DEVELOPMENT OF NEPHRIDIA OF PHERETIMA S3 



secondary nephridia formed from the stray nephridial cells 

 lying beneath the peritoneal membrane of the body- wall. 

 But, as in fig. 17, we have to account for a string of cells; 

 which is not always seen, running from the nephridial rudiment 

 to the ectoderm. It may be that it is a secondary formation 

 leading from the nephridial rudiment to form the terminal 

 duct. 



8. Development of the Pharyngeal Nephridia and 



THEIR Ducts. 



The development of the pharyngeal nephridia of the fourth, 

 fifth, and sixth segments can be followed in all its stages 

 in the same embryos which show the development of the integu- 

 mentary and septal nephridia. The pharyngeal nephridia 

 appear at the same time as the primary integumentary 

 nephridia, but are rather slower in growth than the latter. 

 At a stage of development when the embryo is 5 to 6 mm. 

 long and the integumentary nephridia of some of the segments 

 behind the first six (e. g. seventh, eighth, and ninth) are almost 

 fully formed and have developed their intra-cellular canals, 

 the pharyngeal nephridia are seen as deeply-staining compact 

 masses of cells lying on the body-wall, a pair in each segment, 

 one on each side of the nerve-cord. They develop from the 

 same source as the primary integumentary nephridia, i. e. from 

 the nephridial cells belonging to the original ectodermal 

 nephric row ; but their manner of development is different 

 from the other two types. While in the case of the integu- 

 mentary nephridia the terminal duct is very short and appears 

 rather late in development, forming a lumen at the same 

 time with the rest of the nephridium, the ducts of the pharyn- 

 geal nephridia develop very early. In an embryo 5 mm. in 

 length the nephridia of the fourth, fifth, and sixth segments 

 are small club-shaped solid masses produced into long solid 

 strings of cells leading anteriorly to the lateral walls of the 

 pharynx (fig. 18). The terminal ducts are thus formed earlier 

 than the bodies of the nephridia themselves. This is still 

 more marked at a later stage in an older embryo in which the 



