September ii, 1890] 



NATURE 



471 



prevent development from being a strict recapitulation of an- 

 cestral characters, the mode in which these came about, and the 

 influence which they respectively exert, are matters which are 

 greatly exercising embryologists, and the attempt to determine 

 which has as yet met with only partial success. 



Tlie most potent and the most widely spread of these dis- 

 turbing causes arise from the necessity of supplying the embryo 

 with nutriment. This acts in two ways. If the amount of 

 nutritive matter within the egg is small, then the young animal 

 must hatch early, and in a condition in which it is able to obtain 

 food for itself. In such cases there is of necessity a long period 

 of larval life, during which natural selection may act so as to 

 introduce modifications of the ancestral history, spurious addi- 

 tions to the text. 



If, on the other hand, the egg contain within itself a con- 

 siderable quantity of nutrient matter, then the period of hatching 

 can be postponed until this nutrient matter has been used up. 

 The consequence is that the embryo hatches at a much later 

 stage of its development, and if the amount of food-material is 

 sufficient, may even leave the egg in the form of the parent. 

 In such cases the earlier developmental phases are often greatly 

 condensed and abbreviated ; and as the embryo does not lead a 

 free existence, and has no need to exert itself to obtain food, it 

 commonly happens that these stages _are passed through in a 

 very modified form, the embryo being, as in a four-day chick, in a 

 condition in which it is clearly incapable of independent 

 existence. 



The nutrition of the embryo prior to hatching is most usually 

 effected by granules of nutrient matter, known as food yolk, and 

 embedded in the protoplasm of the egg itself; and it is on the 

 relative abundance of these granules that the size of the egg 

 chiefly depends. 



Large size of eggs implies diminution of number of the egg?, 

 and hence of the offspring ; aud it can be well understood that 

 while some species derive advantage in the struggle for existence 

 by producing the maximum number of young, to others it is of 

 greater importance that the young on hatching should be of con- 

 siderable size and strength, and able to begin the world on their 

 own account. In other words, some animals may gain by 

 producing a large number of small eggs, others by producing 

 a smaller number of eggs of larger size — i.e. provided with more 

 food yolk. 



The immediate effect of a large amount of food yolk is to 

 mechanically retard the processes of development ; the ultimate 

 result is to greatly shorten the time occupied by development. 

 This apparent paradox is readily explained, A small egg, such 

 as that of Amphioxus, starts its development rapidly, and in 

 about eighteen hours gives rise to a free-swimming larva, capable 

 of independent existence, with a digestive cavity and nervous 

 system already formed ; while a large egg, like that of the hen, 

 hampered by the great mass of food yolk by which it is dis- 

 tended, has, in the same time, made but very slight progress. 



From this time, however, other considerations begin to tell. 

 Amphioxus has been able to make this rapid start owing to its 

 relative freedom from food yolk. This freedom now becomes a 

 retarding influence, for the larva, containing within itself but a 

 very scanty supply of nutriment, must devote much of its energies 

 to hunting for, and to digesting its food, and hence its further 

 development will proceed more slowly. 



The chick embryo, on the other hand, has an abundant supply 

 of food in the egg itself; it has no occasion to spend time 

 searching for food, but can devote its whole energies to the 

 further stages of its development. Hence, except in the earliest 

 stages, the chick develops more rapidly than Amphioxus, and 

 attains its adult form in a much shorter time. 



The tendency of abundant food yolk to lead to shortening or 

 abbreviation of the ancestral history, and even to the entire 

 omission of important stages, is well known. The embryo of 

 forms well provided with yolk takes short cuts in its develop- 

 ment, jumps from branch to branch of its genealogical tree, 

 instead of climbing steadily upwards. 



Thus the little West Indian frog, Hylodes, produces eggs 

 which contain a larger amount of food yolk than those of the 

 common English frog. The young Hylodes is consequently 

 enabled to pass through the tadpole stage before hatching — to 

 attain the form of a frog before leaving the egg ; and the tadpole 

 stage is only imperfectly recapitulated, the formation of gills, fjr 

 instance, being entirely omitted. 



The influence of food yolk on the development of animals is 

 closely analogous to that of capital in human undertakings. A 



NO. 1089, VOL. 42] 



new industry, for example that of pen-making, has often been 

 started by a man working by hand and alone, making and 

 selling his own wares ; if he succeed in the struggle for exist- 

 ence, it soon becomes necessary for him to call in others to assist 

 him, and to subdivide the work ; hand labour is soon super- 

 seded by machines, involving further differentiation of labour ; 

 the earlier machines are replaced by more perfect and more 

 costly ones ; factories are built, agents engaged, and, in the end, 

 a whole army of work-people employed. In later times a man 

 commencing business with very limited means will start at the 

 same level as the original founder, and will have to work his 

 way upwards through much the same stages, i.e. will repeat the 

 pedigree of the industry. The capitalist, on the other hand, is 

 enabled, like Hylodes, to omit these earlier stages, and, after a 

 brief period of incubation, to start business with large factories 

 equipped with the most recent appliances, and with a complete 

 staff of work-people, i.e. to spring into existence fully fledged. 



There is no doubt that abundance of food-yolk is a direct and 

 very frequent cause of the omission of ancestral stages from in- 

 dividual development ; but it must not be viewed as a sole cause. 

 It is quite impossible that any animal, except perhaps in the 

 lowest zoological groups, should repeat all the ancestral stages 

 in the history of the race ; the limits of time available for 

 individual development will not permit this. There is a 

 tendency in all animals towards condensation of the ancestral 

 history — towards striking a direct path from the egg to the adult. 

 This tendency is best marked in the higher, the more com- 

 plicated members of a group ; i.e. in those which have a longer 

 and more tortuous pedigree ; and though greatly strengthened 

 by the presence of food yolk in the egg, is apparently not due to 

 this in the first instance. 



Thus the simpler forms of Orbitolites, as 0. tenuissima, repeat 

 in their development all the stages leading from a spiral to a 

 cyclical shell ; but in the more complicated species, as Dr. 

 Carpenter has pointed out, there is a tendency towards precocious 

 development of the adult characters, the earlier stages being 

 hurried over in a modified form ; while in the most complex ex- 

 amples, as in O. complanata, the earlier spiral stages may be 

 entnely omitted, the shell acquiring almost from its earliest com- 

 mencement the cyclical mode of growth. There is no question 

 here of relative abundance of food yolk, but merely of early or 

 precocious appearance of adult characters. 



The question of the relations and influence of food yolk, in- 

 volving as it does the larger or smaller size of the egg, is, 

 however, merely a special side of the much wider question of the 

 nutrition of the embryo, one of the most potent of the disturbing 

 elements affecting development. 



Speaking generally, we may say that large eggs are more often 

 met with in the higher than the lower groups of animals. Birds 

 and reptiles are cases in point, and, if mammals do not now 

 produce large eggs, it is because a more direct and more efficient 

 mode of nourishing the young by the placenta has been acquired 

 by the higher forms, and has replaced the food yolk that was 

 formerly present, and is now retained in quantity by Mono- 

 tremes alone. Molluscs afford another good example, the eggs 

 of Cephalopoda being of larger size than those of the less 

 highly organized groups. 



The large size of the eggs of Elasmobranchs, and perhaps 

 that of Cephalopods also, may possibly be associated with the 

 carnivorous habits of the animals ; for it is of importance that 

 forms which prey on other animals should hatch of considerable 

 size and strength. 



The influence of habitat must also be considered. It has long 

 been noticed as a general rule that marine animals lay small 

 eggs, while their fresh-water allies have eggs of much larger 

 size. The eggs of the salmon or trout are much larger than 

 those of the cod or herring ; and the crayfish, though only a 

 quarter the length of a lobster, lay eggs of actually larger 

 size. 



This larger size of the eggs of fresh-water forms appears to be 

 dependent on the nature of the environment to which they are 

 exposed. Considering the geological instability of the land as 

 compared with the ocean, there can be no doubt that the fresh- 

 water fauna is, speaking generally, derived from the marine 

 fauna ; and the great problem with regard to fresh-water life is 

 to explain why it is that fo many groups of animals which 

 flourish abundantly in the sea should have failed to establish 

 themselves in fresh water. Sponges and Coelenterates abound 

 in the sea, but their fresh-water representatives are extre nely 

 few in number ; Echinoderms are exclusively marine : there are 



