214 BULLETIN OF THE 



located in somite II. The plane of the section from which Figure 15 

 was drawn passes somewhat behind the middle of this somite, so as to 

 show the location of the constriction between the protovertebi'se and the 

 lateral plates. In the middle of the somite, the arrangement of the cells 

 composing the pronephric thickening appears to be that of a fold in which 

 the layers are in close contact. The thickening is composed of three layers 

 of cells, and it is possible to trace the somatic layer of the protovertebra 

 into the outer layer of the thickening. The lateral indifterent somatopleure 

 is continuous at the ventral border of the thickening with the inner or 

 thin layer which lies next to the body cavity. Near the upper border of 

 the thickening this inner layer appears to be folded abruptly on itself to 

 form the middle layer of the thickening. The middle and outer layers 

 are continuous with each other distally, i. e. ventrally.^ This anterior 

 knob of the pronephric thickening (Fig. 15, fnd. 112^/1' st.^) is the fundament 

 of the first nephrostome, a later stage in the development of which is 

 shown in Plate III. Fig. 18 (nph^stmJ). In Figure 15 the three lay- 

 ers are indicated by the arrangement of the nuclei. Of these the two 

 outer form the fundament of the first nephrostomal tubule. The inner- 

 most layer represents the underlying peritoneum. In the region be- 

 tween somites II. and III., it is impossible to distinguish definite layers 

 in the thickening. 



On entering somite III., the pronephric thickening has a far greater 

 breadth, and it consists of three layers, the meaning of which is to be 

 understood by a comparison with the condition in the region of the first 

 nephrostome, just described. 



In somite IV. (Fig. 16) a division of the thickening into a dorsal and 

 ventral part is indicated, near the termination of the dotted line (eras, 

 pr'nph.). The dorsal part is the fundament of the third nephrostome, 

 and the ventral part represents the anterior portion of the segmental 

 duct (more properly, common trunk, see page 228). The ventral por- 

 tion of the thickening can be traced backwards from this point through 

 a distance of about 0.37 mm. Figure 17 is drawn from a section 

 through a region near the anterior boundary of somite VII., and shows 



1 The correlative terms distal and proximal are so frequently employed by Ger- 

 man writers as synonymous respectively with posterior and anterior that it seems 

 advisable to allude to the fact that they are not used in the present paper in that 

 sense, but invariably with their primitive signification ; thus, the distal portion of 

 a process is that part which is most remote from the point of attachment, whether 

 the structure project anteriorly or posteriorly, medially or laterally, dorsally or 

 ventrally. 



