330 142 



the stage in which they are born. During the embryonic development stress is only 

 laid upon the formation of the male organs, the material given lo the egg not allow- 

 ing of a more elaborate development of the organism. 



We have now I presume got some material to elucidate some of the facts 

 which produced the strongly marked reduction of the male sex, most pronounced in 

 the plancton Rotifers. But of the deeper Ij'ing causes why the reduction of the male 

 sex especially in the plancton Rotifers was really a conditio sine qua non for these 

 organisms from the moment, when they were to pass from creeping bottom organ- 

 isms into freeswimming organisms, take possession of the open waters, and eman- 

 cipate themselves from any substratum therein, we have hitherto had no explana- 

 tion at all. — The strongly marked reduction of the male sex is really of the greatest 

 advantage for the species. This will be understood from the following considerations 

 and observations. 



When the quantity of yolk mass which is at the disposal of the female, and 

 which is to be used for the preparation of the males, is distributed over as large 

 a number of individuals as possible, it is obvious that it is possible for the males, 

 in comparison with the females, to come in an enormous majority. Now it is a 

 well-known fact that the male producing females in a lake almost all come up sim- 

 ultaneoush^ viz. in the last part of a maximum, further, that the enormous amount 

 of male eggs all develop almost simultaneously, viz. in the course of about eight days. 

 This again in other words means that the total amount of material of impregnation 

 in these very eight days is distributed over an enormous quantity of individuals. 

 The possibility of pairing between the two sexes is augmented in a very high degree, 

 that phenomenon upon wdiich depends in the first line the formation of resting 

 eggs, which again in many cases is a conditio sine qua non for keeping the locality 

 for the species. 



As the smallest males of the plancton Rotifers cannot be caught with our 

 plancton nets, we have not the slightest idea of the incredible numbers of males 

 developed in the water layers, during the sexual period of a species. But if in this 

 period we catch large amounts of plancton, as far as possible strain away the lar- 

 ger plancton organisms f. i. Daphnids and Copepoda, and let the rest stand over 

 for a night in great jars, the next morning at the surface rim, turning towards the 

 light source, we shall find a milky border, consisting almost entirely of males. This 

 phenomenon I have observed often enough -with regard to Polyarfhra platyptera, 

 Pompholyx sulcata the Anurœa and Brachioniis species. — But it can only be observed 

 in the short sexual periods, never outside these; the occurrence of the sexual period 

 in the plancton Rotifers may be compared with phenomena that are well known 

 in the vegetable kingdom f. i. the period, when the conifera throw their pollen, 

 and the flowering season of the rye. 



In all cases it is the material for impregnation which, in a few days and hours, 

 fills the surrounding medium; the result of the process is in the one case the seed, 

 in the other, the resting-egg. In the autumn the resting eggs come to the surface; 



