PISCES 165 



The eggs of Amia (a holostean ganoid) show perhaps the nearest ap- 

 proach to complete holoblastic cleavage. In that species the clea- 

 vage furrows run meridionally practically from pole to pole, but the 

 equatorial cleavage furrows in the vegetal region are very slow in ap- 

 pearing, giving to the early cleavage stages quite a meroblastic ap- 

 pearance. The egg of Lepidosteus, another holostean ganoid, shows 

 an incomplete cleavage of the vegetal pole, furrows running only about 

 to the equator or a little beyond. Subsequently, however, the fur- 

 rows deepen and the entire egg is broken up by cleavage into true cells. 

 Transitional cases of this sort indicate that the primitive pro-fishes 

 had small isolecithal eggs, Uke that of Anaphioxus, and that, when the 

 first fishes invaded the region of the rapid currents, they had to lay 

 their eggs on the bottom, probably attached to vegetation, or in a 

 prepared nest of some sort. The primitive eggs of the ganoids and 

 Dipnoi are covered with a coating of sticky jelly and are laid on the 

 bottom. Accompanying this habit there was an increase in yolk 

 accumulation and a tendency toward the telolecithal condition and 

 meroblastic cleavage. That the modern teleosts are all meroblastic, 

 even when the eggs are very small and have only a minimum amount 

 of yolk, is doubtless due to their derivation from ancestors that 

 had a very large yolk, larger probably than that of the amphi- 

 bian eggs of to day. Although pelagic fish eggs are small enough 

 readily to admit holoblastic cleavage, they retain the extreme 

 type of meroblastic cleavage characteristic of their large-yolked 

 ancestors. 



The eggs of elasmobranchs, the largest of fish eggs, are laid singly 

 or in pairs at varying intervals over a long breeding season. In some 

 teleosts the number of tiny eggs laid by a single female in the course 

 of a short breeding season of a few days reaches into the mUKons, the 

 extreme case on record being that of a fifty-four pound Ling that laid 

 over twenty-eight milHon eggs. Between these two extremes there 

 are all intergrades. When the eggs are large and few are laid, chance 

 methods of fertilization cannot be relied upon. It is therefore sig- 

 nificant that in the elasmobranchs a sort of copulation occurs during 

 which the male, by the use of claspers, introduces milt into the ovi- 

 ducts of the female and thus accomplishes internal insemination of 

 the eggs. In some cases gestation is also internal, but this is doubt- 

 less a secondary adaptation. When, on the other hand, the eggs are 

 small and extremely numerous, fertihzation is external and haphazard. 



