46 INSECTA. 



structure known as the rachis, which consists of a central axis 

 with a series of half circumscribed ova radiately arranged round 

 it. In the lowest part of the ovary the ova become completely 

 isolated and form separate cells. 



The protoplasm of the ova, which is clear in the terminal 

 division of the ovary, becomes in most forms filled lower down 

 with yolk-spherules secreted in the body of the ova. These 

 commence to appear at the uppermost extremity of the rachis. 



In some instances, e.g. Cucullanus elegans, yolk-spherules are not 

 formed. In the Oxyuridae the ova are directly segmented off from the 

 terminal syncytium of protoplasm without the intervention of a rachis ; 

 and are therefore formed in the same way as amongst Trematodes, etc. 



The origin of the membrane around the ova of the Nematoda has been 

 much disputed. 



At the time when the ovum is detached from the rachis no membrane 

 is present, but it nevertheless appears from Schneider's observations that the 

 region at which it is detached is softer than other parts, so that a kind of 

 micropyle is here formed which disappears after impregnation. A delicate 

 vitelline membrane then appears, around which there is subsequently 

 established an egg-shell, which is usually stated to be formed as a secretion 

 of the walls of the uterus ; but Schneider and Leuckart have given strong 

 grounds for believing that it is really a further differentiation of the vitel- 

 line membrane due to the activity of the protoplasm of the ovum. The 

 originally single membrane becomes as it thickens split into two layers. 

 The outer of these forms the true egg-shell, and the fertilization of the 

 ovum appears to be a necessary prelude to its production. Round the egg- 

 shell the walls of the uterus often secrete a special albuminous covering. 



The egg-shell exhibits in many cases peculiar sculpturings as well 

 as terminal prolongations. 



INSECTA. 



(49) A.Brandt. Ueber das Ei u. seine Bildungsstatte. Leipzig, 1878. 



(50) T. H. Huxley. " On the agamic reproduction and morphology of Aphis." 

 Linnean Trans., Vol. xxn. 1858. Fi'okalso Manual of Invertebrated Animals, 1877. 



(51) R. Leuckart. " Ueber die Micropyle u. den feinern Bau d. Schalenhaut 

 bei den Insecteneiern." Mailer's Archiv, 1855. 



(52) Fr. Ley dig. Der Eierstock u. die Samentasche d. Insecten. Dresden, 1866. 



(53) Lubbock. " The ova and pseudova of Insects." Phil. Trans. 1859. 



(54) Stein. Die -weiblichen Geschleehtsorgane d. Ktifer. Berlin, 1847. 



[Conf. also Glaus, Landois, Weismann, Ludwig (No. 4).] 



The ovum of Insects has formed the subject of numerous 

 investigations, and has played an important part in the con- 

 troversies on the nature of the ovum. 



