Figure 7. — Microphotograph of the embryo shown in Figure 6. Actual size of embryo is 19 mm. 



age number of embryos per female and the average 

 number of fetuses per female seems to indicate 

 there is very little loss (mortality?) between the 

 two stages of development. The size of the young 

 dogfish at the time of birth is 20-30 cm., regardless 

 of the part of the world from which the observa- 

 tions are reported (table 8) . 



Process of Birth 



There are two recent instances of spiny dogfish 

 giving birth to young in tanks at the Bureau's 

 Woods Hole aquarium. In one instance, I suspect 

 the female aborted rather than having a normal, 

 full-term delivery. 



The first dogfish was caught with hook and line 

 October 19, 1962, in water about 27 m. deep. The 



surface water temperature was 14.2° C. The fish 

 was placed in the aquarium the same day, in water 

 of the same temperature, and apparently adjusted 

 well to the tank conditions. On November 22, 

 1962, during the night, she gave birth to three 

 pups that measured 20.0, 20.3, and 20.7 cm. None 

 of the pups had any evidence of the yolk sac ex- 

 cept for a small scar on the ventral surface be- 

 tween the pectoral fins where the yolk sac had 

 been attached. Presumably the yolk had been 

 resorbed and the pups were fully developed. The 

 water temperature in the tank at the time was 8.4° 

 C. The pups did not survive, although the mother 

 continued to live for several weeks until she was 

 sacrificed. 



The second dogfish was collected in a fish trap 

 and placed in the aquarium on July 24, 1963. The 

 next day the aquarium attendant observed the de- 

 livery of two pups. The female was resting 

 motionless at a slight incline on a pile of rocks in 

 the tank. Two pups were delivered simultane- 

 ously, head first, in rhythmical movements that 

 suggested uterine contractions during mammalian 

 birth. Delivery of the two pups took about 10 

 minutes. The female also delivered four more 

 pups, but the aquarium attendant had been called 



LIFE HISTORY OF SPINY DOGFISH 



543 



