124 ]*l\ci-otory Organs in Bdollostonia Stouti 



just described and tlic youngest of stage A, previously studied, but is 

 nearer the latter than the former. It was cut in two in the region of 

 the forty-sixth segment, the anterior part being sectioned sagittally and 

 the posterior part transversely. There are eleven or twelve pairs of gill 

 slits. Mesenchyme is present between the myotomes and the external 

 ectoderm, and the sclerotomes have been converted into very loose mesen- 

 chyme, which shows no signs of segmentation. The splanchnocoel is 

 found from about the ninth to the eightieth segment, and through the 

 greater part of its extent is continuous from segment to segment. In 

 the posterior region, however, it is poorly developed, and here sections 

 are occasionally met with in Avhich it is entirely wanting. 



The excretory system extends on the one side from the twelfth to the 

 eighty-second segment, and on the other from the eleventh to the eighty- 

 first. Through the greater part of its extent it is no longer in contact 

 with the myotomes, mesenchyme having grown in hetween, but towards 



Figs. 13, 14 and 15. — Thi-ee consecutive sections showing one way in which the 



nephi'ocoel, nc, and splanchnoccel, sc, come together. The Section in front of Fig. 13 



shows no splanclinoccpl at all, while Fig. 15 is the only section in which there is a 

 communication hetween the two cavities. 



the posterior end of the system the ends of the myotomes gradually 

 approach the nephrotomes, until, in about the last ten segments, the two 

 are either in contact or are actually joined together. From the anterior 

 end back to about the twenty-eighth or thirtieth segment the nephrocoels 

 have either disappeared or are disappearing, by being merged with the 

 splanchnoccel. Beyond this they are all in connection with the splanch- 

 nocoel, except the very rudimentary ones in the last four segments. 



A good idea of the excretory system as found throughout the greater 

 part of its extent in this embryo may be gained by an examination of 

 Fig. 16, representing a sagittal section through segments forty-four and 

 forty-five. Fig. 17, representing a transverse section through the nephro- 

 tome in segment forty-seven, and Figs. 18 to 20, three consecutive sections 

 through the segmental duct, back of Fig. 17. The plane of the sagittal 

 section does not quite coincide with the long axis of the body, hence the 

 difference in shape of the nephrocoels in Fig. 16, the left or posterior one 

 being nearer the median line than the right. 



