Mesoderm— Reproductive Organs 99 



growing fat-body, by whose splitting up the body- 

 cavity of the adult insect is formed (fig. 62 c,f). 



Blood-system. — The heart, blood-spaces, and fat- 

 body arise by modification of the cells of the mesoderm. 

 The rudiments of the heart are rows of cells which lie 

 within either edge of the primitive streak (fig. 62 h). 

 As the yolk is enclosed, these rudiments take up a 

 more dorsal position until they join along the upper 

 middle line forming the heart and pericardium (fig. 

 62) (53). 



Reproductive Organs. — The development of the 

 ovaries has been carefully traced in the Cockroach 

 (63). At a very early stage, before segmentation be- 

 gins, the primitive germ-cells can be recognised; they 

 arise at the hinder end of the mesoderm, from the 

 ordinary cells of which they may be distinguished by 

 their larger size. As the embryo segments, and the 

 coelomic pouches appear, further germ-cells are 

 developed from their walls, and take up a dorsal 

 position in the coelomic cavities, where they become 

 surrounded with other mesodermal cells, which arrange 

 themselves to form the oviduct and ovarian tubes and 

 threads. Throughout the development the germ-cells, 

 which are to become eggs, are quite distinct from the 

 cells which give rise to the tissues of the ovary 

 (fig. 62). The organs of the male insect are developed 

 in a similar way, the germ-cells which produce the 

 sperm-cells being all along distinct from the cells 

 which give rise to the tissues of the testes and their 

 ducts. A point of great interest is the separation, in 

 the male cockroach, from the developing testes, of a 

 remnant which represents the female part of an 

 originally bisexual state, since it occasionally gives 

 rise to a rudimentary ovary with eggs (63 b). 



The outer genital passages — vagina of the female 

 and ejaculatory duct of the male — are formed, like 



