44 bulletin: museum of comparative zoology. 



crowding of the nuclei, the intensity of the nuclear stain, and the pres- 

 ence of a large amount of pigment throughout the cells. These three 

 peculiarities combine to give these eai-liest fundaments of the meso- 

 nepliros that conspicuousness which characterizes the organ until it 

 becomes functional. 



That both somatic and splanchnic layers of the mesomer participate 

 in the formation of these fundaments seems unquestionable from the 

 appearance of such sections as that shown in Figure 7, where two 

 groups of nuclei will be noticed, one on each side of the point marked 

 ms'mer. Although such a favorable fundament (that is, one showing a 

 clear separation into two masses, a somatic and a splanchnic) is rather 

 rare, I have seen, nevertheless, such a condition often enough to 

 justify the belief that it is not without significance. 



The only additional points to be noted in this stage are the presence 

 of a distinct aorta (Fig. 7) and postcardinal vein {va. sng., F'ig. 8), the 

 open lumen of the duct, and the coalescence of the germ-cell masses to 

 form a more or less continuous ridge, no longer showing any sign of 

 segmentation. Tliis ridge e.xtends from the postei-ior end of somite 10 

 to somite 16 (see Diagram 1, page 56, column A). 



Tlie earliest traces of the mesonephros thus appear as specialized masses 

 of cells differentiated from the p/osterior portion of the raesomers from 

 somite 9 hack to, presumably, the jjosterior end of the dud. The funda- 

 ments are metameric^ there being one for each segment. Tliey are derived 

 from both sp>lanchnoderm and somatoderm. 



Stage VI. 



The larva described as illustrating this stage measured 10 mm. in 

 lengtli. The development of the mesonephric fundament is very rapid 

 in larvae from 9 to 10 mm. long, which explains the fact that among 

 the many specimens sectioned, I have been able to find but one good 

 examjjle of each of the Stages Y. and VI. 



Figures 10, 11, and 12 (Plate 1) illustrate the changes which the 

 somites have undergone. The mesomer, duct, and dorso-median angle 

 of the lateral mesoderm have migi-ated still farther mediad. The 

 remaining portion of the somatic layer of the epimer has been more 

 or less completely converted into mesenchyme, thus obliterating the 

 remains of the epicoelom by depriving it of one of its walls. In the 

 section represented by Figure 12 this process has been nearly completed. 

 In that shown in Figure 10 it is still in progress, the outer wall being 

 very thin. That portion of the somatoderm directly over the duct has 



