No. 1.] VARIATIONS IN LIMULUS POLVPHEMUS. 19 
PaGE 
Relation of these inherent lines and regions of weakness to meta- 
merism, to transverse fission in annelids, and to the morphological 
and physiological differentiation of the body into head, thorax, 
abdomen wetcre was Weasntly Saude we eee cina id eet 61 
Median fusion an indication of structural weakness and degeneration. 61 
In the case of organs near the apex of the body, ze. ocelli, olfactory 
organs, and lateral eyes, those on the most anterior segment show 
the greatest amount of median fusion. Conversely the degree of 
median fusion is an index of the original sequence of organs.......... 62 
Further degeneration of embryos........................ shade de aaed encmease Meee. ease 63 
Reduction to separate head and tail rudiments. .....0........-.eececseceeeeeeee-s 65 
Reduction to separate invapinated ‘Sacs.........c.cc:t..205-s0ls.cseseeeeeeess 65 
Completeidisappearance Gf latter: <..-c:--22:cscreceesacenctc cesta ee 66 
Comparison of same with primitive cumuli and with the primitive 
head and itronk of an annelid larvac 2c). .c:2.c---<c4-ccsessceaececeeseetee sent 66 
Bearaneon. trochosphere. theory: ..-.s2.c.seccc te eee oe 66 
Wid RSIS C0 <1 act Pes Oe BO a ne ee EE ED eee? SEL DW 66 
Absence of multiple embryos in early stages of development............ 66 
Transverse fission and longitudinal fission. Fundamental difference 
betweemitnestwoumethods: 2.0 82)22....09 he ee ee 67 
Transverse fission due to local degeneration and median fusion, 
longitudinal fission to the intercallation of new organs.................... 67 
BFL REID CRSEM TESSIOM 6) ac: a Sdscnnansszacteoxs kage see ee ee 67 
Potential fission plane, indicated in abnormal embryos by: (a) space 
between third and fourth pairs of appendages ; (4) difference in 
the inclination of appendages on either side of this line; (c) con- 
striction in region of fourth thoracic segment, due to median 
fusion and degeneration of adjacent organs. ...............::10:c10-ee 68 
Ostenorn portion) shows preatest vitality. :s:se = ace oe 70 
Similar potential fissure planes in region of chelicerae and cheliceral 
SEPIeN Gm eeeese Nf cesta SO Be See eae) Oe eee eon 71 
Bearing on the morphology of trunk regions throughout segmented 
animals) (see:also SOG: LV) \stoscccetssgeccceseses. aeececee ct oes cones ee meee 71 
B. Longitudinal Fission; Double and Triple Embry u....c1ce2ecceen- 71 
INotidueito:tusion of separate embryos ies ce se.scsess = ss ece eee eee neces ies 
1. Double Embryos. 
Due to intercallation of new halves. New and old halves always 
matchweach) ‘other! exactly. .cs:f 208 oo. .scesoc-ncsssecvescusee seeceee caseeee seescearere eae 72 
ew organs appear in reverse order of their disappearance by median 
fuston. When new parts are added, organs morphologically the 
oldest appear first, as in ordinary growth, but in the reverse direction, 
— that is, from before backwards............-. dain ceieefasav cap tedznasns te scotaagasssewoustace 73 
Wawa formatlon! Of NeW; OLEAN S oer cseee se sec eee este eee meena es 74 
Axis of repetition and axis of differentiation at right angles to each 
OLDER ene ie ate lees sebcebtbaSe conte scnetscesaee eRe ese an rama tee HCL ReE cee 74 
Splitting of unpaired appendages begins at apex and extends towards 
the base of the appendage: this is the exact reverse of what occurs 
MM), MVE AM LUSL OM! < cn cs- ccc. csescnssoncasedasscdasnushecechseetbecccseanwes tec accnenerosstesseoe 75 
