EGGS. lxv 



principal cause which makes eggs subside to the bottom is that their specific 

 gravity is greater than that of the fluid in which they are floating, unless 

 due to some, mechanical arrangement (as the presence of filaments) they 

 are attached to foreign substances, when they would sink or swim in accord- 

 ance with the condition of the body to which they were attached, as the 

 eggs of the marine gar (Belone) ; or a fish (as a perch) may have its ova in a 

 band-like state, when it selects rushes, reeds, or grass growing in the water 

 or a piece of wood or other hard substance, against which it (the female) 

 presses itself until one end of the band has become attached, then swim- 

 ming slowly away the eggs are voided. But sometimes eggs, as of the cod, 

 float in normally saline water, and questions have arisen as to the position of 

 the micropyle. Dr. Ransom in 1854 found that in the trout, salmon, and 

 grayling it corresponded to the centre of the germinal pole. Here the 

 formative yelk or germ collects, and having attached to it some oil drops, 

 always floats uppermost. In the Spanish mackerel and some other American 

 forms a single large oil sphere keeps them buoyant, situated at a point 

 immediately opposite the germinal disk, which is constantly inverted or 

 carried on the lower face of the vitellus, thus acting exactly the reverse to 

 what is observed among the Salmonidce. In the cod no oil drop exists, but 

 the egg is so light that it behaves like the foregoing. It is seen in the cod 

 fisheries that at the period of breeding the egg floats with the micropyle 

 directed downwards, and as a consequence the milters are found to swim 

 lower than the spawners, the milt must consequently ascend. 



It will now be necessary to briefly observe upon the physical changes 

 which fishes' eggs have to undergo prior to their being rendered in a suitable 

 condition to continue the species. If we examine under the microscope the 

 ova of an osseous fish, as a stickleback, as remarked upon by Dr. Ransom, 

 we may perceive around the eggs before they are deposited and holding the 

 mass together is a viscid layer or secretion from the oviduct of the female. 

 This secretion will for some time resist the imbibition of water in the unim- 

 pregnated ova, so that they have been observed to remain flaccid at least 

 two and a half hours after immersion. Subsequently it seems to set round 

 the eggs, making them cohere firmly together. The egg itself may be said 

 externally to have a double cortical layer, the two being divided by an 

 interspace ; the outer of these, which is rather thick, may be termed 

 the yelk sac, and is in immediate contact with the second internal or 

 vitelline membrane which surrounds the yelk ball within the yelk sac. 

 The outer membrane of the egg is distinguished in one spot by a number 

 of cup-shaped or mushroom-like processes, which cover about one-fourth 

 of its surface and mark the .germinal pole. In the centre of these small 

 elevations is the micropyle, consisting of a funnel-shaped pit directed 

 towards the centre 'of the eu\ and continued 'inwards as a narrow tube with 



