A HUMAN EMBRYO OF TWENTY-FOUR PAIRS OF SOMITES. 147 



The chorda dorsalis consists of polygonal and wedge-shaped cells, with large 

 rounded nuclei and considerable granular cytoplasm. In places the cells are 

 arranged radially about the center of the chord, while in other regions this arrange- 

 ment is less distinct. Mitotic figures are numerous among the cells. 



In the center of the chorda a fine lumen can be made out in certain sections 

 (plate 4, fig. 3). This lumen is not continuous throughout, but is present in 

 numerous places, each extending through but a few sections. In size it varies 

 from 2 to 4 microns. That this discontinuous lumen of the chorda is normal in 

 certain young stages of the human embryo seems to be well established, since it 

 has been found by His 18 in his embryo LI of 2.4 mm., by Eternod 5 in three embryos 

 of 1.3 mm., 2.11 mm., the third somewhat larger, and by Watt 48 in twin embryos 

 of 17-19 paired somites. 



An ill-defined cuticular membrane surrounding the chorda dorsalis, such as 

 has been described by Van den Broeck 47 , is present, except where the chorda is 

 fused to the entoderm of the pharynx. 



NEPHRIC SYSTEM. 



In describing the Robert Meyer embryo of 23 somites (Thompson's embryo) 

 Felix 10 states: 



"The pronephros is almost completely developed, so far at least as one may speak 

 of its completion. It consists of a number of tubules and the primary excretory duct. 

 There are in all seven tubules present, the most anterior of which is not united with the 

 succeeding tubule .... The tubules 2-5 have fused so as to form a collecting duct. 

 The tubules 5, 6, and 7 are not yet united, but their union is imminent." 



In my specimen I likewise find a series of tubules representing the nephric 

 system. Although conditions on both sides of the embryo are not identical, for 

 the most part they are quite similar. The following description is based on their 

 arrangement of the left side and the terminology used is the same as employed 

 by Felix. 



On the seven pronephric tubules which are present, the first two are rudi- 

 mentary (plate 4, fig. 4). The first is represented by two or three small clusters 

 of cells at the level of the ninth body-segment. It is indistinct and indefinite and 

 its identity is determinable only by the position which the clusters of cells occupy 

 and by the fact that the cells are more or less isolated from the mesenchyma. 



The second tubule is larger and more distinct. It is composed of a single 

 cluster of cells which form a spherical mass. Its cells are isolated from the mesen- 

 chymal cells by a clear space. It also lies in the seventh body-segment. The 

 remaining pronephric tubules are elongated gland-like tubes of epithelium, rather 

 bulbous at their cephalic extremities and tapering caudally. Each in its course 

 crosses the body-wall obliquely in a ventro-dorsal direction ; thus in its upper part 

 each lies close to the mesodermal lining of the ccelomic cavity, while below it 

 comes in close relation with the ectoderm (plate 4, fig. 4). According to the 

 description of Felix, each of these tubules represents two different portions of the 



