150 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. [1885. 



the yelk -bag over the opercles at py. In other hardened specimens 

 this pouch was observed to be prolonged upwards so as actually to 

 meet its fellow of the oi>posite on the top of the head. In this way it 

 happens that a sort of girdle is formed by the hollow cornua arising 

 from the sides of the yelk-sac, which thus surround the head. 



The next stage. Fig. 3, is from a living foetus liberated from its fol- 

 licle. In this specimen the scales were already developed, though the 

 yelk was not yet absorbed, but was considerably diminished in amount. 

 The heart still had its venous eud directed downwards, gathering the 

 blood from the net- work of superficial vitelline capillaries through a 

 great median ventral vein, the same as in Fig. 1. In a cross- section. 

 Fig. 15, from a specimen a little more advanced than that represented 

 in Fig. 3, the dermal pouches in which the scales are developed are 

 shown at np ; the section has also cut through the more advanced ven- 

 tral at vt and the anal at a. 



A remarkable characteristic of the embryos of Gambusiais to be noted 

 in the fact that they do not develop continuous fin-folds, but, as shown in 

 Figs. 1, 2, and 3, the vertical fius at once appear as distinct folds. This 

 type is therefore not perfectly lophocercal or provided with a continuous 

 eradiate dorso-ventral fin-fold such as is found in the embryos of Gadus 

 of the same relative stage of development. 



Chondrocranium. — The cartilaginous skull of Gamhusia at about the 

 stage represented in Fig. 3 is shown in detail in Fig. 21, Plate X. The 

 brain-box has an incomplete roof; there is a cartilaginous bridge over 

 the piueal region T cr, and one over the medulla oblangata oc. On its floor 

 there is wide pituitary interval, as seen in Figs. 21 and 28, into which 

 the hy[)ophysis Hy, the infundibulum In, and optic nerve II, and crus 

 ch project downward more or less decidedly. The chorda i)rojects a con- 

 siderable distance under the hind-brain, as may be inferred from Fig. 

 10, representing a longitudinal section of its anterior end, the vertebrae 

 vr, vr being already differentiated with their intervertebral ligaments 

 Ij I, defined. 



The auditory vesicle Ati is covered externally by cartilage, and the 

 orbit is limited above by an imperfect supraorbital curved cartilagi- 

 nous bar, below by the pterygoi)alatine and hyomandibular bars. The 

 nasa! fossa? are limited behind by the cartilagiuous orbito-nasal septum, 

 below by the rostrum. The hyomandibular Hni is remarkably long 

 and slender when embraced together with the symplectic Sy. Meckel's 

 cartilage, Mk, is short ; the glosso-hyal G. Hy is long ; the hypo-hyal, 

 Hy, nearly globular ; the cerato-hyal, G. hy, wide and somewhat pro- 

 longed and joined to the hyomandibular by a short interhyal, IR. The 

 branchiosteges Brs originate in perichondria! membrane, and have no 

 cartilaginous rudiments. There are five branchial arches, I, II, III, IV, 

 V, and detached suprabranchial nodules, S. hr. , 



Behind the branchial arches, but more superficially, lies the cartilag- 

 inous coraco-scapular rudiment Got, iSc, with the cartilaginous plate 

 from which the actinosts are formed, abutting against it i)OSteriorly, 



