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one of the cells is larger and more transparent ; the two others are very 

 small and closely applied one to either side of the large cell. At the 

 outer edge of the organ, where it projects into the »uterus«, the three- 

 celled clusters may be seen breaking away and migrating out into the 

 conduits leading to the smaller coecal ramifications of the »ovary«. 

 They ultimately attach themselves to the epithelial walls of the coeca 

 and begin to grow. Soon the two accessory cells fuse with the middle 

 cell, the nucleus of which becomes the germinal vesicle of the ovum. 

 The nuclei of the accessory cells retain their identity for a conside- 

 rable time, gradually enlarging but never attaining to the size of 

 the germinal vesicle. In quite large eggs their faint boundaries may 

 still be traced , one at either pole of the irregularly ellipsoidal ovum. 

 Finally they fade away and become indistinguishable from the general 

 egg-cytoplasm which is just beginning to acquire yolk-granules. The 

 curious and constant arrangement of the cells in groups of three en- 

 ables one to trace their development step by step from their origin in 

 Nansen's organ to their attachment and growth in the coeca of the 

 so called »ovary«. The resemblance of this course of development to 

 the conditions in Chaetopods where the young ova fall into and ma- 

 ture in the body cavity, is at once apparent and constitutes an additio- 

 nal indication that the Myzostomida are degenerate Annelids. It also 

 follows that we are to interpret the so called »ovary« as the true body- 

 cavity which is really well developed in Myzostomes and not a mere 

 rudiment as most authors have assumed. 



The discovery of the true ovaries leads to still further interesting 

 conclusions. Nansen found the »problematical organs« in all the 

 »complemental males« of the species that he studied. I have also seen 

 them in the »complemental males« oî M. glabriwi and M. alatum, so 

 that these small individuals are not really males but hermaphrodites. 

 In M. glahrum I have studied sections of specimens of different sizes 

 wich the following results : 



1) The youngest M. glahrum were those found attached to the an- 

 terior dorsal edge of the medium- sized and large individuals. They 

 have only the male organs in a high degree of development , the sper- 

 matozoa being found in all stages of formation and fully mature. In 

 the true ovaries the cells are not found proliferating, nor do the young 

 ova migrate as yet through the body cavity , which exists only in the 

 form of a small uterus without ramifications. It was these very young 

 Myzostomes which Beard called »complemental males« and to which 

 he emphatically denied any traces of female reproductive organs. 



2) The youngest specimens found attached to the disc of the A7i~ 

 tedon are but slightly larger than those attached to the backs of the 



