilization; (2) individuals may be differentiated 

 into males and females without an evident external mark of 

 differentiation either in the individuals themselves or in their 

 germ-cells. The various differential characters of eggs and 

 sperm-cells and all other visible differences between male and 

 female individuals reveal only their different nature, but do not 

 touch the essence of sex itself. Interesting confirmatory obser- 

 vations in this connection are those of Blakeslee on Mucorinea?. 



Originally there is present only the "determination" in the 

 germ-cells which renders possible the union of some pairs of 

 germ-cells and prevents the union of other pairs. All else is 

 of a secondary character. With few exceptions the higher ani- 

 mals are unisexual. Among the higher plants many different 

 sexual types occur. Neglecting transition conditions, the main 

 types are hermaphrodite forms where stamens and pistils are 

 united in the same flower; bisexual or monoecious forms, where 

 pistils and stamens are separated into female and male flowers ; 

 unisexual or dioecious forms, where the male and female flowers 



