386 Ralph S. Lillie 



of the sarcoplasm seems to become membranous and to contribute to 

 the formation of the peritoneal lining of the bodv-cavity. 



The uephridial riidimeut can as a rule he disting-nished before 

 the septiim has assumed a definite membranous structure (Piate '23, 

 Fig. 31). It is therefore diffienlt to decide whether or not the 

 nephridium is derived chietly from one or other of the two lamellae 

 of which the septum is composed, or from both. From ali appearanees, 

 however, it seems clear that neither lamella is exclusively eoncerned 

 in its formation hut that both contribute in part; and that the post- 

 scptal mesoblast also participates in the process. The protoplasm 

 of the nephridial rudiment and that of the septal cells are at first 

 continous with one auother; it is only later that the boundaries of 

 the two become sharply detìned. The fact, however, that at its 

 very earliest appearance the nephridial lumen communicates with 

 the body-cavity of the preceding somite proves that the mesoblastic 

 wall of this somite euters, in part at least, into the formation of 

 the early nephrostome. It cannot, however, he said that the nephro- 

 stome is formed exclusively from the posterior wall of this somite. 

 The early nephridial rudiment is in fact a continuous and undivided 

 structure without regional delimitations, and it is not uutil much 

 later that the nephrostome becomes distinct from the glandulär 

 portiou of the organ. It is impossible, therefore, to say that one 

 definite portion of the nephridium is derived from one somite and 

 aaother portion from anothcr somite. It is even impossible in very 

 early stages to sharply define the limits of the mesoderm of succes- 

 sive somites, since the differentiation of somites is itself a graduai 

 process, and the nephridial rudiment is already laid down before 

 the segmentai limits are sharply defined. It is, however, true that 

 in its early undiiferentiated condition the nephridium is continuous 

 with mesodermal tissue which later forins portions of two successive 

 somites. The organ, however, is differentiated as a whole without 

 strict reference to segmentai limits. Even cell-limits, as already 

 pointed cut, are not sharply defined until the organ has already 

 acquired many of its most distinctive characters. The whole of the 

 early nephridium is, however, strictly mesoblastic and the ectoderm 

 plays no part whatever in its formation. Later in developmeut, as 

 will be shown, the ectoderm apparently cpntributes to the formation 

 of the terminal vesicle; but as originally laid down in early develop- 

 ment the entire organ is strictly mesoblastic. 



The structure of the early nephridium, as present in the fourth 



