22 WILLIAM MOUTON WHEELER, 



nephroi (Fig. 27 d). In older embryos of this stage the prouephric 

 ducts may be easily traced to their terminations in horizontal sections 

 through the posterior end of the embryo. Two successive sections of 

 this nature are shown in Figs. 42 and 43, PI. 3. In Fig. 42 the 

 pronephric duct (d) is distinctly seen lying between the myotome and 

 the rudiment of the reproductive organ (gon). In the next section 

 posterior (Fig. 43) the duct opens into the mesenteron. By going 

 over all the sections in this scries it is easy to convince oneself that 

 the duct has everywhere emancipated itself from the mesoderm and 

 is beginning to take up „fluid at this time from the l)ody-cavity 

 through the funnels of the pronephros. This may be inferred from 

 the fact that frequently in this and the next succeeding stage the 

 collecting and pronephric duct for some distance behind the pro- 

 nephros may be enormously distended with fluid on one side of the 

 body. A case of this kind is seen in Fig. 41 where the collecting 

 duct cd is much distended. A similar condition is also shown in a 

 slightly older embryo (Fig. 28 cd). The distention here is so great 

 that the originally columnar or cuboidal epithelium of the right 

 collecting duct has become a pavement epithelium. In one embryo 

 in stage 3 the distention extended to the posterior end of the pro- 

 nephric duct. I believe this condition to result from an occlusion of 

 the cloaca after the nephrostomes of the pronephros have begun to 

 take up liquid from the body cavity. The distention is temporary ; 

 it disappears probably as soon as the communication of the cloaca 

 with the outside is established. 



After tracing the development of the pronephros and its duct 

 through the trunk of the embryo one naturally turns to consider the 

 conditions in the branchial region in front of the pronephros in the 

 hope of finding serial homologues of the tubules. Such homologues 

 actually exist in Petromyzon, but I have l)een unable to trace them 

 beyond their inception. In stage 3 there are only two pairs of gill- 

 clefts and a third pair developing, and there is a considerable region 

 between this third pair and the anterior end of the pronephros. In 

 this region the primitive condition of the mesoderm is still undisturbed 

 by invaginations of the ectoderm and evaginations of the entoderm. 

 In Fig. 37 I have represented the seventeenth section in front of the 

 first pronephric tubule. It shows the myomere distinctly separated 

 from the lateral mesoderm. At the lower inner corner of the former 

 there is a hollow diverticulum (scl)^ the sclerotome, which is giving 

 rise to the primitive connective tissue cells. Some of these are al- 

 ready moving up between the chorda and the myotome. This diverti- 



