on the relations of man to the lower animals-第2部分

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result　as　that　at　which　a　human　artificer　arrives　by　his　operations　in
a　brickfield。　　She　takes　the　rough　plastic　material　of　the　yelk　and
breaks　it　up　into　well…shaped　tolerably　even…sized　masses；　handy　for
building　up　into　any　part　of　the　living　edifice。

FIG。　13。Earliest　rudiment　of　the　Dog。　　B。　Rudiment　further　advanced；
showing　the　foundations　of　the　head；　tail；　and　vertebral　column。　　C。
The　very　young　puppy；　with　attached　ends　of　the　yelk…sac　and　allantois；
and　invested　in　the　amnion。

Next；　the　mass　of　organic　bricks；　or　'cells'　as　they　are　technically
called；　thus　formed；　acquires　an　orderly　arrangement；　becoming
converted　into　a　hollow　spheroid　with　double　walls。　　Then；　upon　one
side　of　this　spheroid；　appears　a　thickening；　and；　by　and　bye；　in　the
centre　of　the　area　of　thickening；　a　straight　shallow　groove　（Fig。　13；
A）　marks　the　central　line　of　the　edifice　which　is　to　be　raised；　or；　in
other　words；　indicates　the　position　of　the　middle　line　of　the　body　of
the　future　dog。　　The　substance　bounding　the　groove　on　each　side　next
rises　up　into　a　fold；　the　rudiment　of　the　side　wall　of　that　long
cavity；　which　will　eventually　lodge　the　spinal　marrow　and　the　brain；
and　in　the　floor　of　this　chamber　appears　a　solid　cellular　cord；　the
so…called　'notochord。'　　One　end　of　the　inclosed　cavity　dilates　to　form
the　head　（Fig。　13；　B）；　the　other　remains　narrow；　and　eventually　becomes
the　tail；　the　side　walls　of　the　body　are　fashioned　out　of　the　downward
continuation　of　the　walls　of　the　groove；　and　from　them；　by　and　bye；
grow　out　little　buds　which；　by　degrees；　assume　the　shape　of　limbs。
Watching　the　fashioning　process　stage　by　stage；　one　is　forcibly
reminded　of　the　modeller　in　clay。　　Every　part；　every　organ；　is　at
first；　as　it　were；　pinched　up　rudely；　and　sketched　out　in　the　rough；
then　shaped　more　accurately；　and　only；　at　last；　receives　the　touches
which　stamp　its　final　character。

Thus；　at　length；　the　young　puppy　assumes　such　a　form　as　is　shown　in　Fig。
13；　C。　　In　this　condition　it　has　a　disproportionately　large　head；　as
dissimilar　to　that　of　a　dog　as　the　bud…like　limbs　are　unlike　his　legs。

The　remains　of　the　yelk；　which　have　not　yet　been　applied　to　the
nutrition　and　growth　of　the　young　animal；　are　contained　in　a　sac
attached　to　the　rudimentary　intestine；　and　termed　the　yelk　sac；　or
'umbilical　vesicle。'　Two　membranous　bags；　intended　to　subserve
respectively　the　protection　and　nutrition　of　the　young　creature；　have
been　developed　from　the　skin　and　from　the　under　and　hinder　surface　of
the　body；　the　former；　the　so…called　'amnion；'　is　a　sac　filled　with
fluid；　which　invests　the　whole　body　of　the　embryo；　and　plays　the　part　of
a　sort　of　water…bed　for　it；　the　other；　termed　the　'allantois；'　grows
out；　loaded　with　blood…vessels；　from　the　ventral　region；　and　eventually
applying　itself　to　the　walls　of　the　cavity；　in　which　the　developing
organism　is　contained；　enables　these　vessels　to　become　the　channel　by
which　the　stream　of　nutriment；　required　to　supply　the　wants　of　the
offspring；　is　furnished　to　it　by　the　parent。

The　structure　which　is　developed　by　the　interlacement　of　the　vessels　of
the　offspring　with　those　of　the　parent；　and　by　means　of　which　the
former　is　enabled　to　receive　nourishment　and　to　get　rid　of　effete
matters；　is　termed　the　'Placenta。'

It　would　be　tedious；　and　it　is　unnecessary　for　my　present　purpose；　to
trace　the　process　of　development　further；　suffice　it　to　say；　that；　by　a
long　and　gradual　series　of　changes；　the　rudiment　here　depicted　and
described　becomes　a　puppy；　is　born；　and　then；　by　still　slower　and　less
perceptible　steps；　passes　into　the　adult　Dog。

There　is　not　much　apparent　resemblance　between　a　barndoor　Fowl　and　the
Dog　who　protects　the　farm…yard。　　Nevertheless　the　student　of
development　finds；　not　only　that　the　chick　commences　its　existence　as
an　egg；　primarily　identical；　in　all　essential　respects；　with　that　of
the　Dog；　but　that　the　yelk　of　this　egg　undergoes　divisionthat　the
primitive　groove　arises；　and　that　the　contiguous　parts　of　the　germ　are
fashioned；　by　precisely　similar　methods；　into　a　young　chick；　which；　at
one　stage　of　its　existence；　is　so　like　the　nascent　Dog；　that　ordinary
inspection　would　hardly　distinguish　the　two。

The　history　of　the　development　of　any　other　vertebrate　animal；　Lizard；
Snake；　Frog；　or　Fish；　tells　the　same　story。　　There　is　always；　to　begin
with；　an　egg　having　the　same　essential　structure　as　that　of　the
Dog：the　yelk　of　that　egg　always　undergoes　division；　or　'segmentation'
as　it　is　often　called：　the　ultimate　products　of　that　segmentation
constitute　the　building　materials　for　the　body　of　the　young　animal；　and
this　is　built　up　round　a　primitive　groove；　in　the　floor　of　which　a
notochord　is　developed。　　Furthermore；　there　is　a　period　in　which　the
young　of　all　these　animals　resemble　one　another；　not　merely　in　outward
form；　but　in　all　essentials　of　structure；　so　closely；　that　the
differences　between　them　are　inconsiderable；　while；　in　their　subsequent
course；　they　diverge　more　and　more　widely　from　one　another。　　And　it　is　a
general　law；　that；　the　more　closely　any　animals　resemble　one　another　in
adult　structure；　the　longer　and　the　more　intimately　do　their　embryos
resemble　one　another：　so　that；　for　example；　the　embryos　of　a　Snake　and
of　a　Lizard　remain　like　one　another　longer　than　do　those　of　a　Snake　and
of　a　Bird；　and　the　embryo　of　a　Dog　and　of　a　Cat　remain　like　one　another
for　a　far　longer　period　than　do　those　of　a　Dog　and　a　Bird；　or　of　a　Dog
and　an　Opossum；　or　even　than　those　of　a　Dog　and　a　Monkey。

Thus　the　study　of　development　affords　a　clear　test　of　closeness　of
structural　affinity；　and　one　turns　with　impatience　to　inquire　what
results　are　yielded　by　the　study　of　the　development　of　Man。　　Is　he
something　apart？　　Does　he　originate　in　a　totally　different　way　from　Dog；
Bird；　Frog；　and　Fish；　thus　justifying　those　who　assert　him　to　have　no
place　in　nature　and　no　real　affinity　with　the　lower　world　of　animal
life？　　Or　does　he　originate　in　a　similar　germ；　pass　through　the　same
slow　and　gradually　progressive　modifications；depend　on　the　same
contrivances　for　protection　and　nutrition；　and　finally　enter　the　world
by　the　help　of　the　same　mechanism？　　The　reply　is　not　doubtful　for　a
moment；　and　has　not　been　doubtful　any　time　these　thirty　years。　　Without
question；　the　mode　of　origin　and　the　early　stages　of　the　development　of
man　are　identical　with　those　of　the　animals　immediately　below　him　in　the
scale：without　a　doubt；　in　these　respects；　he　is　far　nearer　the　Apes；
than　the　Apes　are　to　the　Dog。

The　Human　ovum　is　about　l/125　of　an　inch　in　diameter；　and　might　be
described　in　the　same　terms　as　that　of　the　Dog；　so　that　I　need　only
refer　to　the　figure　illustrative　（14　A）　of　its　structure。　　It　leaves
the　organ　in　which　it　is　formed　in　a　similar　fashion　and　enters　the
organic　chamber　prepared　for　its　reception　in　the　same　way；　the
conditions　of　its　development　being　in　all　respects　the　same。　　It　has
not　yet　been　possible　（and　only　by　some　rare　chance　can　it　ever　be
possible）　to　study　the　human　ovum　in　so　early　a　developmental　stage　as
that　of　yelk　division；　but　there　is　every　reason　to　conclude　that　the
changes　it　undergoes　are　identical　with　those　exhibited　by　the　ova　of
other　vertebrated　animals；　for　the　formative　materials　of　which　the
rudimentary　human　body　is　composed；　in　the　earliest　conditions　in　which
it　has　been　observed；　are　the　same　as　those　of　other　animals。　　Some　of
these　earliest　stages　are　figured　below；　and；　as　will　be　seen；　they　are
strictly　comparable　to　the　very　early　states　of　the　Dog；　the　marvellous
correspondence　between　the　two　which　is　kept　up；　even　for　some　time；　as
development　advances；　becoming　apparent　by　the　simple　comparison　of　the
figures　with　those　on　page　249。

Fig。　14。A。　Human　ovum　（after　Kolliker）。　　a。　germinal　vesicle。　b。
germinal　spot。　B。　A　very　early　condition　of　Man；　with　yelk…sac；
allantois；　and　amnion　（original）。　C。　A　more　advanced　stage　（after
Kolliker）；　compare　Fig。　13；　C。

Indeed；　it　is　very　long　before　the　body　of　the　young　human　being　can　be
readily　discriminated　from　that　of　the　young　puppy；　but；　at　a　tolerably
early　period；　the　two　become　distinguishable　by　the　different　form　of
their　adjuncts；　the　yelk…sac　and　the　allantois。　　The　former；　in　the
Dog；　becomes　long　and　spindle…shaped；　while　in　Man　it　remains
spherical；　the　latter；　in　the　Dog；　attains　an　extremely　large　size；　and
the　vascular　processes　which　are　developed　from　it　and　eventually　give
rise　to　the　formation　of　the　placenta　（taking　root；　as　it　were；　in　the
parental　organism；　so　as　to　draw　nourishment　therefrom；　as　the　root　of
a　tree　extracts　it　from　the　soil）　are　arranged　in　an　encircling　zone；
while　in　Man；　the　allantois　remains　comparatively　small；　and　its
vascular　rootlets　are　eventually　restricted　to　one　disk…like　spot。
Hence；　while　the　placenta　of　the　Dog　is　like　a　girdle；　that　of　Man　has
the　cake…like　form；　indicated　by　the　name　of　the　organ。

But；　exactly　in　those　respects　in　which　the　developing　Man　differs　from
the　Dog；　he　resembles　the　ape；　which；　like　man；　has　a　spheroidal
yelk…sac　and　a　discoidalsometimes　partially　lobedplacenta。　　So　that
it　is　only　quite　in　the　later　stages　of　development　that　the　young
human　being　presents　marked　differences　from　the　young　ape；　while　the
latter　departs　as　much　from　the　dog　in　its　development；　as　the　man
does。

Startling　as　the　last　assertion　may　appear　to　be；　it　is　demonstrably
true；　and　it　alone　appears　to　me　sufficient　to　place　beyond　all　doubt
the　structural　unity　of　man　with　the　rest　of　the　animal　world；　and　more
particularly　and　closely　with　the　apes。

Thus；　identical　in　the　physical　processes　by　which　he
originatesidentical　in　the　early　stages　of　his　formationidentical
in　the　mode　of　his　nutrition　before　and　after　birth；　with　the　animals
which　lie　immediately　below　him　in　the　scaleMan；　if　his　adult　and
perfect　structure　be　compared　with　theirs；　exhibits；　as　might　be
expected；　a　marvellous　likeness　of　organization。　He　resembles　them　as
they　resemble　one　anotherhe　differs　from　them　as　they　differ　from　one
another。And；　though　these　differences　and　resemblances　cannot　be
weighed　and　measured；　their　value　may　be　readily　estim