382 THE ANATOMY OF INVERTEBRATED ANIMALS. 



into as many elongated solid cellular bodies as ovarioles are 

 to be formed. The peripheral cells become the peritoneal 

 layer. Each cellular rudiment surrounds itself with a struct- 

 ureless membrane, and then elongates into an ovariole, some 

 of the cells filling the posterior end of which then becomes 

 differentiated into the first primary ovum and its capsule, with 

 or without vitelligenous cells. The contents of each ovariole 

 must therefore be regarded as a column of generative cells, 

 which instead of burrowing in the stroma of an ovary, and 

 becoming divided into ovisacs, as in a vertebrated animal, 

 grows straight backward, and, as it grows, becomes divided 

 into ovisacs, of which the oldest and most advanced is the 

 hindermost. 



Nothing is certainly known respecting the origin of the 

 vagina or the oviducts, though it may be suspected that the 

 posterior prolongations of the primary ovaries give rise to the 

 latter. 



The development of the testes takes place in the same 

 manner as that of the ovaries, but the contents of the testic- 

 ular tubes become converted into spermatozoa. The origin 

 of the vasa deferentia is unknown. 1 



In most insects, the vitellus undergoes partial yelk-divis- 

 ion. In some Poduridm, however, complete division has been 

 observed. The development of the blastoderm takes place in 

 the same way as in other Arthropods, and the cephalic end of 

 the embryo terminates in two procephalic lobes. In many 

 insects, the periphery of the blastoderm, external to the lon- 

 gitudinal thickening which gives rise to the sternal region of 

 the body, and which may be termed the sternal band (" Keim- 

 streif " of the German embryologists), gives off a lamina 

 which grows inward over the sternal face of the embryo, 

 and eventually forms a complete investment thereto. The 

 lamina may be formed by a single layer of cells, or it may, 



1 The account given above of the structure of the ovarian tubes in Blatta 

 and Aphis is based on ray own observations, "which are in pretty close accord- 

 ance with those of A. Brandt, " Ueber die EirOhren der Blatta [Periplaneta) 

 orientalis " (" Mem. de 1' Acad. St.-Petersbourg," tome xxi., 1874). The liter- 

 ature of the subject is somewhat extensive. See especially Leydig, " Der 

 Eierstock unci die Samentasehe der Insecten " ("Nova Acta," xxxiii., 1867); 

 Lubbock, l \ The Ova and Pseudova of Insects " (" Phil. Trans.," 1858) ; Weis- 

 mann, "Die nachembryonale Entwickelung der Musciden" (ZeitscJirift fur 

 iviss. Zoologie, xiv.) ; Bessels, "Entwickelung der Sexualdriisen bei den 

 Lepidopteren " (Zeitschrift fur wiss. Zoologie, 1857); and Von Siebold, 

 " Beitrage zur Parthenogenesis der Arthropoden," 1871. The various forms 

 of the micropyle and the structure of the chorion are dealt with by Leuckart, 

 in his elaborate memoir, " Ueber die Micropyle und den feineren Bau der 

 Schalenhaut bei den Insekteneiern " (" Muller's Archiv," 1855). 



