368 s. HATTA : 



They are the spaces formed by the slackening and fokling of the 

 median peritoneum which coats the pronephric tubules, as above 

 stated (p. 355). The fold, i.e. the space, extends throughout 

 almost the whole length of the pronephros (figs. 100-104) and 

 contains numerous free-cells. But, as the peritoneum finally 

 adheres to the median walls of the tubules in each nephric 

 segment, the space becomes divided into three pairs : the anterior 

 pair, which soon disappears, is found between the tubules of 

 the second and third, the middle is detected between the third 

 and fourth, and the posterior between the fourth and fifth tubules. 

 These spaces communicate directly, — medially with the aorta tract 

 and externally with the tracts of the anterior cardinal veins, 

 which emerge, in later stages, in the pronephros. They are the 

 blood-spaces which, I believe, correspond with the intersomitic 

 arteries demonstrated by Paul Mayer and others in Selachia. 

 AVhen the tubules develop further, the arterial portion of 

 these blood-spaces disappear except the middle portion where it 

 is sacculated and filled up with the mesenchymatous cells 

 (figs. 108-109, gl.). This portion is the structure which is called 

 the glomerulus of the pronephros ; it is found one on each side 

 (see pp. 355 and 358). 



Having followed, in the foregoing pages, the successive pro- 

 cesses which take place in the development of the pronephros 

 in Petronyzon, step by step, I will give a short resume of the facts. 



1. In the earliest part of Stage ii, the ni esoblast consists 

 simply of A\\q parietal (dorsal) and the visceral (median and 

 ventral) layers. The proximal portion of the mesoblast is dis- 



