Od the Life-History and Development of the Genus Myzostoma. 551 



Development. 



The egg. 



The ovaiy of Myzostoma is not a well defined organ. The ova are 

 seattered and lie in and among the connective tissiie. The development 

 of the ovaries is not known, a gap in our knowledge which I hope later 

 to fili up , but since the ovary fills up all the body spaee , not oceupied 

 by the testes and nervous system , between the alimentary canal and 

 the skin, it can be regarded as derived from the epithelium of the body- 

 cavity , which by the great development of the sexual organs has be- 

 come greatly modified so that it has practically as a body-cavity dis- 

 appeared , only the parts concerned in the sexual function remaining. 

 Perhaps rather we may say the whole of the epithelium of the body- 

 cavity has acquired the function of producing sexual cells. 



In a transverse section of a mature hermaphrodite one meets ova 

 in all stages of development lying in nests of connective tissue (PI. 2 

 fig. 36) . The youngest ova are small , possess only a dark staining 

 nucleus, and no germinai spot, and are hardly distinguishable from 

 connective tissue cells. 



Soon a germinai spot is formed by the collection of the chromatic 

 substance of the nucleus, and the growth of the achromatic substance. 

 Thus even in young ova of 0,008mm in diameter we find a darkly 

 staining germinai spot (fig. 36) enclosed in a germinai vesicle of fairly 

 large size. The germinai vesicle has a distinct membrane and clear 

 non-staining contents. The protoplasm of the t^^ is somewhat granular. 

 In the course of its development yolk is formed in the q^^ , perhaps in 

 the way described by Will ^ in Amphibia and Insects , for I bave 

 noticed and figured small bodies in the germinai vesicle and in the 

 protoplasm of the egg (fig. 39 k. /, jj) which seem to proceed from the 

 germinai spot. These bodies appear to me to be exactly analogous to, 

 and to have the same characters as those in Amphibia and Insects. My 

 friend's complete paper is not yet published, so I will only draw atten- 

 tion to the fact. In the ripe egg (fig. 1) one sees usually a dark- 

 coloured mass in the protoplasm , which I identify as the so-called 

 yolk-nucleus of other animals. It is situated at the vegetative pole of 

 the egg , and has usually not very well defined outlines ; but in some 

 cases of hardened and stained eggs it seems to have a curved sharp 



1 Will, Über die Entstehung des Dotters und der Epithelzellen bei den Am- 

 phibien und Insecten. Zool. Anzeiger. 1884. No. 167. 168. 



