STAGES IN THE DEVELOPMENT 



row, and by careful exploration the egg mass, 

 which is soft and yielding, can be obtained. It 

 must be handled gently. (Sometimes a second 

 fish is flushed from the nest and rushes past the 

 collector's arm as he attempts to find the egg 

 mass.) 



The adults work over the eggs very actively 

 (Breder, 1935), fanning them with their fins, 

 taking them in their mouths and ejecting them, 

 and in general handling them rather roughly. 

 Apparently almost constant agitation is re- 

 quired to assure normal development of the 

 eggs, since eggs removed to still water usually 

 will die within a day or so. Two simple methods 

 were devised for handling the eggs; either will 

 usually give normal development of the major 

 share of the eggs of a cluster. The cluster may be 

 placed in a seven-inch finger bowl in tap water 

 flowing from a rubber tube of which the lower 

 three inches rests on the bottom of the finger 

 bowl directed to give a swirling flow of the 

 water in the bowl. There should be a flow of 

 water adequate to agitate the eggs slightly but 

 not to carry them with the current. A constant 

 flow of water can best be obtained through a 

 petcock without a washer. In a washered outlet, 

 the flow reduces as the washer swells. The sec- 

 ond method is illustrated in Figure 1, which 

 shows a strainer to which a metal tube is 

 soldered in such a position that the water flows 

 from below through the egg mass resting in the 

 strainer. The egg masses should be examined 

 at least twice a day and any dead eggs removed, 

 since these quickly develop mold which extends 

 to the adjacent eggs and kills them. 



The laying season starts about the same time 

 in central New York State and on Cape Cod, 

 Massachusetts— i.e., about the middle of May. 

 The latest that newly laid eggs were found on 

 Cape Cod was July 28. Adults in aquaria will 

 lay spontaneously, with the opening and closing 

 dates of the spawning somewhat later than with 

 fish in the wild. Females early in the winter 

 carry eggs which in appearance and size are close 



to maturity. We induced spawning and obtained 

 fertilized eggs out of season by injecting into a 

 pair of bullheads crushed pituitaries from ripe 

 female alewives, but additional work must be 

 done to standardize the procedures if uniform 

 success is to be achieved. 



The eggs measure about 3 mm. in diameter. 

 Those from the same egg mass are remarkably 

 uniform in size but there are differences in size 

 from one egg mass to another. The subchor ionic 

 space is wide and accounts for about one-third 

 of the diameter of the egg. The chorionic mem- 

 brane is transparent, naked, and relatively thin 

 for the size of the egg. The membrane can be 

 removed readily without injury to the develop- 

 ing embryo merely by tearing the membrane 

 with fine forceps. After the blastopore is closed, 

 the embryos survive in running tap water with- 

 out any special treatment; in fact, they are less 

 susceptible to death than if left in the mem- 

 branes where the mechanics of aeration are 

 more difficult. The embryos hatch spontaneous- 

 ly in Stage 43 and will swim actively on the floor 

 of their container. In the earlier swimming 

 stages, the embryos can be kept in an open 

 dish with a gentle stream of water circulating 

 through the dish. In the more advanced stages, 

 they become negatively geotropic and are lost 

 from an open dish, but they can be kept in a 

 battery jar closed with a strainer fitted with a 

 tube through which water can be introduced 

 from above (Figure 2) . The strainer must be 



FIGURE 1 



FIGURE 2 



