Whence the Shadows? 239 



object. The mother tugs against the pull of the tendrils, easing the pas- 

 sage of the capsule. 



Whitley tells of an Australian shark, the Spotted Catshark (Chiloscyl- 

 liiim punctatiim), that anchors its eggs with silky fibers that are looped 

 around weeds. A story goes that the mother Catshark weaves the loop 

 with her own lips, but Whitley believes it is far more likely that the 

 mother forms the loop by swimming around the object to which she 

 wants the egg capsule anchored. The Port Jackson shark of Australia 

 (HeterodojjtKS porttis-jackso?ji) lays spiraled eggs which sometimes are 

 found so tightly wedged between rocks on the sea bottom that the only 

 way to loosen them is literally by unscrewing them, as one would a cork- 

 screw. 



The majority of sharks are either viviparous or ovo viviparous, which 

 means that one way or the other they give birth to living young. The 

 viviparous shark, like the viviparous mammal, develops its young within 

 itself. In nearly all mammals, the embryo and the mother are linked by 

 an umbilical cord and placenta. Some sort of connection exists between 

 the mother and the embryo in viviparous sharks, but this connection is 

 not, strictly speaking, a placenta. Early in its development, the embryo 

 feeds upon the yolky portion of its ovum. After a while, this part of the 

 ovum becomes a distinct yolk-sac joined to the embryo by a long, thin 

 neck. Eventually, the yolk-sac forms a close attachment to the womb, 

 or uterine wall, and nourishment passes from the maternal blood stream 

 to the embryo via the yolk-sac. This complex arrangement, which seems 

 to be an evolutionary prelude to the more complex structure of the mam- 

 mahan placenta, is called the yolk-sac placenta. 



In ovoviviparous sharks, there is no connection between the yolk-sac 

 and the womb. A temporary shell is formed around the new embryo. 

 Then the temporary shell ruptures (usually it is rolled up in the uterus) 

 and the embryo continues its development within the womb, nourished 

 by secretions deposited by the mother. 



The newborn pup enters the sea fully equipped to wrestle with its 

 dangers. Even the Hammerheads (family Sphyrnidae) and the Spiny 

 dogfish {Sqiialus acanthias), which bear spike-like quills in front of the 

 dorsal fins, are born alive and fully formed without injury to the mother. 

 The head of the newborn Hammerhead is pliable and the hammer-lobes 

 fold back during birth. The Spiny dogfish's quills, or spines, as they are 

 usually called, are covered with small knobs of cartilage when the shark 

 is born. The knobs are sloughed off right after birth, so that the dogfish 

 is able to use its weapons. 



Whether the new shark emerges from an &gg capsule or is born alive, 

 it is fully prepared to be an adult, no matter how small. It knows no play- 

 ful puppyhood, no parental care, no nest. It is a hungry, restless creature, 

 the latest descendant of a primal, ageless breed. 



