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small, in accordance with the force of the mother animal. This fact is again connected 

 with the phenomenon that the individuals, on coming out of the eggs, are very much 

 smaller than the parents and are in a stage of development far from that in which 

 the parents were. It is only through a series of larva stages that these young 

 ones, by means of food reception, slowly grow up to the mature stages. Often they 

 take about a year before these are reached (f. i. many Copepoda which are hatched 

 in the autumn months). In some plancton organisms we find care for the offspring, 

 the young ones living for a long time in the brood-room of the mother animal. 



Now as far as I know, among all Metazoa, the Rotifera are the only plancton 

 organisms which neither possess any metamorphosis nor show any care for the young. 



Deriving from fresh water turbellaria where a metamorphosis does not exist, 

 thej^ were originally found to live in a medium where larva stages are either totally 

 suppressed or at any rate only play a very inconspicuous rôle. As well known, 

 with regard to the pelagic region of the sea and the freshwaters, there is the great 

 difference that, whereas the firstnamed teems with larva stages, especially belonging 

 to benthonic animals, these are almost quite absent in the pelagic region of the 

 freshwaters, where the Benthos hardly ever produces pelagic larva stages and where, 

 of the plancton organisms, these as a rule occur only in the Copepoda. — This 

 lack of metamorphosis in the Rotifera must enter into our calculations, if we will 

 try to understand the main causes of the reduction of the male sex. 



In those cases where we possess a more thorough knowledge with regard to 

 the rate of development of the male eggs in the Rotifera, we know that this may 

 be finished in the course of only 24 — 36 hours. Only one or two days after the 

 egg has left the mother animal and has been attached to the posterior part of its 

 body, the cleavage process is finished, the egg shell bursts, and the male is a free- 

 swimming organism. We are further able to substantiate that the spermatozoa in 

 the testis are in full motion for several hours before the eggs are hatched. Finally 

 if we have some Polyaithra or Aniirœa females (male producers) in a drop, we will 

 see that in the very same hour in which they are hatched the pairing will take 

 place. The males so to speak rush from the eggshell directly upon the females, ready 

 to pair in the moment of birth. 



From the two now etablished facts, that the total amount of yolkmass of the 

 female is to be distributed over many individuals, and that this is just the case 

 with a group of animals where metamorphosis does not occur, I find some clues 

 to the explanation of the reduction of the male sex in the Rotifera. It is in the first 

 place dependent on the fact that the amount of yolk mass, which the egg is provided 

 with in the plancton Rotifers as well as in almost all other plancton organisms, is 

 extremely small. This caused the diminution in size of the male sex. The great 

 reduction of the common organisation is based upon the second phenomenon, that 

 the Rotifera belong to those organisms which are destitute of a metamorphosis, larva 

 stages like those of so many freshwater organisms being wholly unknown. After 

 birth the males get no time to build up their bodies by the reception of food beyond 



