8 



T. H. BURLEND, 



a tliickeiiinj? of the somatic 

 mesoderm lining the coelom (see 

 Plate 1 Fig. 5). 



The development on the left side 

 is not so distinct as on the right side, 

 especially in the anterior region. 



in segments V and VI, although 

 the protovertebra has not become 

 clearly differentiated from the more 

 laterally situated mesoderm in 

 this region (see Plate 1 Fig. 4). 



Hence the segments where the groove is present on the left 

 side are III, IV, V, VI and VII; and on the right side II(?}, Ill, 

 IV, Fand F/; it is thus evident that the rudiment of the kidney does 

 not usually extend so far forward, but extends further caudalwards 

 on the left side than it does on the right side. 



The mesoderm of the nephrotome and lateral plate regions is 

 more or less differentiated into somatic and splanchnic layers from 

 2,12 to about 3,18 on the left side, and from 2,5 to about 3,5 on 

 the right side. There is no indication of a kidney duct in this 

 embryo. 



The following points are of importance as exhibited in embryos 

 A and B. 



1. The first trace or rudiment ("Anlage") of the kidney appears 

 as a "primitive kidney groove" in the somatic mesoderm on each side 

 of the body about the time when 5 or 6 protovertebrae are re- 

 cognizable. 



2. The "primitive kidney groove" is from the first a continuous 

 structure lying almost immediately lateral to the protovertebra, 

 except in places where the structure is less marked, in which case 

 it is most evident in the intersegmental regions of the mesoderm, 

 and indistinct or absent in the mesoderm in regions opposite the 

 middle of the protovertebrae. 



3. The formation of the groove seems to be concomitant with, 

 or to follow closely after, the differentiation of the mesoderm into 

 somatic and splanchnic layers in the anterior region of the body, 

 and to precede by a short interval of time the transverse con- 

 strictions which divide up the mesoderm into protovertebrae. If 

 these phenomena have any palingenetic significance they imply that 

 the primitive kidney groove in this region of the body is present 

 in a primitive ancestor in which no segmentation of the trunk into 

 somites has occurred. . 



4. From the figures of sections of these embryos it will be seen 

 than the ectoderm immediately above the primitive kidney groove 



