THE EMBRYONIC CCELOM. 85 



In the amniota when the somites are first formed they display usually no actual 

 cavity, but we must consider that one is morphologically present, since we can 

 easily observe the line of contact between the opposite walls of the segments. As 

 observed in transverse sections, the somites are seen to become triangular in outline. 

 The base of the triangle extends along the side of the medullary canal; the apex 

 of the triangle lies next to the splanchnocele, and at the point of the triangle the 

 somatic and splanchnic mesoderm separate widely from one another. Very soon 

 the apex of the triangle forms a narrow piece (Fig. 46, N), which is known com- 

 monly as the nephrotome or intermediate cell-mass. While the nephrotome is being 

 marked off the proximal portion of the segment enlarges, the cells assume a more 

 distinctly epithelial character (Fig. 46, My), enclosing a considerable space, which, 

 however, is completely filled by a mass of cells, C, which arise by a proliferation 



/ JS>*S'_<il3tlt**S,,^iQ J , _ SOITl 



Spl 



FIG. 46. TRANSVERSE SECTION OF THE MESODERM OF A CHICK EMBRYO WITH ABOUT EIGHTEEN SEGMENTS. 

 Only the mesoderm of one side has been drawn. The section passes through a recently formed segment. My, 



Secondary segment. C, Core of the segment. W.d, Wolffian duct. N, Nephrotome. Cos, Ccelom. Som, 



Somatic mesoderm. Spl, Splanchnic mesoderm. X 227 diams. 



of the cells from the lower side of the segment. The line around this mass of 

 cells marking it off from the other wall of the segment indicates the morphological 

 cavity. In the sheep and the chick it has been observed that the cavities of the 

 first four segments can be traced through the nephrotome to the splanchnocele. 

 This represents a primitive condition, one which we find in all the segments of 

 some of the lower vertebrates. Did we know the development of the amniota 

 only, we should not have been able to identify the cavity of the somite as mor- 

 phologically a portion of the crelom. The development in fishes shows conclusively 

 that it must be so regarded. 



The Separation of the Nephrotome. The nephrotome early loses its connection 

 on the one side with the enlarged central portion of the somite, 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 nephrotome assumes 



