Ceo. C. Price 



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The nepliroeiiL'l has increased in size, especially in its dorso-ventral 

 diameter, and the dorsal wall of the nephrotome as seen in transverse 

 section (Fig. 17), is arched, as if being evaginated to form a tubule. 

 As will appear later this is true only in part. A glance at the sagittal 

 section (Fig. 16) proves that we are still dealing with nephrotomes and 

 that a tubule in the strict sense of the word has not yet appeared. 



Extending from one nephrotome to the next is a solid rod of cells 

 (Fig. 16, d), a part of the segmental duct. At one point, about half 

 way between the nephrotomes, the duct is sligtly smaller than else- 

 where, and here the nuclei are much less numerous. This is brought out 

 still more clearly in the transverse sections (Figs. 18 to 20). Fig. 19 

 corresponds to the point a- in Fig. 16, and Figs. 18 and 20 are the sections 



Fig. 16. — Sagittal section through the the nephrotomes in segments forty-four and 

 forty-five, the one in forty-four being to the right, d, forming segmental duct ; nc, 

 nephrocoels ; x, place where the two nephrotomes have joined together. 



Fig. 17.- — Transverse section through the nephrotome in the forty-seventh segment of 

 the same embryo from which Fig. 16 was taken, nc, nephrocoel ; sc, splanchnocoel. 



Figs. 18, 19 and 20. — Three consecutive sections through the duct between the 

 nephrotomes in the forty-seventh and forty-eighth segments. Fig. 19 corresponds to 

 the point x in Fig. 16 and represents the place where the ends of two nephrotomes have 

 united. 



next on either side. The relation of the duct to the nephrotomes, and 

 a comparison with the younger embryo suggest very strongly that it has 

 been formed from the posterior end of one nephrotome and the anterior 

 end of the next, the point x being the place where the two have united. 

 This point of union is easily made out in almost all of the segments. 



The description just given answers in all essential respects for about 

 three-fourths of the segments, or to be exact, for all except four at the 

 posterior end and twelve or fifteen at the anterior end. However, as we 

 approach the posterior end, in the region where in the older embryos the 

 tubules are rudimentary and early disappear, the nephrotomes become 

 quite a little smaller. 



