HALL : MESONEPIIROS AND MULLEPJAN DUCT IN AMPHIBLV. 63 



reached the cloaca. About fifteen somites are recognizable. No blood- 

 cells have as yet appeared. 



As showu in the tigure, splanchuoderm [spVdrrn. l.) and somatoderm 

 (so'drm. I.) are pressed together, so that the arrangement of the cells 

 and the presence of deeper pigmentation alone indicate the line of 

 demarcation. Both layers are seen to be continnous throughout the 

 three regions of the mesoderm, — epimer, mesomer, and lateral plates. 

 Toward the posterior end of each mesomer the coelom is represented by 

 an actual lumen. 



Figui-e 14 (Plate 2) represents a section through the sixth somite 

 of a lai'va 4.5 mm. in length and coiTesponding approximately to the 

 stage in Amblystoma illustrated by Figure 2. The Wolffian ducts are 

 just opening into the cloaca, and blood-cells are numerous in the ventral 

 and lateral lacunae of the entoderm. Occasionally one is seen to have 

 migrated dorsad into the fundament of the blood-vessels (va. sjuj.y In 

 the fitrure, the sclerotome is seen as a proliferation of the mesomeric 

 splanchnoderm. In sections posterior to the one figured, it extends to 

 the dorsal angle of the epimer. In the outer angle of the epimer is seen 

 a small lumen, caused by the separation of the somatic and splanchnic 

 layers. Later, this lumen is obliterated, as in Amblystoma, by a disin- 

 tegration of its lateral Avail (compare Fig. 4, Plate 1). The mesomer 

 (^ms'mer.), leaving out of account the sclcrotomic portion, is less clearly 

 marked than in Amblystoma, but careful study shows that it contains 

 tissue belonging to both somatic and splanchnic layers. 



In the larva represented by Figure 4, the obliquity of the somites 

 has increased to such an extent that a section through the centre of one 

 cuts the next anterior one as well. The lower somite in this figure is 

 the ninth, the upper one the tenth. The larva measures seven milli- 

 metres and shows a well-developed glomus and ciliated nephrostome. 



That Figures 6 and 14 represent sections anteinor to the position of 

 the future mesonephros is immaterial, as there is no essential difference 

 at those stccjes between the mesonephric region and that immediately in 

 front of it. However, in the larva now under discussion and in older 

 ones, sections through the mesonephric region and those through the 

 region anterior to it present very different aspects, for, while anteriorly 

 the mesomer remains inconspicuous, in the mesonephric region it becomes 

 more and more massive, as may be seen from Figures 4, IG, and 1.5, fnd. 

 mg'nph. The statement that the mesomer becomes massive is not quite 

 accurate. "What really happens is that the mosoniers of successive 

 somites fuse to form a flattened band of tissue, the mesonephric blastema, 



