476 The Embryology of Hylodes Martinicensis 



As development advances beyond stage IV, the legs of the embryo 

 elongate, the area of the tail increases, and the brain and sense organs 

 are further differentiated. At about stage VI (Fig. 10), pigment appears 

 at isolated points over the dorsal surface of the embryo and in a dermal 

 fold which covers the base of the anterior legs. 



In stage IX (Plate II, Fig. 11), the eyes are large and heavily pig- 

 mented. The olfactory pits are plainly seen in an anterior view of the 

 head, and are so far apart as to show from the side. The anterior legs 

 lie close to the head (reaching well to the middle of the eyes when looked 

 at from above). They are covered to the foot by the pigmented dermal 

 fold. In some views, the outlines of toes can be seen. The tail is nearly 

 twice as long as the hind leg, and is spread out under the egg membrane 

 in a thin vascular sheet. The axis of the tail divides it into more or less 

 unequal parts, the part lying towards the head end of the embryo being 

 smaller than that lying on the opposite side of the axis. The tail is not 

 pigmented, but the pigment in the embryo has increased in density, and 

 is extending over the yolk. 



Between stages IX and XII, the egg as a whole increases in size, and 

 t:he embryo manifestly develops at the expense of the yolk. It is possible 

 to see the increasing complexity of the brain until a period when the 

 pigment renders the skin opaque. The fore-legs, as they increase in 

 length, grow around the head (meeting ultimately under the chin), and 

 the dermal fold, by which they have been partially covered, recedes. The 

 hind-legs grow around the yolk, until they almost touch the fore-legs, 

 and the fingers and toes become jointed and provided with suckers. The 

 area of the tail increases. By the time stage XII (Plate II, Fig. 12) is 

 reached, the embryo with the exception of the tail is uniformily though 

 not deeply pigmented ; in the tail, there are only a few scattered pigment 

 cells. The lower lip, or rather the chin, is very thick, so that the head is 

 less fiat than before; on the upper lip is a single median black horny 

 protrusion or beak. 



In stage XIII (Plate II, Fig. 13), the frog no longer appears to be 

 encircling an egg, but what yolk remains is incorporated in the body. 

 Near each thigh is a pale triangular area, raised slightly above the sur- 

 rounding surface. This is seen again in XIV (Fig. 14), where similar 

 raised patches show on the sides of the neck. 



The embryo in stage XIV has hatched, and looks like a somewhat bloated 

 miniature adult frog. It measures 2.3 cm. in length; the adult frog 

 measures 5 to 6 mm. The boundaries of the lymph spaces appear as 

 pale lines in the skin, forming two seams on each side of the embryo 

 and a triangle around each fore-leg, joined by two seams near together 



