108 EDWARD MCCRADY, JR. 



The changes just described occur in cephalocaudal pro- 

 gression, and are correspondingly most advanced in the an- 

 terior mesonephric segments. In stage 28 the tubule anlagen 

 are S-shaped from the tenth somite (fig. 36, section F) back to 

 about the eighteenth. From the eighteenth to approximately 

 the twenty-fourth the tubules are separate from each other 

 but are not long enough to be very much twisted, and the most 

 posterior of them are more sac-like than tubular. Caudad 

 from this point they merge into a continuous nephrogenic 

 cord much like that described for the region of the eleventh 

 somite in stage 24. It was a section through a tubule anlage 

 in this cord that Selenka (pi. XXII, fig. 13) mistook for the 

 wolffian duct. And finally, the cord merges into the undifferen- 

 tiated mesoderm beyond the last somites. At the level of the 

 notochordal canal (W.I.C. 16149, sections 913 to 918) this 

 undifferentiated mesoderm merges with the endoderm which 

 has no basement membrane. 



The pronephros at this time is about as well developed as 

 it will ever be. It consists of five or six very slender, solid 

 'tubules' which are connected with both the coelomic epi- 

 'thelium and the wolman duct. They are situated between the 

 seventh and tenth somites (fig. 36, section E), and occur at 

 both somitic and intersomitic levels. In specimen 16149 on 

 the left side there are about thirty-six nephric tubules in all, 

 of which the first six are pronephric, and the last thirty meso- 

 nephric. This count includes the most posterior anlagen 

 which are not separate from one another except in their 

 lumina. 



The liver diverticulum. A diverticulum from the foregut 

 in the midventral line at the level at which the omphalomesen- 

 teric veins empty into the sinus venosus, is the first anlage 

 of the liver. It is shown in cross section in figure 36, section 

 D. During stage 28 it is a simple diverticulum without any 

 Jiver cords branching from it. It is found also in a few of the 

 late specimens of stage 27, that is, specimens in which the oto- 

 cysts and anterior neuropore are still open. 



Fig. 36 Lateral reconstruction and cross sections of stage 28. The recon- 

 struction is from 16150, the sections are from 16149. 



