i INTRODUCTION 19 



in development, and turning to the developmental process itself, we 

 find that every animal passes through two phases in its passage to 

 the adult form. In the first of these phases the young organism is 

 sheltered from external influences either by an egg-shell or by the 

 body of the parent, or by both. Its food is supplied in the first 

 instance by the deuteroplasm or food-yolk embedded in its own 

 substance, supplemented in many cases by maternal secretions. In 

 the second phase the young animal, after escaping from its shelter, 

 is obliged to seek its own food, but it never is exactly like the 

 parent, and some time elapses and considerable growth takes place 

 before it attains the adult form. In the first phase it is known as 

 an embryo, in the second as a larva. 



In different life-histories the embryonic and larval phases vary 

 enormously in their relative lengths. Sometimes, as in Echinoder- 

 mata, the young organism is thrown on its own resources at an extra- 

 ordinary early period of development, a very long larval life 

 ensues, and the young animal is at first utterly unlike the parent. 

 In other cases, as in the case of Man, when the young organism leaves 

 the parent it resembles the adult in all essential features. In 

 such cases it is customary to say that the larval stage has been 

 omitted. But the practice of confining the term " larva " to cases 

 where the free-living young differ markedly from the parent is not 

 logical. The baby is very different from a full grown man, and so is 

 the young child ; for example, the proportions of the limbs are 

 markedly different, and a continuous series of stages can be found 

 between differences of this kind and differences as great as those 

 which divide the larva from the adult Echinoderm. We may assert 

 with confidence that all animals pass through first an embryonic and 

 then a larval phase of development, and nothing is gained by calling 

 a larval stage which closely resembles the adult a " brephic " or 

 " neanic " stage, as was originally suggested by Hyatt and has been 

 adopted by some English zoologists. 



Of course, development goes on throughout both embryonic and 

 larval phases, and the form of the organism is constantly changing ; 

 but there is one great group of animals, the Arthropoda, in which the 

 organism is confined within a rigid envelope derived from its own 

 secretions, and in this case, for a definite period of time, the external 

 form appears to be unchanged; only when the dead envelope is burst 

 and cast off, do the internal changes which have been going on 

 manifest themselves in a change of form. Hence we can appropri- 

 ately speak of these periods of fixity of form as a series of larval 

 stages, or, as Sharpe (1895) has suggested, we might call them 

 instars. In other groups of the animal kingdom where this 

 rigidity of form does not obtain, there are, nevertheless, crises in 

 development when great changes take place very rapidly, accom- 

 panied in many cases by the casting off of portions of the body 

 of the larva. These crises are termed metamorphoses, and the 

 stages, of quiet growth preceding and succeeding them are looked 



