1885.] PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 153 



tine ; are,iu fact, thickened ridge-like swellings of the sides of the mes- 

 entery. 



The renal system lies almost wholly exterior to the abdominal peri- 

 toneum of the embryo, and is the only organ, with the partial exception 

 of the air-bladder, which does so. The renal or Wolffian organ of Gam- 

 busia consists of a remarkably well-developed anterior mesonephric por- 

 tion |rn, Fig. 7, which is crowded up against the auditory vesicle poste- 

 riorly and richly supplied with bloodvessels. The pronephros was not 

 observed, as the stages studied by me were already too far advanced^ 

 but the posterior part of the segmental duct was still essentially pro- 

 nephric in character, as no segmental tubules opened into it. At the. 

 hinder aspect of the urinary vesicle al the segmental duct of the one 

 side joined its fellow of the opposite side, and shortly after opens into 

 the bladder, which has a distinct outlet, which lies in a line contiuuous 

 with that of the segmental tubes when the latter are viewed from the 

 side. In Fig. 23, Pn, a cross-section of the anterior mesonephric organ 

 of Gambusia is shown, and in Fig. 25 the segmental or Wolffian ducts 

 Wd.are shown in cross section through the region of the posterior part 

 of the abdomen. 



The brain. — A vertical longitudinal section of the brain of a f(Btus of 

 Gambusia, which has attained about the development of the one shown 

 in Fig. 3, is shown in Fig. 28. The cerebellum cer is unusually wide 

 an tero-posteriorly, resembling in this respect the cerebellum of Amiimis 

 of about the same age. The oral epithelium is shown in place at oep, and 

 at Hi there is an involution which seems to represent the last vestige of 

 the hypophysial pouch from which the hypophysis Hy has been de- 

 veloped, and from which that structure has bei-ome disconnected. 



The muscular system presents nothing essentially different from that 

 10 be met with in other fish embryos. The so-called lateral dermal mus- 

 cle Dm, Fig, 23, is distinguishable as a distinct layer of fibers external 

 to the myotomes m, w, m, but it is remarkable tl|at this muscle still 

 shows the segmented condition seen in the latter, a feature which it does 

 not share in common with the dermal muscles of higher animals. It is 

 therefore seriously to be doubted if it has any very strong claim to be 

 called a dermal muscle. 



The courses of the fibers of the muscles of the body are shown in 

 Fig. 7, which represents a longitudinal section considerably oif of the 

 median line. 



The muscles of the jiectoral develop as usual on either side of the 

 coraco- scapular plate cs. Fig. 7, and are probably derived as in the 

 Elasmobranchs from buds given oft" by the muscular segments above 

 the rudiment of the girdle. It will be noticed, however, that there are 

 no muscles developed around or below the yelk-sac, so that there are 

 no recti-abdominales muscles yet formed, the only investment of the 

 yelk being the outer epiblastic covering and the deeper mesoblastic one 

 developed in connection with the vascular net-work which ultimately 

 unites with the heart anteriorly. 



