MUSEUM OF COMPARATIVE ZOOLOGY. 133 



eggs of Abramis brama, Chondrosteus nasus, Squalinus argenteus, Cobitis 

 barbatula, Gobio fluviatilis, Cyprinus rufus, and Gasterosteus pungitius. 

 In all these species he found the appendages inserted in a very thin 

 membrane, which ultimately lies just outside the zona radiata and which 

 makes its appearance before the latter. 



The most important paper on Pygosteus is that of Ransom ('67). He 

 studied Gasterosteus pungitius and G. leiurus, and found that the eggs 

 of the two species do not differ greatly. He says that in the oviduct the 

 eggs are surrounded by a viscid layer, and that the zona radiata lies be- 

 low this layer. The zona is in contact with the yolk except in ripe eggs, 

 in which a thin homogeneous membrane covers the yolk and follows the 

 constrictions at the time of cleavage. The micropyle and the dotted 

 appearance of the egg membrane were first made out in eggs t^q'' thick, 

 and in eggs ^Stf" m diameter the membrane could be separated from the 

 yolk. The button-shaped processes can be made out in eggs somewhat 

 less than j^js ,f (0.17 mm.) in diameter. They are attached to the outer 

 surface of the yolk-sac by a bright, highly refractive point.' In the case 

 of the smallest ova there are on an average seventy buttons, in that of 

 the largest two hundred and seven. They serve to attach the egg to 

 foreign substances. Eansom describes and figures the micropyle. 



Owsjannikow ('85) found that in ovarian eggs the granulosa cells cover 

 the micropyle. In fully grown eggs only a single membrane is present, 

 while in the younger ones the zona seems differentiated into two layers, 

 owing to the fact that the zona is laid down by successive additions. 

 The pores do not appear till the membrane has attained considerable 

 thickness, and they are then much finer than in the ripe egg. The 

 mushroom-shaped processes are maintained by him to be cells that pos- 

 sess nuclei which are colored red with carmine. From the base of the 

 process a thread can be traced into the zona radiata. In young eggs 

 the processes consist principally of a nucleus attached to a filament. He 

 does not believe that they are derived from the zona, but thinks they 

 come from the granulosa ; why he thinks so is not stated. Inside the 

 zona he has found the zonoid layer of His. 



I have examined ovarian eggs of fishes taken in November, December, 

 and April. A few days after the spawning, in early April, the ovaries 

 contain a considerable number of eggs (about 0.55 mm. in diameter) in 

 which the formation of the yolk is well advanced. These are evidently 

 destined to be laid before the recurrence of the next annual spawning 

 season, for they are much larger than any of the ovarian eggs found in 

 December. These eggs show no signs of degeneration, and their pres- 



