Geo. C. Price 



137 



In some of the segments farther forward this is not the case, the two 

 being in contact or actnally fused together. This union is to be looked 

 upon as secondar}'. 



Stages A and B previously described follow in natural sequence upon 

 the one we have just been considering, but before proceeding to the gap 

 between B and C, it will be well to pause for a time, and describe a certain 

 phase in the early differentiation of the segmental duct and tubules 

 and also notice some of the mistakes of the earlier paper. 



Beginning with an embryo not a great deal older than the last, and 

 corresponding well with the youngest of stage A, it is found that a con- 

 striction has occurred in the nephrotome, the most obvious result of which 

 is to divide the nephroeoel into three parts, a dorsal part, which helps to 

 form the lumen of the segmental duct, a ventral part, which later forms 



Fig. 22. — Transverse section through the tubule and duct in the forty-fifth segment 

 of an embryo corresponding to stage A of the previous worlj. one, what will later be 

 the cavity of the Malpighian corpuscle ; Id, lumen of the segmental duct ; Itj lumen of 

 the tubule. The line a — & corresponds to the plane of Fig. 23. 



Fig. 23. — Oblique longitudinal section of the segmental duct and tubule in the seg- 

 ment from which Fig. 22 was taken, reconstructed on millimeter paper. The plane of 

 the section corresponds to the line a — & in Fig. 22. cmc, what will later form the cavity 

 of the Malpighian corpuiscle ; Id, lumen of the segmental duct ; It, lumen of the tubule. 



Fig. 24.- — Section of a tubule in the anterior region where the nephrocoels have dis- 

 appeared, of the same embryo as the one from which Fig. 22 was taken, t, tubule. 



the cavity of the Malpighian corpuscle, and a middle part, which forms 

 the lumen of the tubule proper. This is not apparent in a single trans- 

 verse section, such as Fig. 22, passing through the forty-fifth segment, 

 but it is brougfit out clearly by the study of a series of sections through 

 a segment, and still better in a longitudinal section such as Fig, 23. 

 This was reconstructed on, millimeter paper, and represents an oblique 

 section, the plane of which corresponds to the line a — & in Fig. 22. 

 The constriction has affected the lower part of the nephrotome, but not 

 to so great an extent as the middle part. A comparison of Fig. 23 with 

 the right hand nephrotome in Fig. 16, from the younger embryo, will 



