hall: mesonephkos and mullerian duct in amphibia. 45 



also become very thin, or has entirely disappeared. Occasionally, how- 

 ever, as at so'drm. t., Figure 12, this layer is still visible as a thin mem- 

 brane connecting the mesomer with the lateral portions of the epimeric 

 somatoderm. Aside from this slender connection, the mesomer has been 

 completely severed from the overlying epimer. The all-important change 

 to be noted is the fusion of the successive mesomer s to form a continuous 

 cord. That portion of the cord extending from somite 9 to somite 19 

 or 20 is the mesonephric blastema. This blastema is not of uniform 

 diameter throughout its length, but presents swellings extending from 

 the posterior third of each somite backward to a point opposite the 

 anterior third of the following one. These swellings present all the 

 characteristics of the mesonephric fundaments described in the preced- 

 ing stage, and are presumably identical with them. That each has moved 

 slightly backward from the position in the posterior part of each somite 

 where it arose, thus bringing its greatest diameter into an intersegmental 

 position, is not strange, as the mesomer is now practically free from the 

 rest of the somite. Cross-sections of two of these swellings are shown in 

 Figures 10 and 11 {fnd. ms'nph.). In Figure 10 may still be seen a 

 condition suggestive of an origin from the two layers, splanchnoderm and 

 somatoderm (compare Fig. 7). 



Opposite the middle of each somite, from the ninth to the twelfth, 

 there is an additional swelling which is quite evident, though much 

 smaller than the ones just described. Opposite somite 13 there seem 

 to be at least two of these smaller thickenings.^ For convenience I 

 shall refer to all of these smaller swellings as swellings or units of the 

 "second orde?;" to the larger ones as those of the "Jirst order." Figure 

 12 {fnd. vis'nph.) shows a cross-section of one of the most distinct of 

 the former, that opposite the middle of the twelfth somite. The 

 units of the second order are probably developed from the tissue of 

 the anterior portion of the mesomer just before or shortly after the 

 fusion of the mesomers to form the continuous blastema. 



A word should be said in respect to the relation of the blastema to the 

 lateral plates. In Figures 11 and 12 there is shown an intimate fusion 

 between the blastema and the lateral plates, here represented by the 

 germ-cell mass. This is true only as far forward as the germ-cell mass 

 extends, — to somite 11, There are about six thickenings anterior to 

 that point, which seem in this larva to have no connection with the 

 lateral plates. 



1 Unfortunately I do not possess the portion of this larva back of somite 13. 



VOL. XLV. NO. 2 2 



