PISCES 163 



anterior end of the primary axis and of the secondary or dorso-ven- 

 tral axis, especially of the dorsal (apical) part of this axis. The broad, 

 flat types are to be thought of as distinctly more senescent types in 

 which the whole system is more profoundly inhibited, but in which 

 the head parts and those nearest to the head recover most com- 

 pletely, while the tertiary axis, especially the apical parts of it, ex- 

 presses itself at the expense of both primary and secondary axes. 



The physiological explanation of the development of inert structures 

 like armor, heavy spines, and even of massive bones and flesh, is that 

 a lowered rate of chemical action allows of the precipitation of inert 

 compounds, which, when the rate is higher, would be used up in dy- 

 namic activity, such as rapid locomotion or rapid growth. Giant 

 size then may, on this theory, be just as definitely a product of racial 

 sensecence as is heavy armor. 



In this volume it would scarcely be appropriate to pursue this 

 theory of vertebrate morphogenesis further. It is, however, the 

 writer's opinion that the theory is as applicable to all of the verte- 

 brate classes as it is to the fishes. The parallel between the condi- 

 tions seen in the teleost fishes and in the birds is especially close and 

 would repay detailed examination. 



EGGS, REPRODUCTION, AND BREEDING HABITS OF FISHES 



The eggs of living species of fishes vary within very wide limits both 

 in size and in form, as is well shown in Fig. 96. The largest eggs are 

 those of some Elasmobranchii, which compare favorably in size with 

 those of birds. They contain a large accumulation of yolk and have 

 a hard chitinous shell. The smallest eggs are the pelagic eggs of many 

 of the teleosts, which are less than 1 mm. in diameter. Greatest 

 egg-size is found in that group which we have been considering the 

 most primitive; least egg-size, in the most highly specialized group. 

 Are we justified then in believing that the primitive fish egg was of 

 large size and that the course of evolution has been steadily in the 

 direction of a smaller and smaller size of ovum? Certain other con- 

 siderations demand a negative answer. 



Curiously enough both the largest and the smallest fish eggs are 

 decidedly telolecithal and show an advanced type of meroblastic 

 cleavage, while other groups of fishes, such as the Chondrostei, Holo- 

 stei, and the Dipneusti have eggs of medium size with considerable 

 yolk, and exhibit various degrees of incomplete holoblastic cleavage. 



