336 s. HATTA : 



At that point of the visceral layer of the mesoblast, where the 

 Anlage of the pronephric tubule passes over to the lateral plate, 

 it is alwa3^s many cells deep (figs. 55 and 50, c.p.), and the pro- 

 jection in question is brought aljout by repeated division of these 

 cells. The projection formed is consequently seen in each somite 

 and thus shows a segmentai arrangement. Its component cells 

 are soon re-arranged into an epithelium, and the pouches thus 

 formed push their way between the myotome and the hypoblast. 

 The coelomic projection appears, at first sight, to be homo- 

 logous with the coelomic pocket described by Price ('97) in 

 Bdellostoma. The coelomic pocket is, however, according to Price, 

 the product of both the parietal and visceral layers of the lateral 

 plate and is afterwards converted into the cavity between the 

 glomerulus and Bow3ian's capsule of the Malpighian cor- 

 puscle ; the fioor of the pocket forms Bowman's caj)sule, 

 and its roof together with a part of the pronephric tubule is 

 transformed into the cover of the glomerulus ('97, p. 213). The 

 coelomic projection in Petromyzon is, on the contrary, formed 

 out of the visceral layer of the distal half of the somite and gives 

 rise, as just stated, to the radix of the mesentery, from which 

 partly the mesonephrie tubule and partly the gonads are formed.^' 



Fiîïs. 59-63 are from a series of sections throuo-h an older 

 embryo of the same stage. In this series of sections, a further 

 development of the coelomic projections is clearly seen ; the first 

 figure (fig. 59) shows the section through the second tubule, fig. 

 60 through the third, and so forth. In the first 3 figures and 

 on the right of fig. 62, the coelomic projection {cp.) presents an 



1)1 will not here fiirtlier discuss this structure, as I iutend to do so in a future paper 

 iu whicli tlie developuieut of the iiiesonephros in Pdromij-ym. will l)e delt with. 



