THE OVUM. 37 



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 Oxyuridse 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 Trematodey, 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 front 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 <nven stronif 



O O 



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. Ui'ber das Ei u. seine Bildungsstatte. Leipzig, 1878. 



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

 Lin-neon Trans., Vol. xxn. 1858. Vide also Manual of Invertebrated Animals, 1877. 



(5t) R. Leuckart. "Ueber die Micropyle it. den feineru Bau d. Schalenhaut bei 

 den Insecteneiern." Miiller's Archiv, 1855. 



(52) Fr. Leydig. Der Eierstock it. die Samnttasche d. Tnsecten. Dresden, 1866. 



(53) Lubbock. "The ova and pseiidova of Insects." Phil. Trans. 185V., 



(54) Stein. Die weiblichen Gesch'echtsorgane d. Kafer. Berlin, 18i7. 



[Couf. also Glaus, Landois. Weismanu, Ludwig (No. 4).] 



The ovum of Insects has formed the subject of numerous 

 investigations, and has played an important part in the controversies 

 on the nature of the ovum. 



The ovaries are paired organs, rarely directly connected, each 

 consisting of more or fewer ovarian tubes which open into a common 

 oviduct. The oviducts unite into a vagina, usually provided with 

 a spermatheca and accessory glands, which need not be further 

 alluded to. Each ovary is invested by a peritoneal covering, which 

 assumes various characters, and either forms a loose network covering 

 the whole or a special tunic round each egg-tube. It is continuous 

 with the general peritoneal investment. Each ovarian tube (fig. 17) 

 consists of three sections: (1) a terminal thread, (2) the terminal 

 chamber or germogen, (3) the egg- tube proper. 



The whole egg-tube is invested in a structureless tunica propria. 



