ELASMOBRA NCHS. 235 



arch, usually free, but occasionally (raiae) connected with the 

 vertebral column, or rarely with the skull. The pelvic girdle 

 consists of a transverse ventral bar without dorsal or iliac 

 processes. 



A con us with two or more rows of valves occurs in connec- 

 tion with the heart ; the aortic arches and the chief arteries and 

 veins are of the primitive type, and a cardinal sinus (p. 195) 

 usually occurs. 



The eggs are few and large, and in the recent forms undergo 

 internal inpregnation, more or less complicated structures (the 

 ' claspers '), which serve as intromittent organs, being developed 

 in the pectoral fins of the male. The spermatozoa is carried to 

 the exterior by means of the Wolffian duct, while the Miillerian 

 duct serves as oviduct, the fused nephrostomes of the rudimen- 

 tary pronephros serving as the ostium tubae. In many sharks 

 and in some of the rays a' portion of the oviduct becomes enlarged 

 into a uterus, and in some species of CarcJiarias and Mustelns, a 

 placenta is formed, at first sight strikingly similar to that of the 

 mammals, but developed from the yolk sac rather than from an 

 allantois (see mammals). The outer surface of the yolk sac 

 in these forms becomes richly vascular ; and this becomes con- 

 nected with the uterine walls, so that the growing embryo 

 receives nourishment from the blood of the mother. 



The segmentation is restricted to a portion of the upper 

 surface, i.e., is meroblastic. The result of this is the formation 

 of a circular layer of cells, the blastoderm, resting upon the yolk. 

 Inside of these cells is a space corresponding to the segmen- 

 tation cavity of the typical egg (p. 211). Owing to the great 

 amount of yolk, the process of gastrulation' becomes greatly 

 modified. At one end of the blastoderm the cells turn in be- 

 tween the blastoderm and the yolk, and these ingrowing cells 

 become the entoderm. At the place where this ingrowth occurs 

 an arcuate elevation appears, terminating in a pair of swell- 

 ings, the tail swellings, on the margin of the blastoderm. With 

 growth the distance between the arched elevation (which marks 

 the tip of the head) and the tail swellings increases, forming 

 the rest of the head and the trunk region of the body. At first 

 this embryonic area forms a broad, shallow medullary plate ; but 



