MUSEUM OF COMPARATIVE ZOOLOGY, 217 



tion occurs immediately dorsal to the fundament of the segmental duct, 

 which, as I have shown, is continuous anteriorly with the ventral half 

 of the thickening appearing in somite IV. A series of measurements 

 from the dorsal median line shows that the ventral portion of the pro- 

 nephric thickening remains at a nearly constant level, so that the pro- 

 tovertebrse must reach a somewhat more ventral position in the posterior 

 region than in somites II.-IV. 



Figure 11 (Plate II.) represents a frontal section through the dorsal 

 part of the pronephric pouch in one of the oldest embryos of this stage. 

 It shows the course of the earliest fundaments of the three tubules which 

 emerge from the somatopleure beneath protovertebrte II., III., and IV. 

 The most anterior outgrowth, arising in somite II., inclines outward and 

 backward into the region of somite III. ; the second outgrowth proceeds 

 from its origin beneath protovertebra III. directly outward ; and the 

 third outgrowth inclines forward, so that its distal extremity also lies in 

 the region of somite III. As the review of the previous stages has 

 shown, these fundaments of the tubules have not arisen as separate out- 

 growths from the somatopleui*e, but have been differentiated from the 

 originally continuous pronephric thickening, the three fundaments being 

 confluent distally. 



In this section the nuclei are abundant along a central band, but scarce 

 or wholly absent in peripheral parts. This peculiar arrangement be- 

 comes intelligible when we consider that the plane of the section passes 

 almost tangentially through the curved dorsal wall of the pouch. As we 

 have seen in transverse sections, the nuclei lie close to the inner lumen 

 of the pouch ; it is therefore only in the deeper central parts of the sec- 

 tion that they are encountered. In a section 0.03 mm. farther ventral 

 (Fig. 12), the lumen of the pouch can be made out, though it is not con- 

 spicuous. It is difficult to say whether at this stage the lumen is contin- 

 uous throughout the whole structure. In many embryos the evidence of 

 such continuity seems indubitable ; whereas in others, apparently quite 

 as far advanced in other respects, the lumen seems to consist of uncon- 

 nected portions. In some instances where no trace of a separation of the 

 walls could be seen, a line of pigment indicated the position of the lumen. 

 Occasionally I have met with a distinct prolongation of the body cavity 

 into the pronephric mass. This condition has been most frequently en- 

 countered in the case of the fundament of the first tubule. I am not, 

 however, inclined to place much weight on such observations as proving 

 the claim that the lumen of the pronephros forms as an ingrowth of the 

 coelom pi'oceeding from the nephrostomes and advancing into the duct. 



