170 MR. E. W. SHANN ON THE EMBRYONIC 



that water may enter and be expelled through this channel. 

 Such a supposition offers an explanation of why no matter 

 (whether nutritive or faecal) is found as a rule in the uterus on 

 capture. It also ofiers a solution to the problem of respiration ; 

 for, as has been shown, the gills are fully developed and apparently 

 functional. The uterine wall is highly muscular, so that a 

 potential mechanism for pumping water in and out of the uterus 

 is present. Dr. Williamson, in a letter to me, says : " Mr. Ennson 

 observed that the pregnant female dogfishes (piked dogs) when 

 brought up in the trawl had the abdomen distended and sea- water 

 poured out of the cloaca on to the deck." 



Although the mode of embryonic nutrition described above, so 

 far as I can ascertain, is unique among Elasmobranch fishes, 

 certain observations by G*idger (3) on the Batoids of Beaufort, 

 IST.C, are worth consideration in this connection. Speaking of 

 Dasyatis say Gudger remarks: — 



" The young are found bathed in a substance of the color and 

 consistence of rich yellow Jersey cream." 



" The older embryos had the large intestine filled with a 

 chlorine-yelloAv substance, evidently the milk-like food secreted 

 by the villi and taken in probably through the spiracles." In 

 the younger stages it is taken in by the long external gill- 

 filaments. 



" Notwithstanding the fact that the umbilical cord entered the 

 alimentary tract at the junction of the small with the large 

 intestine, and that the material in the anterior part of the large 

 intestine was lighter in colour than in the middle and hinder 

 regions, it is reasonably sure that it was not yolk." The material 

 is described as " finely divided flocculent grading to large plate- 

 like masses." 



In this Ray and in Pteroplatea machtra when the uteri are 

 gravid the ovaries are insignificant. The lumina of the ovaries 

 were filled with " an abundant yolky material which probably 

 came from the breaking down of some of the ova." 



Gudger does not describe the contents of the upper parts of 

 the oviducts : the question arises, did they contain any of the 

 " yolky material " found in the ovaries ? If not, how are we to 

 account for the "large plate- like masses" which were found in 

 the intestine : surely they do not come from a purely milky 

 nutrition? Whether the nutrient matter is taken in through 

 the spiracles (which in a Ray have sufficiently large openings) or 

 through the mouth is a minor consideration ; the important 

 point is that food is actually swallowed and digested, according to 

 Gudger, instead of being merely absorbed through gill-filaments. 

 In the Rays, however, the supply does not exceed the demand, 

 and, consequently, no yolk stomach is produced. The fact that 

 in these Rays two batches of young are produced each season 

 precludes the possibility of a protracted intra-uterine develop- 

 ment such as occurs in Tjcimna cormiMm. 



