lxvi BREEDING. 



the inner end opened. This outer covering of the egg is changed by the 

 imbibition of water from an easily torn membrane into a firm elastic one. 

 The yelk-ball, or that portion surrounded by the vitelline or inner membrane, 

 contains those essential portions of the egg which are subsequently directly 

 transformed into the germ, and into which the micropyle opens. Between 

 the two layers is a space, small in the unimpregnated egg prior to the imbi- 

 bition of water, but which becomes filled, forming what has been termed the 

 breathing chamber, a space increased probably also in size by a contraction 

 of the yelk. In the stickleback this absorption of water from the outside 

 commences near the micropyle, and gradually extends throughout the 

 chamber ; but in most osseous fishes, Ransom observes, water enters freely 

 through the yelk sac, and the breathing chamber may commence simul- 

 taneously at all parts of the surface. It is only during this period when 

 absorption is going on that fertilization can take place, and the sper- 

 matozoon obtain access by the micropyle to the germinal spot. When the 

 air-chamber has been filled with water the outer covering of the egg hardens 

 and become elastic ; it is no longer soft and adhesive, and it " frees " itself 

 from what it is attached to. 



In this place it is unnecessary to go further into the embryology of 

 osseous fishes, neither will it be required to prove that the elements for 

 resph'ation must be received through the outer coat from the surrounding- 

 water. Here, however, it becomes needful to point out that as oxygen 

 has to be imbibed through the outer covering of the egg certain mechanical 

 influences may be at work to prevent this absorption, and so to decrease 

 or altogether cut off the necessary aeration. In some fishes the breathing 

 chamber is very large, swelling the egg to as much as double its 

 original size, and it is evident that were these eggs fixed close together 

 prior to distension, one of two things must occur, either their due expan- 

 sion would be checked by one pressing against another, or some must 

 give way. . 



I have already mentioned the eggs of the common smelt which possess 

 filaments that adhere to contiguous objects : if these filaments are torn off 

 the egg dies. Should many be placed together prior to imbibition they give 

 the appearance under the microscope after the air chamber has become 

 filled that they are honeycombed, which is due to the number of facets the 

 eggs show owing to pressure one against another. Irrespective of injury 

 from pressure, it is obvious that due aeration of the yelk will be stopped, 

 and as a result death will most probably ensue. 



Fish ova, as of the Sahnonidce and of some other forms, have been trans- 

 ported long distances, as from Europe to the Australian Colonies, Canada, 

 and the United States. The first experiment of employing ice for the 

 purpose of conveying trout and salmon eggs through the tropics is recorded 



