124 



Part III. — Sixteenth Annual Report 



February, when the eggs were not fully mature, weighed lib. 15Joz., 

 and was 16| inches long. The gross volume of the fish was 951 c.c. ; 

 the ovaries, which weighed 6Joz., had a volume of 170 c.c, so that the 

 volume of the body was 781 c.c. The number of eggs in the ovaries was 

 computed to be 2,733,000, the volume of which in the mature state 

 would be about 1114 c.c. 



The expansion of the pelagic ovum at maturation is therefore a direct 

 cause of the prolonged spawning period in those species whose eggs are 

 pelagic. In conclusion, it may be said that a comparison of the change 

 in the demersal and pelagic egg does not support the hypothesis that 

 the pelagic ovum is the more primitive type, because the latter 

 exhibits a more specialised development of a feature that is common 

 to both, and whose primitive significance is unconnected with the flotation 

 of the ovum. 



EXPLANATION OF PLATE L 

 Fig. 1. Section of ovary of haddock (Zeiss A. oc. 4). 

 Fig. 2. „ „ „ X 200. 



Fig. 3. ovum of gurnard x 200. 



Fig. 4. , , ova of plaice x 200. 

 Fig. 5. ,, ,, (Zeiss A. oc. 4). 



Fig, 6. ovum of eel, showing oil vacuoles. 



Fig. 7. „ solenette x 120. 



Fig. 7a. Germinal vesicle of young ovum in same section. 



Fig. 8. Ovum of flounder, showing the presence of the internal vitelline 

 membrane. 



Fig, 9. The same, more magnified. 



Fig. 10. The same, showing its presence in the unruptured egg. 



Fig. 11. Showing the germinal vesicle escaping by pressure. 



Fig. 12. „ „ „ „ 



Fig. 13. ,, „ ,, between two ova on slide. 



Fig. 14. Section of mature hyaline egg of plaice, with a small part of the 

 yolk still undissolved. The egg-capsule is shown too thick and 

 swollen. 



Fig. 15. Section of mature hyaline egg of plaice, the yolk-spherules completely 

 dissolved. 



Fig. 16. Section of unripe ovum of herring, showing the yolk-spherules, germinal 

 vesicle, three-layered zona. 



Fig. 17. Ripe egg of herring, showing the partial fusion of the yolk-spherules 

 and the aggregation of the germinal protoplasm ; germinal vesicle 

 disappeared. 



Fig. 18. Germinal vesicle of lemon sole in the fresh condition, showing fibrillse 

 passing among the yolk-spherules. 



Fig, 19-24. Transition stages from the nearly spent ovary of a flounder, showing 

 various appearances. 



Fig. 25. Surface markings on zona radiata of sole (A) and tusk (B) ; a, of opaque 

 eggs ; &, of translucent mature eggs. 



