Oovolopiiient of tin.' Kxcrotory {)r;^iiis of IMollostDinn stoiiti [^ ickirisîtoii. 209 



embryo lies tiat upon the yolk, like a yoiiii^ chick embryo. The stage 

 of development of the myotomes, the spinal cord, the spinal nerves, 

 and the notochord shows that the embryo has already passed through 

 the earliest stages. On the right the section passes through one of 

 the short segmental tubules (s. t), which projects as a dorso-lateral 

 evagination from the narrow body-cavity. On the left, it passes through 

 the solid anläge of the segmental duct (s. d), which here lies surrounded 

 by mesenchyme, and just above what will be spoken of as the inner 

 angle of the body-cavity, that is, the place where the somatic and 

 splanchnic layers of the coelomic epithelium pass into each other. 



In nearly all cases the tubules occur in the same sections with 

 the spinal ganglia, as is the case in Fig. 1. As the ganglia recur of 

 course at regular intervals, they have been found to be the most con- 

 venient landmarks for determining relative positions. In longitudinal 

 sections the myotomes might be used for this purpose, but in trans- 

 verse sections, and I have no other, this would be difficult. In the 

 following description, when a part is spoken of as occurring in a 

 certain segment, it is simply meant that it occurs in the sections 

 passing through the corresponding spinal ganglion , without any re- 

 ference as to whether it lies opposite a myotome, or between two 

 myotomes. For instance, the ganglion in Fig. 1 is the twenty-seventh 

 from the anterior end, and the tubule would be said to be in the 

 twenty-seventh segment. 



There are four embryos of stage A. In one of these, which was 

 hardened in Flemming's mixture, the parts stand out rather more 

 distinctly than in the others, and it will therefore be used as a basis 

 for the description of the excretory system in this stage. 



The first indication of the system occurs here in the eleventh 

 segment, and consists of a simple thickening of the somatic layer of 

 the coelomic epithelium (Fig. 2), which extends through seven sections. 

 As will be seen from the figure, the thickening has not been caused 

 by a proliferation of cells, but by certain cells having assumed the 

 form of columnar epithelium, while the adjoining cells have retained 

 the form of flat epithelium. A comparison with the corresponding 

 region in older embryos, and also with other parts of the system in 

 the same embryo, enables us to say that later an evagination will 

 here take place to form a segmental tubule, and a slight concavity 

 of the lower surface of the thickening may indicate the beginning of 

 such a process. The anläge that has just been described is entirely 

 isolated from the rest of the system, for on following the series caudal- 



