162 



MR. E. W. SHANN ON THE EMBRYONIC 



which piocess distends the abdominal wall of the body to an 

 incredible extent. The yolk- stomach so formed was erroneously 

 described by previous writers (including myself) as a yolk-sac. 

 The semi-solid matter is derived from the ovary of the mother 

 and is actually swallowed by the embryo in the uterus. Feeding 

 continues in this manner for a long time, certainly for more than 

 a year, during which time the yolk-stomach continues to increase 

 in size (unlike a yolk-sac) until it assumes gigantic proportions. 

 It will be shown that there is a regular increase in the bulk of 

 the yolk-stomach from the embryo of 35 cm. to that of 55 cm. 

 Embryos have been found in the uterus measuring as much 

 as 75 cm. (Pennant, 5); unfortunately, no record is available 

 of the measurements of the paunch, though it is said to be 

 very large. 



The smallest free-living specimens of Lamna measure 82 cm. 

 (Day) and 87 cm. (Williamson); these, however, have lost all 

 trace of the yolk-stomach, and in external appearance possess in 

 every detail the character of the adult. Birth presumably takes 

 place when the young shark measures about 80 cm. But to what 

 purpose is the great accumulation of food in the stomach put? 

 Certainly it is not used in body-building, for the free-living 

 young are only a few cm. longer than the paunched embryos. It 

 seems possible that the accumulation of nutrient matter in 

 the embryo is used for the expensive purpose of building the 

 reproductive organs, and that the 3'oung at birth are already 

 mature*. 



Such a condition would be unique among chordate animals ; 

 but, indeed, the very mode of nutrition of the embryo is unique. 

 Viviparity among Elasmobranch fishes is by no means uncommon, 

 and three general methods of nutrition are in vogue : either the 

 yolk-sac forms a pseudo-placental connection with the uterine 

 Avall ; or the latter secretes a nutrient fluid which is absorbed by 

 means of external gill-filaments ; or, again, the uterine wall itself 

 produces long secretile villse which enter the alimentary canal of 

 the embryo by way of the spiracles. From all these recognized 

 methods of embryonic nutrition the condition in Lamna forms a 

 fundamental departure ; moreover, whereas in other forms the 

 maternal nutriment is used up at once in body-building, here 

 the vast majority of it accumulates in the stomach as a reserve 

 store. 



After consulting the "Zoological Record," as well as from con- 

 versations with several eminent ichthyologists, I believe that I am 

 now in possession of ail the outstanding facts at present known 

 regarding the embryonic development of Lamna cornuhica. I 

 have attempted in the following pages to arrange these facts in 



* At the same time Dr. Williamson, in a letter to me, saj^s : "A female 3ft. 6 in. 

 long, in October, was immature. Further, a Porbeagle 6 ft. long, in December, 

 appeared to be a male, but it had only a slight indication of the external male 

 <;haracteristics." 



