THE EMBRYONIC C(ELOM. 



81 



brates. Did we know the development of the amniota only, we should not have 

 been able to identify the cavity of the segment as morphologically a portion of 

 the coelom. The development in fishes shows conclusively that it must be so 

 regarded. 



The Separation of the Nephrotome. The nephrotome early loses its connec- 

 tion on the one side with the enlarged central portion of the segment, and on the 

 other with the mesodermic walls of the splanchnocele, so that each nephrotome 

 forms a little mass of cells isolated from, but in definite topographical relation to, 

 the other parts of the mesoderm. It may be noted that during these early 

 stages one can always find the anlage of the Wolffian duct on the ectodermal side, 

 and on the entodermal side the anlage of a blood-vessel. Very soon the nephro- 



Am. 



Som. Ao. Nek. 



FIG. 33. SECTION OF A VERY YOUNG CAT EMBRYO. (Transverse Series 413, section 181.) 

 Am, Amnion. Ao, Aorta. Md, Medullary tube (spinal cord). My, Outer, My' , inner wall of primitive 

 segment. Nch, Notochord. A T ephr, Nephrotome (segmental vesicle). Som, Somatopleure. Spl, 

 Splanchnopleure. Ve, Blood-vessel. W.D, Wolffian duct. X 5 diams. 



tome assumes a rounded form, and a cavity appears in its interior; it is then 

 often called a segmental -vesicle (Fig. 33, Nephr). The exact details of the process 

 by which the nephrotome is separated from the other parts of the middle germ- 

 layer have not yet been carefully studied. Each nephrotome is the anlage of 

 one of the excretory tubules of the Wolffian body. 



' The portion of the primitive segment which is isolated by the formation of 

 the nephrotome lies, of course, next to the medullary canal. The term primitive 

 segment (as also proto- vertebra) is often applied to this structure as well as to 

 the original primitive segment before the separation of the nephrotome, but it 

 would be better to refer to it as the secondary segment.* The secondary seg- 



* This is a new term, here proposed for the first time. 



