72 



NA TURE 



[September ii, 1890 



no fresh-water Cephalopods, and no Ascidians ; and of the 

 smaller groups of Worms, Molluscs, and Crustaceans, there are 

 many that do not occur in fresh-water. 



Direct experiment has shown that in many cases this dis- 

 tribution is not due to inability of the adult animals to live in 

 fresh water ; and the real explanation appears to be that the 

 early larval stages are unable to establish themselves under 

 such conditions. This interesting suggo'-.tion, which has been 

 worked out in detail by Prof. Sollas,i undoubtedly affords 

 an important clue. To establish itself permanently in fresh 

 water an animal must either be fixed, or else be strong enough 

 to withstand and make headway against the currents of the 

 streams or rivers it inhabits, for otherwise it will in the long run 

 be swept out to sea, and this consideration applies to larval 

 forms equally with adults. 



The majority of marine invertebrates leave the egg as minute 

 ciliated larvje ; and such larvae are quite incapable of holding 

 their own in currents of any strength. Hence, it is only forms 

 which have got rid of the free-swimming ciliated larval stage, 

 and which leave the egg of considerable size and strength, that 

 can establish themselves as fresh-water animals. This is effected 

 most readily by the acquisition of food yolk — hence the large 

 size of the eggs of fresh-water animals — and is often supple- 

 mented, as Sollas has shown, by special protective devices of a most 

 interesting nature. For this reason fresh-water forms are not so 

 well adapted as their marine allies for the study of ancestral 

 history as revealed in larval or embryonic development. 



Before leaving the question of food yolk, reference must be 

 made to the proposal of the brothers Sarasin, to regard the yolk 

 cells as forming a distinct embryonic layer, the lecithoblast," 

 distinct from the blastoderm. I do not desire to speak dogmati- 

 cally on a point the full bearings of which are not yet apparent, 

 but I venture to think that this suggestion will not commend 

 itself to embryologists. The distinction between the yolk granules 

 and the cells in which they are embedded is a real and funda- 

 mental one ; but I see no reason for regarding the yolk cells as 

 other than originally functional endoderm cells in which yolk 

 granules have accumulated to such an extent that they have in 

 extreme cases become devoted solely to the storing of food for 

 the embryo."* 



Of all the causes tending to modify development, tending to 

 obscure or falsify the ancestral record, food yolk is the most 

 frequent and the most important ; its position in the egg deter- 

 mines the mode of segmentation ; and its relative abundance 

 affects profoundly the entire embryonic history, and decides at 

 what particular stage, and of what size and form, the embryo 

 shall hatch. 



The loss of food yolk is another disturbing element, the full 

 influence of which is as yet imperfectly understood, but the pos- 

 sibility of which must be always kept in mind. It is best 

 known in the case of mammals, where it has led to apparent, 

 though very deceptive, simplification of development ; and it 

 will probably not be until the embryology of the large-yolked 

 Monotremes is at length described, that we shall fully under- 

 stand the formation of the germinal layers in the higher placental 

 mammals. 



Amongst invertebrates we know but little as yet concerning 

 the effects of loss of food yolk. It has been suggested that the 

 extraordinary nature of the segmentation of the egg of Peripatus 

 capensis, made known to us through Mr. Sedgwick's admirable 

 researches, may be due to loss of food yolk ; a suggestion which 

 receives support from the long duration of uterine development 

 in this case. 



Our knowledge is very imperfect as to the ease with which 

 food yolk may be acquired or lost ; but until our information is 

 more precise on this point, it seems unwise to lay much stress on 

 suggested pedigrees which involve great and frequent alternations 

 in the amount of food yolk present. 



Of causes other than food yolk, or only indirectly connected 

 with it, which tend to falsify the ancestral history, many are now 

 known, but time will only permit me to notice the more im- 

 portant. These are distortion, whether in time or space ; sudden 

 or violent metamorphosis ; a series of modifications, due chiefly 



' " On the Origin of Freshwater Faunas," Scientific Transactions of the 

 Royal Dublin Society, vol. iii. Ser. ii. i886. 



* " Ergebnisse naturwissenschaftlicher Forschungen auf Ceylon," Bd. ii. 

 Heft, iii., 1889. 



3 Cf. E. B. Wilson, " The Development of Renilla," Phil. Trans., 1883, p. 

 755- 



NO. 1089, VOL. 42] 



to mechanical causes, and which may be spoken of as develop- 

 mental conveniences ; the important question of variability in 

 development ; and finally the great problem of degeneration. 



Concerning distortions in time, all embryologists have 

 noticed the tendency to anticipation or precocious de- 

 velopment of characters which really belong to a later stage 

 in the pedigree. The early attainment of the cyclical form 

 in the shell of Orbitolites complanata is a case in point ; and 

 Wiirtemberger has specially noticed this tendency in Ammonites. 

 Many early larvae show it markedly, the explanation in this case 

 being that it is essential for them to hatch in a condition capable 

 of independent existence, i.e. capable, at any rate, of obtaining 

 and digesting their own food. 



Anachronisms, or actual reversal of the historical order of 

 development of organs or parts, occur frequently. Thus the 

 joint surfaces of bones acquire their characteristic curvatures 

 before movement of one part on another is effected, and before 

 even the joint cavities are formed. 



Another good example is afforded by the development of the 

 mesenterial filaments in Alcyonarians. Wilson has shown in 

 the case of Renilla that in the development of an embryo from 

 the egg the six endodermal filaments appear first, and the two 

 long ectodermal filaments at a later period ; but that in the 

 formation of a bud this, order of development is reversed, the 

 ectodermal filaments being the first formed. He suggests, in 

 explanation, that, as the endodermal filaments are the digestive 

 organs, it is of primary importance to the free embryo that they 

 should be formed quickly. The long ectodermal filaments are 

 chiefly concerned with maintaining currents of water through 

 the colony ; in bud-development they appear before the endo- 

 dermal filaments, because they enable the bud during its early 

 stages to draw nutrient matter from the body fluid of the parent ; 

 while the endodermal filaments cannot come into use until the 

 bud has acquired both mouth and tentacles. 



The completion of the ventricular septum in the heart of higher 

 vertebrates before the auricular septum is a well-known anachron- 

 ism, and every embryologist could readily furnish many other 

 cases. 



A curious instance is afforded by the development of the teeth 

 in mammals, if recent suggestions as to the origin of the milk 

 dentition are confirmed, and the milk dentition prove to be a 

 more recent acquisition than the permanent one.^ 



But the most important cases in reference to distortion in 

 time concern the reproductive organs. If development were a 

 strict and correct recapitulation of ancestral history, then each 

 stage would possess reproductive organs in a mature condition. 

 This is not the case, and it is clearly of the greatest importance 

 that it should not be. It is true that the first commencement of 

 the reproductive organs may occur at a very early larval stage, 

 or even that the very first step in development may be a division 

 of the egg into somatic and reproductive cells ; and it is possible 

 that, as maintained by Weismann, this latter condition is a 

 primitive one. Still, even in these cases the reproductive organs 

 merely commence their development at these early stages, and 

 do not become functional itntil the animal is adult. 



Exceptionally in certain animals, and as a normal occurrence 

 in others, precocious maturation of the reproductive organs 

 takes place, and a larval form becomes capable of sexual re- 

 production. This may lead to arrest of development, either at 

 a late larval period, as in the Axolotl, or at successively earlier 

 and earlier stage.";, as in the gonophores of the Hydromedusse, 

 until finally the extreme condition seen in Hydra is produced. 



We do not know the causes that determine the period, 

 whether late or early, at which the reproductive organs ripen, 

 but the question is one of great interest and importance and 

 deserves careful attention. The suggestion has been made that 

 entire groups of animals, such as the Mesozoa, are merely larvse, 

 arrested through such precocious acquiring of reproductive 

 power, and it is conceivable that this may be the case. Mesozoa 

 are a puzzling group in which the life-history, though known 

 with tolerable completeness, has as yet given us no reliable 

 clue concerning their affinities to other animals — a tantalizing 

 distinction that is shared with them by Rotifers and Polyzoa. 



Distortion of a curious kind is seen in cases of abrupt 

 metamorphosis, where, as in the case of many Echinoderms, of 

 Phoronis, and of the metabolic insects, the larva and the adult 

 differ greatly in form, habits, mode of life, and very usually in 



^ Cf. Thomas Oldfield, " On the Homologies and Succession of the Teeth 

 in the Dasyuridae, with an attempt to trace the history of the evolution of 

 the Mammalian teeth in general," Phil. Trans., 1887. 



