130 SEGMENTAL DUCT. 



behind this there maybe seen the segmental duct with a distinct 

 lumen, and also a pair of segmental involutions (PL xi. fig. 14 a). 

 In the still posterior sections the segmental duct would be quite 

 without a lumen, and would closely adjoin the epiblast. 



It seems not out of place to point out that the modes of 

 the development of the segmental duct and of the segmental 

 involutions are strikingly similar. Both arise as solid involu- 

 tions, from homologous parts of the mesoblast. The seg- 

 mental duct arises in the vertebral segment immediately in 

 front of that in which the first segmental involution appears ; 

 so that the segmental duct appears to he equivalent to a single 

 segmental involution. 



The next stage corresponds with the first appearance of the 

 external gills. The segmental duct now communicates by a 

 wide opening with the body-cavity (PI. XI. fig. 9 sd). It pos- 

 sesses a lumen along its whole length up to the extreme hind 

 end (PI. XI. fig. 9 a). It is, however, at this hinder extremity 

 that the most important change has taken place. This end has 

 grown downwards towards that part of the alimentary canal 

 which still lies behind the anus. This downgrowth is begin- 

 ning to shew distinct traces of a lumen, and will appear in the 

 next stage as one of the horns by which the segmental ducts 

 communicate with the cloaca (PI. xi. fig. 9 h). All the anterior 

 segmental involutions have now acquired a lumen. But this 

 is still absent in the posterior ones (PI. xi. fig. 9 a). 



Owing to the disappearance of the body-cavity in the 

 region behind the anus, the primitive involutions there remain 

 as simple masses of cells still disconnected with the segmental 

 duct (PL XI. fig. 9 6, 9 c and 9 d). 



Primitive Ova. The true generative products make their 

 first appearance as the primitive ova between stages I and K. 



In the sections of one of my embryos of this stage they are 

 especially well shewn, and the following description is taken 

 from those displayed in that embryo. 



They are confined to the region which extends posteriorly 

 nearly to the end of the small intestine and anteriorly to the 

 abdominal opening of the segmental duct. 



Their situation in this region is peculiar. There is no trace 

 of a distinct genital ridge, but the ova mainly lie in the dorsal 

 portion of the mesentery, and therefore in a part of the mesoblast 



