478 



NA TURE 



[September ii, 1890 



The early larval stages in the development of animals, and 

 more especially those that are marine and pelagic in habit, have 

 naturally attracted much attention, since in the absence, probably 

 inevitable, of satisfactory palajontological evidence, they afford 

 us the sole available clue to the determination of the mutual 

 relations of the large groups of animals, or of the points at 

 which these diverged from one another. 



In attempting to interpret these early ontogenetic stages as 

 actual ancestral forms, beyond which development at one time 

 did not proceed, we must keep clearly in view the various dis- 

 turbing causes which tend to falsify the ancestral record, such as 

 the influence of food yolk, or of habitat, and the tendency of 

 diminution in size to give rise to simplification of structure, a 

 point of importance if it be granted that these free larvae are of 

 smaller size than the ancestral forms to which they correspond. 



If, on the other hand, in spite of these powerful modifying 

 causes, we do find a particular larval form occurring widely and 

 in groups not very closely akin, then we certainty are justified in 

 attaching great importance to it, and in regarding it as having 

 strong claims to be accepted as ancestral for these groups. 



Concerning these larval forms, and their possible ancestral 

 significance, our knowledge has made no great advance since the 

 publication of Balfour's memorable chapter on this subject ; and 

 I propose merely to allude briefly to a few of the more striking 

 instances. 



The earliest, the most widely spread, and the most famous of 

 larval forms is the gastrula, which occurs in a simple or in a 

 modified form in some members of each of the large animal 

 groups. It is generally admitted that its significance is the same 

 in all cases, and the evidence is very strong in favour of regard- 

 ing it as a stage ancestral for all Metazoa. The difficulty arising 

 from its varying mode of development in different forms is, how- 

 ever, still unsolved, and embryologists are not yet agreed whether 

 the invaginate or delaminate form is the more primitive. In 

 favour of the former is its much wider occurrence ; in favour of 

 the latter the fact that it is easy to picture a series of stages lead- 

 ing gradually from a unicellular protozoon to a blastula, a di- 

 blastula, and ultimately a gastrula, each stage being a distinct 

 advance, both morphological and physiological, on the preceding 

 stage ; while in the case of the invaginate gastrula it is not easy 

 10 imagine any advantage resulting from a flattening or slight 

 pitting in of one part of the surface, sufficient to lead to its 

 preservation and further development. 



Of larval forms later than the gastrula, the most important by 

 far is the Pilidium larva, from which it is possible, as Balfour 

 has shown, that the slightly later Echinoderm larva, as well as 

 the widely spread Trochosphere larva, may both be derived. 

 Balfour concludes that the larval forms of all Coelomata, exclud- 

 ing the Crustacea and vertebrates, may be derived from one 

 common type, which is most nearly represented now by the 

 Pilidium larva, and which "was an organism something like a 

 Medusa, with a radial symmetry." The tendency of recent 

 phylogenetic speculations is to accept this in full, and to regard 

 as the ancestor of Turbeliarians and of all higher forms, a jelly- 

 fish or ctenophoran, which, in place of swimming freely, has 

 taken to crawling on the sea bottom. 



Of the two groups excluded above, the Crustacea and the 

 Vertebrata, the interest of the former centres in the much dis- 

 cussed problem of the significance of the Nauplius larva. There 

 is now a fairly general agreement that the primitive Crustacea 

 were types akin to the phyllopods, i.e. forms with elongated and 

 many-segmented bodies, and a large number of pairs of similar 

 appendages. If this is correct, then the explanation of the 

 Nauplius stage must be afforded by the phyllopods themselves, 

 and it is no use looking beyond this group for it. A Nauplius 

 larva occurs in other Crustacea merely because they have in- 

 herited from their phyllopod ancestors the tendency to develop 

 such a stage, and it is quite legitimate to hold that higher 

 crustaceans are descended from phyllopods, and that the 

 Nauplius represents in more or less modified form an earlier 

 ancestor of the phyllopods themselves. 



As to the Nauplius itself, the first thing to note is that, though 

 an early larval form, it cannot be a very primitive form, for it is 

 already an unmistakable crustacean ; the absence of cilia, the 

 formation of a cuticular investment, the presence of jointed 

 schizopodous limbs, together with other anatomical characters, 

 proving this point conclusively. It follows, therefore, either 

 that the earlier and more pi-imitive stages are entirely omitted 

 in the development of Crustacea, or else that the Nauplius repre- 

 sents such an early ancestral stage, with crustacean characters, 



which properly belong to a later stage, thrown back upon it and 

 precociously developed. 



The latter explanation is the one usually adopted ; but before 

 the question can be finally decided, more accurate observations 

 than we at present possess are needed concerning the stages 

 intermediate between the egg and the Nauplius. 



The absence of a heart in the Nauplius may reasonably be 

 associated with the small size of the larva. 



Concerning the larval forms of vertebrates, it is only in 

 Amphioxus and the Ascidians that the earliest larval stages are 

 free-living, independent animals. In both groups the most 

 characteristic larval stage is that in which a notochord is present, 

 and a neural tube, open in front, and communicating behind 

 through a neurenteric canal with the digestive cavity, which has 

 no other opening to the exterior. This is a very early stage, both 

 in Amphioxus and Ascidians ; but, so far as we know, it cannot 

 be compared with any invertebrate larva. It is customary, in 

 discussions on the affinities of vertebrates, to absolutely ignore 

 the vertebrate larval forms, and to assume that their peculiarities 

 are due to precocious development of vertebrate characteristics. 

 It may turn out that this view of the matter is correct ; but it has 

 certainly not yet been proved to be so, and the development of 

 both Amphioxus and Ascidians is so direct and straightforward 

 that evidence of some kind may reasonably be required before 

 accepting the doctrine that this development is entirely deceptive 

 with regard to the ancestry of vertebrates. 



Zoologists have not quite made up their minds what to do with 

 Amphioxus : apparently the most guileless of creatures, many 

 view it with the utmost suspicion, and not merely refuse to accept 

 its mute protestations of innocence, but regard and speak of it as 

 the most arcful of deceivers. Few questions at the present day 

 are in greater need of authoritative settlement. 



That ontogeny really is a repetition of phylogeny must, I 

 think, be admitted, in spite of the numerous and various ways 

 in which the ancestral history may be distorted during actual 

 development. 



Before leaving the subject, it is worth while inquiring whether 

 any explanation can be found of recapitulation, A complete 

 answer can certainly not be given at present, but a partial one 

 may, perhaps, be obtained. 



Darwin himself suggested that the clue might be found in the 

 consideration that at whatever age a variation first appears in the 

 parent, it tends to reappear at a corresponding age in the off- 

 spring ; but this must be regarded rather as a statement of the 

 fundamental fact of embryology than as an explanation of it. 



It is probably safe to assume that animals would not re- 

 capitulate unless they were compelled to do so : that there must 

 be some constraining influence at work, forcing them to repeat 

 more or less closely the ancestral stages. It is impossible, for 

 instance, to conceive what advantage it can be to a reptilian or 

 mammalian embryo to develop gill-clefts which are never used, 

 and which disappear at a slightly later stage, or how it can 

 benefit a whale, that in its embryonic condition it should 

 possess teeth which never cut the gum, and which are lost before 

 birth. 



Moreover, the history of development in different animals or 

 groups of animals, offers to us, as we have seen, a series of in- 

 genious, determined, varied, but more or less unsuccessful efforts 

 to escape from the necessity of recapitulating, and to substitute 

 for the ancestral process a more direct method. 



A further consideration of importance is that recapitulation is 

 not seen in all forms of development, but only in sexual de- 

 velopment, or, at least, only in development from the egJ. In 

 the several forms of asexual development, of which budding is 

 the most frequent and most familiar, there is no repetition of 

 ancestral phases ; neither is there in cases of regeneration of 

 lost parts, such as the tentacle of a snail, the arm of a starfish, 

 or the tail of a lizard ; in such regeneration it is not a larval 

 tentacle, or arm, or tail, that is produced, but an adult one. 



The most striking point about the development of the higher 

 animals is that they all alike commence as eggs. Looking more 

 closely at the egg and the conditions of its development, two 

 facts impress us as of special importance : first, the egg is a 

 single cell, and therefore represents morphologically the Proto- 

 zoan, or earliest ancestral phase; secondly, the egg, before it 

 can develop, must be fertilized by a spermatozoon, just as the 

 stimulus of fertilization by the pollen-grain is necessary before 

 the ovum of a plant will commence to develop into the plant- 

 embryo. 



NO. 1089, VOL. 42] 



