20 WILLIAM MORTON WHEELER, 



the tubules {pron^ and pron'') show that these must be lengthening 

 rapidly. In Figs. 31 and 32 two transverse sections are represented, 

 the former through the long third proiiephric tuljule {pron'^), the 

 latter through the collecting duct {cd) where it unites the third 

 and fourth tubules. In both sections a fold of the splanchnic meso- 

 derm extends up between the pronephros and is apparently continuous 

 with the sclerotome (Fig. 32 ad). On the outer side between the 

 pronephros and the ectoderm a smaller but similar fold may be ob- 

 served. Both folds are obviously formed by a lengthening of the 

 prouephric tubules. These are pushed down into the body cavity but 

 their lips, or nephrostomes , remain firmly attached to the epithelial 

 splanchnopleure on the one hand and to the somatopleure on the other. 



c) The Formation of the Pronephric Duct. 

 With the formation of the pronephric tubules and the collecting 

 duct we may leave the description of these organs for the present 

 and consider the pronephric duct which is forming in the segments 

 immediately behind the pronephros. Beginning with stage 3 we find 

 the mesoderm less and less advanced as we pass backwards. For 

 some distance the myotomes are distinctly recognizable, but in the 

 posterior trunk region the mesoderm is not yet segmented. Just back 

 of the pronephros we find that the mesoderm spreads out over the 

 surface of the yolk horizontally. The dorsal region of the embryo is 

 moulded to the convex spherical surface of the yolk in such a way 

 that ordinary cross-sections fail to give a true idea of the development 

 of the pronephric duct and it becomes necessary to make cuneate, or 

 wedge-shaped sections. In Figs. 10—24, PI. 1 I, have represented a 

 series of such sections through the mesoderm between the myotomic 

 and parietal regions in three successive segments in the middle of 

 the trunk. Beginning with Fig. 10 from a section passing through 

 the middle of a segment, it is seen that the mesoderm lateral to the 

 myotome contains a small cavity at cœ, which is the equivalent of a 

 uephrocœle in a pronephric segment. It does not join the cavity of 

 the myotome, nor does it as yet extend out between the mesoderm cells 

 lying on the yolk {lm)\ so that the somatopleure and splanchnopleure 

 can be recognized as discrete layers only for a very short distance. 

 The somatopleure next to the myotome extends upwards and outwards 

 as a pointed projection {d) consisting of a few cells and containing 

 the rudiment of a cavity confluent with the small triangular nephro- 

 cœle {cœ). In the next section, Fig. 11, the projection of the soniato- 



