PISCES 



167 



mLh" 



to the promiscuous fertilization of eggs when numbers of males and 

 females spawn in schools. 



It seems highly probaljlc 

 that the habit of nesting, 

 such as is seen in Amia and 

 the Dipneusti is close to 

 the primitive condition and 

 that there has been a spe- 

 cialization, in one direction, 

 of few large eggs and in- 

 ternal fertilization, and in 

 the other direction, as in 

 fishes living in open waters, 

 of an increase in numbers 

 and decrease in size of eggs 

 and consequent haphazard 

 fei-tilization. For primitive 

 breeding habits therefore, I 

 would be inclined to look 

 to Amio, and the Dipneusti, 

 where the conditions are 

 not so very different from 

 those in Amphibia. 



The fundamental embry- 

 ological changes following 

 holoblastic and those fol- 

 lowing meroblastic cleav- 

 age are decidedly different, 

 and an example of each, 

 chosen from the fishes, maj^ 

 serve to illustrate some 

 important developmental 

 principles. 



The case of Ncocerato- 

 dus, the most primitive of 

 the modern Dipneusti may 

 be taken to illustrate holo- 

 blastic cleavage and its ap- 

 propriate tj-pe of gastrulation. 



J>'* 



y.s 



Fig. 9S. — Development of a teleost (Sal- 

 mon.) A, four-cell stage; B, multicellular 

 blastoderm (an early blastula stage) ; C, blas- 

 toderm (bl.) beginning to overgrow the yolk; 

 D, gastrulation beginning and germ-ring (r) 

 formed; E, and F, embryo formed by concres- 

 cence of germ ring and germ ring one-third 

 and two-thirds around yolk; G and H, early 

 and advanced embryos (einb.) with lilastoderm 

 surrounding yolk-sac (,;;. s.); I, just hatched 

 larva wth remains of yolk-sac (ij- s.). (From 

 Parker and Haswell, after Henneguy.) 



As described by Semon for the rela- 



