1885. J PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 149 



through the yelk substance. At the time of fertilizatiou it is probable 

 that the germinal matter is aggresated at one pole of the yelk. The 

 latter in Gamhusia is orange colored, and embedded between it and in 

 the periblast superficially are a great number of refringent oil globules 

 of small size. 



The body of the young fish lies in a groove or furrow on the surface 

 of the yelk at about the time the tail is beginning to bud out, as shown 

 in Fig. 13, Plate IX, which represents the youngest stage of Gambu.sia 

 which I have been able to observe. The somites or segments of muscle 

 plates had been developed for some time, and the heart, brain, intes- 

 tine, and organs of sense were well defined. 



The next stage observed was that represented in Fig. 1, Plate VI, figured 

 from the fresh embryo removed from its follicle. 



The mouth is not yet well open and the pericardiac cavity seems to 

 have its anterior or inferior wall supported by the end of the snout. 

 The heart lies on the floor of this cavity on the yelk, with its venous end 

 directed downward and forward. The vitelline net- work of vessels arise 

 behind from the caudal vein, which bends down when it reaches the 

 yelk, when it divides and sends a trunk forward along either side of 

 the latter to join the rudimentary Cuvieriau duct which runs down upon 

 the yelk in the vicinity of tbe liver where the portal vein anastomoses 

 with the latter. 



The air bladder ah, Fig. 1, does not yet fill up the peritoneal cavity 

 pp, and the liver L is quite large and lies on the left side. The intestine 

 is not yet much bent or coiled upon itself. 



The auditory vesicle and otoliths are well defined, and the eye and top 

 of the head are pretty well pigmented. 



The lateral line organs, or the neuromasfs of E. Ramsay Wright, are 

 present at this stage over each muscular segment, as shown by the small 

 circles nh of Fig. 1. But at a later stage similar organs are found at 

 the anterior border of the opercles, lodged in depressions of the skin as 

 shown in section much enlarged in Fig. 20. These last-mentioned ter- 

 minal sensory disks or neuromasU in the skin over the cheeks appear 

 to derive their nerve supply from the great fifth pair. 



The fins at this stage consist of the pectoral, dorsal, anal, and cau- 

 dal; the ventral is not yet developed. Eays have commenced to form 

 in the caudal ; the others do not yet present any well-marked indica- 

 tions of rays, which only become well defined during the later stages 

 shown in Figs. 2 and 3, when the ventral first appears as a papilliform 

 rudiment on either side a little in front of the vent. No sexual differ- 

 entiation of the anal fin was observed in any of the embryos taken 

 from the follicles, and it is therefore inferred that such differentiation 

 occurs during post-foetal life. 



The next stage observed is that represented in Fig. 2, which is drawn 

 from a specimen hardened in chromic acid, and is consequently a sur- 

 face view. The striking peculiarity which this specimen renders a])- 

 pareut is the prolongation upwards and forwards of a process of 



