CHAPTER XVI 

 DEVELOPMENT 



Gametes. Eggs of Cyclostomes and Selachians. Eggs of Bony Fishes: 

 pelagic and demersal eggs. Segmentation. Yolk. Embryonic 

 development. Viviparous fishes. Connection between embryo and 

 parent. Embryonic development of a Shark. Development of Salmon. 

 Larvae and larval organs: .^mmoco^/g^; external gills. Larval Sun-fishes, 

 Deal-fishes, "Sword-fishes," Gar-fishes, etc. Larval development and 

 metamorphosis of Fresh-water Eel. Metamorphosis of Flat-fishes. 

 Hybrids. 



By the term development is understood all those changes that 

 take place in the egg or ovum from the moment it is fertilised 

 until maturity. As has been already pointed out, the process of 

 fertilisation consists in the union of the two kinds of sex-cells 

 or gametes, the ovum of the female and the spermatozoon of 

 the male. The early history of these gametes cannot be detailed 

 here, and it must suffice to point out that these are at first 

 similar to the ordinary body-cells, but later become specialised 

 for the duty of perpetuating the race. Thus, each gamete is 

 a single cell, consisting of a clear ground substance or cytoplasm 

 with its contained nucleus. The ovum is large, being distended 

 with yolk, and is immobile. The spermatozoon, on the other 

 hand, has no such food reserve, and is consequently very much 

 smaller: it consists of a head, which may be globular, elliptical, 

 wavy, or rod-Hke in shape, and is composed largely of nuclear 

 material; and a propelling, whip-like tail (Fig. 115), which 

 enables it to seek out the ovum. The latter is generally provided 

 with a minute aperture or micropyle in its surrounding mem- 

 brane, situated at one of the poles, through which the head of 

 the spermatozoon makes its entry, the tail, having played its 

 part in bringing the two together, being left outside to die. 

 The entry of the male element stimulates the ovum into 

 activity, and it begins to divide into two, four, and eight cells, 

 and so on, until gradually the embryo is built up. Another 

 important feature of fertilisation lies in the fact that the fusion 

 of the nuclei of the two gametes results in the mixing of the 

 characters of the two parents in the offspring, these characters 

 being believed to be carried by the microscopic bodies known 



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