126 



GREGORY. 



[VOL. I. 



one section was found (Fig. 6) in which it is evident from the 

 deeper stain of the nuclei that karyokinesis has just taken 

 place between the ectoderm and the duct. It is not possible 

 that these are two adjacent cells dividing longitudinally, since 

 the sections immediately preceding and following show no such 

 condition. 



While the duct is separating from the ectoderm in the older 

 regions, the point of division is very often marked by project- 

 ing points of tissue on both duct and ectoderm. Sometimes 



FIG. 5. 



this maybe noticeable only on the one or the other. Drawings 

 of two such sections are represented (Fig. 7, <?, b}. 



A very few division figures were found in the duct, but its 

 increase in size while separated from all other tissues would of 

 itself indicate that such growth must take place. Although 

 the duct frequently lies for much of its length close against 

 the somites, there is always a distinct line of separation. The 

 nuclei in the outer mesodermal layer are also very constantly 

 found at the inner end of the cells, as may be seen from the 

 sections figured. In frontal sections the entirely different and 

 independent arrangement of cells in the duct and in the somites 

 is also very marked. 



Fig. 8 is a reconstruction from frontal sections of ah embryo 

 6^-7 mm. long, with about 38 somites (two gill slits open). 

 This shows that the pronephros on the right side consists of 

 six tubules, the first of which is developed from the 7th somite, 

 with four aortic branches running between. At either end is 



