MORPHOLOGY OF CYCL0ST03IATA. 419 



to the chacal part, and as having been, in the anterior rer/ion, 

 replaced by gilh and, in the posterior, converted into the .segmental 

 duct. 



Ill the two anterior segments which l^elong to the hranchial 

 region, the free ends of the tubules are brought into close contact 

 with theepiblast but this germinal layer has, in this region, no 

 share in the formation of the system. The hind extremity of the 

 segmental duct, however, strikes against the epiblast and has every 

 appearance of receiving some cells out of it. These facts allow 

 us to infer that all the tubules once had each an independent 

 external opening until they were secondarily united with one 

 another by the intcrso initie duet. 



The visceral layer of the nephrotome becomes evaginated 

 medianwards and forms a series of segmental pouches on either 

 side of the subchorda ; but this feature is temporary, and the struc- 

 ture is soon smoothed by their Ijecoming confluent with one another. 

 This series of pouches is, I l^elieve, the remnant of the primitive 

 segmental cœlonie, and gives rise to the gonads and the meso- 

 uephros. 



If the accounts given above be correct, the primary rnesoblast 

 is, during early development, divided into two distinct portions : («) 

 the larger proximal portion ivhich is segmented, and {b) a ><mall 

 distcd jyortion which is unsegmented. The former is differentiated 

 into the sclero-myotome and the nephrotome, and the latter forms 

 simply the jjeritonecd linings. 



The pronephric vessels acquire their définitive form in much 

 later stages ; when established, they are intersomitic in position. 

 The posterior part is transformed into a pair of the glomeruli 

 of the pronephros. 



Feiromy.:o/i has for a long time been looked upon as being 



