250 BULLETIN OF THE 



tebral plate. It is probable that this slightly thickened somatic layer 

 is the first indication of the pronephric thickening. 



Stage II. 



Plate V. Fig. 44. 



Embryos of this stage measure nearly 4 mm. in length ; the medul- 

 lary tube has become entirely separated from the superficial ectoderm, 

 and three protovertebrse can be distinguished in longitudinal sections. 



The fundament of the pronephros forms in this stage (Plate V. Fig. 44) 

 an evident thickening of the somatic mesoderm lying immediately lateral 

 to the protovertebral plate. Throughout the greater part of the thick- 

 ening, the layer is obviously two cells thick, and occasionally three nuclei 

 may be seen in a line perpendicular to its surface. The cells constitut- 

 ing the thickening are closely compacted, and do not appear to form 

 definite layers. The fact that the thickening passes through a stage in 

 which it is only two cells in thickness precludes the possibility of its 

 being a disguised fold with closely applied walls, for in that event there 

 must be at least three layers of cells involved. Neither the anterior nor 

 the posterior limit of the thickening can be clearly determined at this 

 stage. I am also uuable to state definitely its relations to the protover- 

 tebra?, inasmuch as these cannot be adequately made out in transverse 

 sections, and the extent of the thickening cannot be satisfactorily ob- 

 served in such longitudinal sections as pass through both the protover- 

 tebrre and the pronephric thickening. The latter may be traced for a 

 distance of about 0.5 or 0.G mm. Each protovertebra at this stage 

 measures about 0.27 mm. in length, so that the thickening extends 

 through a length of about two protovertebra;. 



In slightly older embryos the pronephric thickening becomes in gen- 

 eral three cells in thickness ; but it is still a solid proliferation, with no 

 indication of extensions of the ccelom between the layers. 



Stage III. 



Plate VII. Figs. 55, 56. 



At this stage the young Amblystomas are about 4.3 mm. long and dis- 

 tinctly elongated in shape ; but they show as yet no trace of a tail. 

 They are further characterized by the possession of about eight well 

 marked protovertebra?. 



In all the embiyos of this stage the pronephric thickening is at least 

 three cells in depth, and has a definite ventral boundary. The thickening 

 extends as far forward as the front face of somite III., and posteriorly 



