416 EDGAR J. ALLEN. 



the adult, as has already been shown, and lies upon the front 

 end of the ovary (fig. 10). 



The Posterior Region op the Thorax. — The condition of 

 the body-cavity in this region may be gathered from figs. 18 

 and 19. The central and lateral cavities (be, b. lat.) are 

 similar to those in the anterior region, whilst dorsal to these 

 the pericardial chamber lies (per.). This chamber is sepa- 

 rated from the central body-cavity, as is already well known, 

 by the pericardial septum {p. sep.), and it contains the heart. 

 In the larva the pericardial cavity appears to be bounded dor- 

 sally by the ectoderm, but I consider it probable that a layer 

 of mesoderm lies within the ectoderm, although I have been 

 unable to detect it. Such a layer is undoubtedly present in 

 the adult, although it is even then very thin, and has extremely 

 small nuclei. 



At the front end of the pericardial cavity, immediately 

 beneath the pericardial septum, the genital organs are situ- 

 ated (fig. 18, gen.). In the just hatched larva and in late 

 embryos these consist of two masses of cells with large nuclei, 

 each mass being enclosed in a sheath of mesoderm. At this 

 stage the presence of the latter, on account of the extreme 

 tenuity of the layer, can only be ascertained by the presence of 

 occasional spindle-shaped nuclei which lie outside the larger 

 cells, but in later stages this mesodermal investment becomes 

 much more marked. Bobretzky (see No. 14, p. 379) has 

 already stated that the genital organs originate in the region 

 in which they are found in the larva. 



I have not been able to find any trace of the genital ducts 

 in young larvse, and can give no information as to how these 

 organs originate. 



The Abdomen. — With regard to the abdomen I have no 

 new facts to add, but my sections confirm the accounts given 

 by Milne Edwards (No. 9) and Claus (No. 4). There are two 

 principal sinuses which run along its whole length — a dorsal 

 sinus in which the intestine lies, and a ventral one which con- 



