212 BULLETIN OF THE 



the trunk, immediately behind the ganglion nodosum, there is a marked 

 thickening (eras, pr'nph.), which ends abruptly in front, but gradually 

 thins out into indifferent somatopleure behind. This thickening is 

 distinctly present through a length of 0.5 mm., which is slightly greater 

 than the extent of protovertebrse II., III., and IV. Still farther ven- 

 trally, the antero-posterior extent of the thickening is much diminished, 

 the reduction taking place from both ends, so that in passing ventrally 

 the region in which the structure is last visible is situated approxi- 

 mately beneath protovertebra III. 



Another pair of embryos, one of which was 2.5, the other 2.G mm. in 

 length, presented a condition of the pronephros somewhat more advanced 

 than that just described (Plate I. Fig. 5). In these embryos the lips of 

 the medullary fold in the most advanced region were in contact, but had 

 not yet fused. The anterior limit of the pronephric thickening was the 

 same in position as in the younger pair of embryos, lying near the middle 

 of somite II. A study of the arrangement of the nuclei in this region 

 made it evident that there were at this stage in general three layers in the 

 thickening. The innermost of these is the thinnest, and is destined to be- 

 come the peritoneal covering of the pronephros ; the other two represent 

 the two walls of the pronephric pouch, soon to be described. The prone- 

 phric thickening ic the region of the anterior face of somite IV. is shown 

 in Figure 5. The section gives a somewhat false impression as to the 

 somatic layer of the protovertebra, unless the relation of the section to 

 the successive somites be borne in mind. The considerable thickening 

 which this layer apparently undergoes on passing into the protovertebra 

 is due to the circumstance that the section here passes obliquely through 

 a portion of the anterior wall of somite IV. Sections through the middle 

 of a protovertebra show a gradual thinning of the somatic layer as far a.; 

 the dorsal angle of the mesoderm (compare Plate II. Fig. 15, which is 

 a cross section of the following stage), where this layer is almost pave- 

 ment-like. The pronephric thickening extends rather farther posteriorly 

 than in the former pair of embryos, and while it is manifestly difficult 

 to set a limit to the structure, I am confident that the thickening ex- 

 tends into somite V. This posterior extension of the thickening is to 

 be regarded as the fundament of the pronephric, or, according to the 

 later nomenclature of Balfour, the segmental duct. 



The corresponding series of frontal sections shows six well differen- 

 tiated protovertebra?, representing somites I. -VI. The same group of 

 cells which I interpreted before as the last remnant of a rudimentary 



