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Show 384 EMBRYOLOGY. (CHAP. XIII. lowly organised than they were in the larval condition. But in some genera the larvre become developed either into hermaphrodites having the ordinary structure, or into what I have called complemental males : and in the latter, the development has assuredly been retrograde ; for the male is a mere sack, which lives for a · short time, and is destitute of mouth, stomach, or other organ of importance, excep_ting for reproduction. We are so much accustomed to see di:fi'erences in struc-ture between the embryo and the adult, and likewise a close similarity in the embryos of widely different animals within the same class, that we might be led to look at these facts as necessarily contingent in some manner on growt~. But there is no obv~ous reason why, for instance, the wing of a bat, or the fin of a porpoise, should not have been sketched out with all the parts in proper proportion, as soon as any structure beca1ne visible in the embryo. And in some whole groups of animals and in certain members of other groups, the embryo does not at any period differ widely from the adult: thus Owen has remarked in regard to cuttle-fish, "there is no metainorphosis ; the cephalopodic character is manifested long before the parts of the embryo are completed;" and again in spiders, " there is nothing worthy to be called a metamorphosis." The larvre of insects, whether adapted to the most d~verse and active habi~s, or quit.e inactive, being fed by thmr parents or placed In the midst of proper nutriment, yet nearly all pass through a similar worm-like stage of development ; but in some few cases, as in that of AJ>his, if we look to the admirable drawings by Professor Huxley of the development of this insect, we see no trace of the vermiform stage. .How, then, can we explain these several facts in em-bryology,-- namely the very general, but not universal difference in structure between the embryo and the adult; -of parts in the same individual embryo, which ultimately become very unlike and serve for diverse purposes, being at this early period of growth alike ;-of" embryos of different species within the same class, generally, but not universally, resembling each other ;-of the structure CHAP. XIII.] EMBRYOLOGY. of the embryo not b . 385 of ~xisten?e, except e~We~l~~ly related to its condition period of life active and ha e cmb.ryo becomes at s e?lbryo apparently havin.,. s to p;ov1de for itself. of ~fY tion than the mature . 5 sometnnes a hi her 0 ' . 1 e I believe that all thes~n}m~l, into which it ~s dev~fam~ai on the.view of descent w~fus can.be explained as folfe . It 18 commonly as 1 dmodification. ' ows, often affecting the emsbume 'perhaps frmn monstr 't' }' ht · · ryo at a OS1 leS ~ Ig. variations necessarily a very early period, that period. But we have littl . ppear at an equally e I t~e evidence rather points e th:ldence on this head-inda:e~ rlO.us that breeders of cattle other way ; for it is notoan~ malls,hcannot positively tell ~r~~s, and various fancy anima as been born what ·' n 1 ~orne tirno after the mate~y turn out. We' see thi!s m~nts ~r form will ultidren' we cannot always tell tlainly In our own chil-tall or short or what 't :V ether the child ·will b question is ndt, at what \~?Jec1s~ features .will be. Th: caused, but at what p~rio~ ?{ l.Ife any variation has been cause may have acted and I I l.s fully displayed. The ;:ven before the embr,Yo is fo beJ1~Ve generally has acted e due to the male and fe~~e ' and the variation may !h;'l affected by the conditio!setose~!l'lh el.ements having eir ancestors, have been w IC either parent or effect ~hus caused at a ver r e~posed: Nevertheless ' an formation of the embr o J eady penod, even before th h hereditary diseas!, ~hl~h a~pear lat~ in life ; as whe~ d:s t?een cl ommunicated to the Jlea~s 1nf old age alone c lVe e ement of one ar spring .rom the re ro: ~~%s ohf cross-bred cattlepha!~\ Or _;;gain, as when ~be e orns of either ar een auected by the sha e ~~~nimal, as long ~8 i~U:~m~~~ ~h~telfare of a ve~y b . e egg, or as lon 't , 1~ 1 smother's womb r.1tB parent, it must bg as~ 1S ~OUrlsbed and rotecte' ? 1~B characters are fulle ;_~1te .ummportant whelher mos~ ::b~e. ~t would not slrnifyq~Ire~ a little earlier or later t~Ined Its food best by h '· or Instance, to a bird which nito 1t as sume d a beak of tha'v.1 n0;:t; a. lo ng b ea1r , whether or was fed by its parents. l!EartiCular length, as Ion<>' as 17 . ence, I conclude, that [t if |