496 The Embryology of Hy lodes Martinicensis 



The notochord is the axis of the tail ; the fiat surfaces are morpholog- 

 ically the right and left sides. Thus the vascular lamella? of the tail are 

 in the position of the dorsal and ventral fins of the tadpole. To which- 

 ever side the tail is turned in the egg the part which lies cephalward from 

 the notochord is dorsal, and the part on the opposite side of the noto- 

 chord is ventral. These relations are clearly established by the relative 

 positions of the notochord, spinal cord, and dorsal aorta 

 (Fig. Q) . While the tail after stage lY becomes broader 

 and thinner, as described from the surface view, the con- 

 nective tissue becomes spongy, and through its meshes is 

 a network of capillary blood-vessels. The ventral part 

 is more richly supplied with blood-vessels than the dor- 

 sal, a condition perhaps correlated with the fact that it 

 contains the dorsal aorta which carries the main supply 

 sectio'n^of^Thc ^^ blood. The aorta extends with the notochord nearly 

 *^^I'tt \" 1^1*^^^ f*3 the end of the tail : no main efferent vessel runs for 



> iJl. bv, blood- ' 



lar^lortR-'^'m '^^^-^ distance through the tail, but the blood is collected 

 muscle: n. noto- jn the postorior vena cava near the union of the tail 



Chora: sc, spinal ^ 



^^o""*!- with the body. 



At the base of the tail the diameter of the notochord from stages YI to 

 XII far exceeds its diameter in the body of the embryo. In the tail itself 

 the notochord diminishes in size toward its distal end. The diameter 

 of the spinal chord is greatly reduced as compared with its size in the 

 body, especially in late stages. Surrounding the notochord is a layer of 

 muscle that is connected with the muscles of the body and like them is 

 segmented. Xo vertebras are found in the tail. 



In stage XIY the notochord, muscles and spinal cord have become 

 reduced, and the tail is more solid but still very vascular; in stage XY 

 when the tail is almost entirely absorbed the tissue is compact. 



6. DERMAL FOLD. 



The fold of skin, which in several embryonic stages partially covers the 

 arms, will be described in more detail. The anterior leg lies in a space 

 which is the angle between the head of the embryo and the yolk (see 

 figure). Across this space stretches the dermal fold whose free anterior 

 edge reaches from the head to the side of the yolk. The fold is composed 

 of two layers of ectoderm separated by mesoderm. The folds of the two 

 sides are entirely independent of one another. The fold differs from the 

 operculum of the tadpole, in that it is continuous with the epidermis of 

 the body posteriorly, and its free edge is directed anteriorly. But the 

 anterior legs of Hylodes, for some time after their first appearance, are 



