18 PATTEN. iVoL. sce 
PaGE 
Method of fusioniand degeneration: 2. scccscccscececcseeresncrennnz-scecersusasaecsese acs 42 
Resultant froma rhomboid esos ceee ce seecses ss coceeccenesnenes cereee seaesrersnaeaecs 42 
Median fusion, cranial and caudal flexures a necessary result of apical 
FeO 9) Oy eee eee Be eR ee orto ee ae eee OS Sacer bee Lae 43 
Contrast between arrangement of organs on a half-metamere and 
the arrangement of metameres in a whole animal.........0....22..2... 43 
Apical growth of half-metameres on a spherical surface necessarily 
results in differential growth-tension, which determines general 
shape of the body and its division into regions having different mor- 
phological characters and different physiological potentialities... 44-45 
Concrescenceof Margin of Mesoderm1e Axed, nese ccsstsnasassteec tsetse 45 
Lateral Apical growth of each mesodermic segment from a Jateral 
primitive-streak-like proliferation. ........... ...-----:csccscceecceessseeeseseeceeeeeeess 46 
Mareini of mesodermicianea acest. cae se cease cease heen, eee ese teeter enne ee eee 46 
Concrescence of same to form post-anal cloud...............::e::seeeseeeeees 47 
Description and comparison of special CaSeS.........-...-..-0ce-cessseesecsesceees 48 
Relation to apical growth, primitive streak, and to telopore................ 49 
Explanation “of Spiralise pm ents: sees ceree areas ene eee ee 49 
Polarity of the embryo is determined by the arrangement of half- 
metameres and the conditions produced by their mode of growth.. 49 
DUlus trations Of Fusion ees eee eee eee ee 52-56 
Exceptions to general law of median fusion: fusion greatest at 
posterior end'; or same at both vends.-22 22 scene eae ee 53 
Abdomen unusually perfect and well developed in embryos whose 
anterior portion is greatly reduced by median fusion and degenera- 
tion. 
Change in Shape of Fused and Degenerating Appendage’. .......2...0.0.0000-+ 56 
Increased length and diminished diameter of newly fused appen- 
dages. Irregular shape of same, etc. 
V. (GENERAL "PROGRESSIVE SDEGENIER A1IO Nip ceeece: eee eee eee ee 56 
Is probably due not to external conditions, but to either a general or 
a local lackof formative emerge: sessee een teeet ces eecee tech meaner eee 56 
Four varieties of general progressive degeneration: (1) Reduc- 
tion in rate of development. (2) Reduction in size of embryo. 
(3) Local reduction of organs. (4) Reduction in range of devel- 
opment of the entire organism, or parts of same, followed by com- 
plete retrogressive degeneration. Result is a new form of death 
forhighly orzanized*animals: ie. Seer ee ee ee 57 
Absence of organs does not affect character of remaining ones.......... 58 
The trunk is a colony of half-metameres...c:-20201-icccofes secs recto 58 
Originally absent organs are probably not restored. 
The variations described due to internal conditions, not to variations 
invexternal “factors vives cases es cee A A 58 
Frequent absence of chelicerae. Bearing on small size of same in 
allarachmids.5.20:5: 0.00 See te) Se Oona Oe oe ee 59 
Weakness of anterior part of thoraxe. 0o toe a 59 
Weakness of anterior margin of cephalic lobes.................-::000eee-se0uees 59 
Weakness of anterior end of abdomen..3.2) 0 59 
