MUSEUM OF COMPARATIVE ZOOLOGY. 141 



Esox reticulatus. 



The egg membrane of Esox was first described by Aubert ('53). He 

 says the shell of the egg is a thin, transparent punctate membrane, which 

 closely envelops the yolk and in sections exhibits radially placed streaks. 

 After lying in water some time, an outer very thin granular membrane 

 makes its appearance. 



Lereboullet ('54) describes two membranes, the outer of which is 

 pierced by microscopic tubes. The inner is a simple extremely thin and 

 amorphous envelope, which has no homologue in the perch. 



Eeichert ('56, p. 94) states that the membrane discovered by Aubert 

 surrounding the zona radiata is to be found on all eggs of this species, 

 but that it is in the fresh condition entirely homogeneous. 



Kolliker ('58, pp. 84 and 85) maintains the existence of a thin outer, 

 resistant layer in all fish eggs, and was able to isolate it in fresh eggs of 

 Esox. 



Ransom ('68) says that in Esox the egg membrane is similar to that 

 of Gasterosteus ; he also, as I think erroneously, supposes the thin 

 outer membrane to be homologous with the " Eikapsel " of the perch. 

 He figures the micropyle. 



Finally, His ('73) described for the zona radiata concentric as well as 

 radial strise. 



The eggs examined by me were taken from the ovary in February. 

 Leaving out of consideration the smallest eggs, 0.063 mm. and less in 

 diameter, which have no membrane except the granulosa, the ovary 

 contained eggs in three stages of development, respectively about 0.50, 

 1.00, and 1.50 mm. in diameter. In eggs of the first stage the zona 

 radiata is about 3 /x thick and very faintly striate. There is no evidence 

 of its being differentiated into concentric layers. At the micropyle (Fig. 

 35, Plate III.) it reaches a thickness of 7 p. Very generally the yolk is 

 more or less retracted from the zona by the action of the hardening re- 

 agents, so that a narrow space, which varies a good deal in thickness over 

 different parts of the egg, is left between the two structures. Spanning 

 this interval are numerous fine threads, which have the appearance of 

 being prolongations of the substance of the yolk continued into pore- 

 canals of the zona. This is a condition which remains at subsequent 

 stages, aud will therefore be discussed further on. The granulosa cells 

 are still thin, and their nuclei much flattened. 



In the second stage (Fig. 36, Plate III.) the zona has a total thickness 

 of 11 or 12 ix, and is distinctly differentiated into two layers, the outer of 



