78 BULLETIN OF THE 



Rathke's ('Gl) posthumous work on the development of vertebrates 

 evidently treats the subject from the standpoint of thirty years before, 

 when little was known about the matter j and 



Lereboullet ('61, pp. 120, 121, 123) does not greatly add to the 

 knowledge of the zona radiata when he says that the chorion of the 

 trout egg is thin and very soft at the moment the egg is laid, and does 

 not present the resistance and elasticity which it acquires after it has 

 remained for some time in the water. For the absorption of water and 

 the passage of gases necessary for the respiration of the egg and em- 

 bryo, the chorion is pierced, he says, by an infinite number of excessively 

 narrow, short parallel tubes, which give a striate appearance to perpen- 

 dicular sections of the shell. 



Owen ('66, pp. 593-595), although following the accounts by Ran- 

 som and Thomson, fails to recognize one of the important points estab- 

 lished by the latter author, for he does not distinguish between the 

 " ectosac " in the perch, and that of salmon and other fishes. More- 

 over, this " ectosac" (evidently the zona radiata) "is composed of close- 

 set series of hollow columns." (!) As Ransom ('67, p. 3) has since 

 pointed out, Owen also erroneously states, possibly under the influence 

 of Rathke's exposition, that the villi are formed after the ova escape into 

 the cavity of the ovary. 



Buchholz ('G3, pp. 71-81, and 63 a , pp. 367-372) was the first to 

 describe a very peculiar appearance of the egg membranes in Osmerus 

 eperlanus. In addition to the porous membrane (zona radiata), which 

 continues to invest the egg after it is laid, there is a second one external 

 to the first, and like it traversed by similar pore-canals. Buchholz states 

 that these canals are much more readily recognized to be pore-canals in 

 the case of this fish than in that of other fishes (p. 73). When the egg 

 has lain for some time in water, the outer envelope, or at least a portion 

 of it, is found to be attached to the inner membrane around the circum- 

 ference of the micropylar canal, whence it depends as a loose funnel- 

 shaped frill with its originally inner surface now directed outward. The 

 pore-canals which traverse these two membranes, instead of being cylin- 

 drical, are funnel-shaped, the wider end being directed outward. By 

 treating the fresh ovarian egg with acetic acid the outer membrane, 

 which at first lies closely in contact with the inner, is made to swell 

 up with irregular foldings until it becomes entirely separated from the 

 inner ; but a striate appearance, which is visible for a moment, becomes 

 quickly obliterated by the action of the acid. If, however, the eggs 

 are first treated for twenty-four hours with very dilute chromic acid, 



