[11] EMBRYOGRAPHY OF OSSEOUS FISHES. 4G5 



Such an effect upon the micropyle is conceivable, though we are at a 

 loss to understand the function of the pore-canals, should it be true 

 that the entrance of the spermatozoon would cause the micropyle to re- 

 main permanently open. Johannes Miiller believed that he had dem- 

 onstrated that they were tubes, and with careful manipalation it has ap- 

 peared to the writer that he could see through the individual canals 

 when a piece of canaliculated egg membrane was placed flatwise under 

 the microscope and viewed with a high power of good definition. This 

 is also the opinion of Allen Thomson, expressed in the article "Ovum" 

 in Todd and Bowman's Cyclopediae of Anatomy and Physiology. 



The imbibition of water by an unimpregnated fish egg appears to be 

 largely due to osmotic action. If an impregnated egg is ])laced in a 

 fluid having a great affinity for water, such as alcohol or glycerine, the 

 membrane tends to collapse at first, owing to the rapid extraction of the 

 water from the cavity of the enveloj^ing membrane; as soon, however, 

 as the balance of osmotic action is restored and the contents of the en- 

 velope become as dense as the surrounding fluid, the membrane becomes 

 full and tense as at first. This tendency of the membrane to leassume 

 the tense or full condition is doubtless due to the capillary action of the 

 pore canals of the membrane, where such exist. 



In the freshly laid unimpregnated ova of both the cod and the shad 

 one may look in vain for a germinal disk such as had been described 

 by various authors in the freshly laid eggs of other species of fishes. 

 Even after the egg of the cod has lain in water for some hours there is 

 very little change in the relation of the germinal matter and the yelk. 

 The former covers the yelk as a thin layer of absolutely uniform thick- 

 ness, as shown in Fig. 1, ani differs from the yelk only in color, be- 

 ing yellowish, tending towards amber in tint. The distinctness of this 

 external germinal or cortical layer of authors in the cod's egg is more 

 decided than in any Teleostean ovum which it has been my privilege to 

 study. The germinal protoplasm shows a double contour under the 

 microscope outside of the yelk, and is as sharply limited as if it were an 

 independent envelope. This germinal layer is further distinguished 

 from the yelk which it incloses in that it has refringent globules or ves- 

 icles imbedded in its substance, as shown in Fig. 1. In Figs. 2 and 3 these 

 vesicles are shown more magnified, and are seen gathered together in 

 clusters. In Fig. 4 a portion of the germinal layer is shown in oj^tic 

 section, and represents the vesicles lying next its outer surface. The 

 disappearance of these vesicles is comparatively rapid, and appears to 

 be effected in the following manner: Those in close proximity to each 

 other coalesce and form larger vesicles, and are thus reduced in num- 

 ber, and finally disappear altogether by rupturing their outer walls next 

 the outer surface of the germinal layer, possibly expelling their contents 

 into the respiratory or breathing chamber surrounding the vitellus. 

 This view is only theoretical, however, as the writer failed to discover 

 what became of them by actual observation. As long as the egg was 

 S. Mis. 46 30 



