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Show EMBRYOLOGY. [CnAP. XIII. 382 . 1 b of some vertebrate anima , forgotten to ticket the e~ r~o b that of a mammal, bird, he cannot now tell ·w~et er 1 1 . e of moths flies, beetles, or repti.l e. Th e ..v,,.. el 'IDifonn alh v mm ore closel' y t h an d o the &c resemble each othehr roue of larvm the embryos are 1na.t,u re 1· nsects,· b u t in td ae tceadse f or specia'l h·n es of 11· i1' e .. A active, and have been ~byronic resmnblance, SOinetlnles trace of the law of e . 3 thus birds of the same gcnll:s, lasts till a rather !ate age . . often resemble each other In and of closely alhed gefm a, e . as we see in the spotted their first and second p umag 'In the cat tribe, most of feathers in the thrusk grou1ted in lines . and stripes can the species a:e ~trip_e or ~~o the whelp of the lion. W. e be plainly distlnguishe·~lu see something of this kind Ill occasionally though rai Y. leaves of the ulex or furze, plants: th.us the embryfn~he phyllodineous acacias, ar.e panindn atthee orf idr~iVt I· ale ea dvfI· ~ eo the ordinary leaves of the legu.m i-nosre. . hich the embryos of w1de- . The points o.f structu:eth~ "':arne class resem~l~ each ly different ammals,d~~ct relation to their conditi~ns of other, often have no t .£ 'nstance suppose th~t In the existence. We canno ' or h eculfar loop-like course of embryos of the vertebrata th.elp l'ts are related to similar the arteri.e s near the branc Ia sm Ia l which is noun.s h ed I.n conditions,-i~ the yhung. :~he egg of the bird which is the womb of Its mot ~r, Ih ' wn of a frog under water. hatched in a nest, and In t t e sh:lieve in such a relation, We have no more ~eason ~ the same bones in the hand than we have to beheve th~ fi f a porpoise are related of a man, wing ?f. a bat, .af£ n No one will s~ppose that to similar conditiOns of If" r or the spots on the the stripes on. the whe}p a~ ~s:o~~ these animals, or are young blackbird, ~r~ o y h" h the are exposed. related to the condition.s ~·ffi I~t whrn an animal during The case, however, IS . I ere l·s active and has to f •t bryonic career ' any part o I s em . d of activity may come on provide for itself. ':f'he per;o henever it conles on, the earlier or later in life; b':t ':onditions of life is just as paedrafpetcatt ioannd o fa st hbee aluart.iv fua ltoa s1 ~n the adult anin1a1. From 1 CH.AP. XIII.] EMBRYOLOGY. 383 sucJ.t special adapta.tions,. the s~milarit y of the larvre or active embryos of allied animals 1s sometimes much obscured; and cases could be given of the larvre of two species, or of two groups of species, differing quite as much, or even more, from each other than do their adult parents. In most cases, however, the larvre, though active, still obey more or less close} y the law of c?rnmon em brJ:onic res em bl:m ce. ~irripedes afford a good Instance of th1s: even the Illustrious Cuvier did not perceive that a barnacle was, as it certainly is, a crustacean ; but a glance at the larva shows this to be the case in an unmistakeable manner. So again the two main divisions of cirripedes, the pedunculated and sessile, which differ widely in external appearance, have larvre in all their several stages barely distinguishable. The embryo in the course of development generally riseS in organisation: I use this expression, though I am aware that it is hardly possible to define clearly what is meant by the organisation being higher or lower. But no one probably will dispute that tlie butterfly is higher than the caterpillar. In some cases, however, the mature animal is generally considered as lower in the scale than the larva, as with certain parasitic crustaceans. To refer once again to cirri pedes : the larvre in the first stage have three pairs of legs, a very simple single eye, and a proboscitbrmed mouth, with which they reed largely, for they increase much in size. In the second stage, answering to the chrysalis stage of butterflies, they have six pairs of beautifully constructed nata tory legs, a pair of magnificent compound eyes, and extremely complex antennre ; but they have a closed and imperfect mouth, and cannot feed: their function at this stage is, to search by their well-developed organs of sense, and to reach by their active powers of swimming, a proper place on which to become attached and to undergo their fi.na} metamorphosis. When this is completed they are fixed for life: their legs are now converted into prehensile organs ; they again obtain a well-constructed mouth; but they have no ante:imre, and their two eyes are now reconverted into a minute, single, and very simple eye-spot. In this last and complete state, clrripedes may be considered as either more highly or more |
