420 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1962 



matozoa from the cores and setting them free to make their way up 

 the genital tract. 



There are two uteri, one on each side of the abdominal cavity. They 

 are lined with a vast number of small tags or trophonemata that in 

 many other elasmobranch fishes secrete nourishment for the young, 

 which develop inside the uterus until they are ready to be born alive. 

 By analogy, therefore, it is believed that the basking shart is vivip- 

 arous. On the other hand, the ovary of the basking shark is strik- 

 ingly different from that of other elasmobranchs, which produce only 

 a few large yolky eggs, whether or not development takes place within 

 the mother. The ovary of the basking shark produces a very large 

 number of minute eggs containing a comparatively small amount of 

 yolk, and in this particular resembles that of most bony fishes. It 

 contains at least 6 million eggs 0.5 mm. or more in diameter, the largest 

 not more than 5.0 mm. in diameter. They appear to be ripe at the 

 latter size, and are released from the ovary through a complex system 

 of branching canals. It is improbable that they grow larger before 

 being released, for the upper part of the oviduct, in contrast to the 

 enormous uterus, is extremely constricted. It has tough inelastic 

 walls giving a canal only 2 to 3 mm. in diameter, so that even an egg 

 no more than 5 mm. in diameter must undergo considerable distortion 

 in passing through this narrow passage. 



The basking shark thus shows the paradox of a large ovary con- 

 taining great numbers of minute eggs as in fishes that spawn, and a 

 large uterus lined copiously with trophonemata (threadlike projec- 

 tions for nourishing embryos) as in viviparous elasmobranchs. Nev- 

 ertheless it is probable that the fish is viviparous — no elasmobranch is 

 known to spawn, and in those that lay eggs there is a large nidamen- 

 tary gland which produces the albumen and the shell, and there is 

 no proliferation of trophonemata in the uterus. The basking shark 

 has a small, almost rudimentary, nidamentary gland, and a great pro- 

 fusion of trophonemata. 



The paradox could readily be resolved were a pregnant female ex- 

 amined; but no pregnant female has ever been seen. It is evident, 

 therefore, that after insemination in the early summer the females 

 must, before any embryo is recognizable, refrain from basking and 

 either swim near the bottom or leave inshore waters, or both. Where 

 and when the young sharks are born is unknown, but the smallest 

 specimens recorded measured about 6 feet in length; evidently that 

 is the size at which they are born. They have always been found in 

 early summer, and consequently it is possible that they are born during 

 the inshore movement of the adults at that time. 



There is space only to mention a final point among the many un- 

 answered questions about this elusive fish. In the commercial catch. 



