390 Ralph S. Lillie 



nuclei tbus formed would complete the process. The stnicture of 

 the larvai excretoiy cells vaiies but slightly at different stages. In 

 the adult uephiidium the cells liave become very numerous and 

 greatly compressed in a lateral direction, assuming a marked coliimnar 

 structure; in consequence of this, apjtarently, the different regions 

 of individuai cells may present more marked differeuces from one 

 another than are fouud in the broad cubical cells of the larvai 

 nephridium. In essential histological character, however, the larvai 

 and adult cells seem to differ but slightly from each other 

 (see p. 370). 



Nephrostome and its Associated Blood-Vessels. 



In the formation of the nephrostome both the anterior region of 

 the originai nephridium and a portiou of the adjoining septum take 

 part. From its earliest appearance, as above shown, the nephridium 

 possesses an anterior opening into the body-cavity of the next somite; 

 but at first the marginai region borderiug this opening is in no 

 respect different from the remainder of the oigan. When subdivision 

 into cells beging the cells of the anterior region at first entirely 

 resemble the others in structure and appearance; as growth pro- 

 ceeds, however, subdivision progresses more rapidly in these cell8 

 than in those behind and as a result the more anteriorly situated 

 cells become smaller and more cubical. At the same time they lose 

 their distinctively excretory character and the protoplasm becomes 

 more homogeneous in appearance, while the vacuoles and excretory 

 granules disappear (Plates 24 and 25, Figs. 37, 39, 42). 



In Figs. 37 and 39, a portion of the adjoining septum appears 

 to be assuraing the character of a cubical epithelium and thus to 

 he contributing to the extension of the dorsal lip of the funnel. The 

 limits between septum and nephridium proper are in fact not sharply 

 defined in early stages (Piate 24, Fig. 34); and until the fuunel- 

 epithclium is definitely formed, it is impossible to say where the 

 two become continuous. Its hinder part however is unquestionably 

 formed from the anterior cells of tlie nephridium proper (Figs. 37 — 39, 

 42), while its marginai cells, especially those of the dorsal lip, seem 

 to be formed in the region of the originai septum. The close rela- 

 tion of the dorsal lip to the septal structures, especially the blood- 

 vessels (a relation which persists throughout life), is thus explaiued. 

 The anterior portion of the nephridium becomes in this manncr 

 composed of au epithelium of small cubical ciliated cells (shown in 



