DEVELOPMENT AND LIFE-HISTOEIES OF TELEOSTEAN FISHES. 671 



cod, *06 ; grey gurnard, "055, and oil-globule, '0116; lemon dab, '053; flounder, "038 ; 

 common dab, "033 ; skulpin, '025 to '030 ; sprat, '044 in long diameter, '039 in short 

 diameter ; sole, '045. 



The Egg- Capsule, with Remarks on the Reproductive Organs and 



Period of Spawning. 



Few points in the constitution of the ovum afford more matter for controversy than 

 the origin and significance of the external protective membrane. 



The twofold division of egg-membranes, due to Prof. E. van Beneden (No. 24, 

 pp. 228-30), and founded upon their derivation, is both natural and convenient, viz., (1) 

 membranes differentiated from the cortex of the egg-mass itself; (2) membranes formed 

 ab extra by the cells of the ovarian follicle. It is generally agreed that the egg-capsule 

 of. Teleostean ova belongs to the first division. Cunningham, however, does not adopt 

 this view, and the " vitelline membrane " of his earlier papers he now considers to 

 be an extra-vitelline product — developed by the cells of the follicular epithelium 

 (No. 47). Other protective structures may lie outside the egg-capsule proper, such as 

 the mucous layer in Perca Jluviatilis, the gelatinous matter surrounding the floating ova 

 of Lophius piscatorius, and others, but they are probably ovarian, oviducal, or other 

 secretions, and do not belong to the ovum proper. Further, it seems most in accord 

 with present results to regard the external capsule as a single membrane — variously 

 styled Eikapsel (Muller, His, &c), Eihaut (Kupffer), Chorion (Lereboullet), Ectosac 

 (Owen), outer yelk-sac (Ransom), and zona radiata (Waldeyer). G. Brook, again, 

 describes in Trachinus a thin membrane (his vitelline membrane) outside the zona. 

 Such has not been seen in any of our pelagic eggs. It is generally hyaline, tough, and 

 slightly resilient, and varies in thickness in different species — thus approximately in 



Anarrhichas lupus, it is '00143 to '00162 in. 



Gastrostcus spinachia, „ '0015 „ 



Gadus morrhua, „ '000312 „ 



„ aiglcfinus, „ '000440 „ 



Gadus merlangw, it is '000310 in. 



Pleuronectes jlesus, „ '000125 „ 



„ limanda, „ '000104 „ 



Trigla gurnarclus, „ "000333 „ 



In pelagic ova it is so exceedingly thin and translucent that the developmental 

 changes in the germ are visible through the capsule,* yet in demersal ova it is not only 

 denser, but presents in many species marked structural features, such as projecting knobs, 

 filamentous processes, reticulations, and the like, all of which, however, must be looked 

 upon merely as modifications of the single capsular membrane — the zona radiata. It is 

 very thin and transparent in the sprat, the egg of which generally shrivels when put in 

 spirit. The zona usually presents laminae, which Sars observed and counted in Gadus 

 morrhua ; but such does not imply the existence of separate layers, for chitinous structures 

 of this kind often show a stratified condition. Ryder could only make out the laminae 



* In undeveloped and dying eggs the growing opacity of the vitelline mass is readily seen. This opacity of the 

 egg-contents Andr£ wrongly attributes to the capsule itself, which he says becomes opaque (No. 4, p. 397). 



