MUSEUM OF COMPARATIVE ZOOLOGY. 215 



the thickening {eras. jt)?*'?i/j/i.) near its posterior termination. The mass 

 is evidently a thickening in situ of the soniatopleure. On either side of 

 the fundament of the segmental duct the somatopleure is one cell thick, 

 whereas in the fundament itself it is two or three cells in thickness. If 

 the additional cells arose by a free backward growth from the anterior 

 pronephric mass, we should expect to find them lying on the external 

 face of a continuous somatopleural layer. But, as a matter of fact, no 

 such continuous inner layer exists ; on reaching the thickened region, the 

 somatopleure merely becomes several qells in thickness, the outer cells 

 presenting really a somewhat more compact condition and a more linear 

 arrangement than the inner ones. 



The constrictions between the protovertebrse and the lateral mesoderm 

 can be distinctly made out only in intersegmental regions. As is shown 

 in Figure 15, between somites II. and III. the level of the constriction 

 is immediately dorsal to the nephrostomal portion of the pronephric 

 mass. In the region between somites III. and IV. the division occurs 

 at a corresponding position. This series of sections shows no sharp sepa- 

 ration between protovertebral and. lateral mesoderm posterior to somite 

 IV., the protovertebral plate being here only partly broken up into suc- 

 cessive metameric blocks, which do not as yet possess sharp ventral 

 boundaries. 



In frontal sections, the pronephric thickening shows a similar condition 

 (compare Figs. 11-14) to that which obtains in the case of the embryo 

 described under Stage II. (page 213), the most noticeable difference 

 being an increase in the thickness of the pronephric mass. The longi- 

 tudinal extent of the thickening corresponds approximately to that of 

 five somites, though the posterior limit is of necessity somewhat un- 

 certain. The posterior portion has every appearance of having arisen in 

 the same way as the part lying beneath somites II., III., and IV. The 

 latter, however, represents, as we have seen, the future pronephros ; the 

 former is the fundament of the segmental duct. 



In an embryo slightly older than those last described, the evidences of 

 an incipient canalization of the pronephric system are more pronounced. 

 In the region of somites II.-IV., the two outer layers of the pronephric 

 thickening are separated from the peritoneal layer by a distinct line of 

 division. In the intersegmental regions, the outline of these two layers is 

 that of an elongated ellipse, the nuclei being disposed, for the most part 

 alternately, on either side of its major axis. The significance of this 

 distribution becomes apparent on studying later stages, in which a lumen 



