66 EOBERT SOHAEFP. 



Leuckart 1 makes mention of an outer membrane-like boun- 

 dary of the zona, through which canals are continued, and 

 Aubert 2 as well as Remak 3 speak to the same effect. 



A few other observers such as His, Lereboullet, 4 Solger, 5 

 and Ransom, 6 do not make any reference to a layer externally 

 to the zona, but admitting at the same time the porous struc- 

 ture of the latter ; while Haeckel, 7 speaks of a structureless 

 vitelline membrane only, having delicate dark spots. Ovsian- 

 nikov 8 again, the most recent writer on the ova of osseous 

 fishes, mentions three layers as occurring in Perca fluvia- 

 tilis. Prolongations from the follicular cells into the canals 

 of the zona were seen distinctly. No doubt they serve for 

 nourishing the egg. There is a distinct zona radiata externa 

 inOsmerus eperlanus, while in Gasterosteus there is only 

 one layer. 



Lindgren, 9 who has recently examined the structure of the 

 mammalian ovum, found that the zona pellucida, which corre- 

 sponds to our zona radiata, is pretty often completely homo- 

 geneous in ripe eggs. He believes that this is due to different 

 physiological conditions of the egg. Author used the highest 

 powers available. 



Johann Muller 10 was to my knowledge the first to identify 



1 Leuckart, " Ueber die Mikropyle bei Insekteueiern (also Fishes)," 

 ' Miiller's Archiv,' 1855. 



2 Aubert, "Beitrage z. Entwicklungsgeschichte d. Eische," 'Zeitsch. f. 

 wiss Zool.,' vol. v, 1854. 



3 Remak, " Ueber Eihiillen und Spermatozoeu," ' Miiller's Archiv,' 18 55. 



4 Lereboullet, " Resume d'un travail d'embryologie comparee sur le devel. 

 du brochet, &c.," ' Ann. Sci. Nat.,' vol. i, 4th ser., 1854. 



5 Solger, " Dottertropfen in d. intracapsularen Flussigkeit v. Fischeiern," 

 ' Arch. f. Mikr. Anat.,' vol. xxvi. 



6 Ransom, loc. cit. 



7 Haeckel, "Ueber d. Eier d. Scomberesoces," ' Miiller's Archiv,' 1855. 



8 Ovsiannikov, loc. cit. 



9 Lindgren, " Ueber d. Vorhandensein v. wirklichen Porenkanalchen in d. 

 Zona pellucida v. Saugethieren," 'Arch f. Anat. und Pbys. Anat. Abth.,' 

 1877. 



10 Muller, " Ueber Zahlreiche Porenkanale in d. Ei-Kapsel d. Fische.' ' 

 •Miiller's Archiv,' 1854. 



