HISTORY Of Tin: QBBM LAI 1:1: i BIO 



a paired >t i net ure. and compares it t> tin- lx>dy-saC8 

 which are developed in Invert l.y BVagil I'-nn the 



e.elenteron. U.u.nu'K ju>tly all."_r'- that tln> oii.'inally s.-'M con- 

 dition of the two fundaments can have no W6lghi a-^ain-t In- i: 



pretation, since in numerous instances organs \vliich < i^lit properly 

 utain cavities arc developed s >lid. and subsequently become 

 hollow, as, for example, in many Echinoderms one encounters solid 

 cell masses in place of hollow ova -/mat ions of the en 



Led 1'V theoretical con-idTat ion> similar to tlm-e of tlie ' 

 inorphilo^i>ts, my brother and ], ly a t hoi oiiL'li CMmparison of de- 

 velopmental and anati inical conditions, and with due n-^-ird to the 

 niorpliolo^ical and histolo^it-ai structure of oi-ranisii:s. then en- 

 deavored to l)i ini: to a solution this <|iie>tion of 1 he day. the (juestin 

 < f the development of the body-cavity and the middle ^'Tin-layers, 

 l.v systematic invest iirations (pnlilished in "Studien xtir IJl;ittT- 

 tlieorie"), which <'\tended over Invertebrates and Yertel>: 

 The results of the-- series of in\e-t iirations \vei-e puh]i<he 1 in 

 articles: (1) in t ho " Coelomtheorie, Versuch -iner Krklarun< des 

 mittleren Keimhlattes." and ('2) in the " Entwickluiii: des mitt ! 

 Keimblattes der \VirleHhi< 



In tin- fir>t pa pei-. in order to prepare the way, we wen- oon 

 i, r ive the fi'i-iti ;/"i m ni-lnyer a more precise definition. We ! 

 as such a layer of tinliri/onic reify /'/,/'>-// are arra/^/''<f like an 

 cfnthi-lium <ntd serve JOT tJn' fi/xifufion of the surface* of the h 

 At the close of segmentation there is only one ifmn-fn */!' ;>< 

 namely, the rjiif/it-lii'm <>f ///" AA/X////I/. The remaininff f/erm-layers 

 from it by the jyrocesscs of iiirni/imifimi <m<l <-r<i'i ,<> itin. The 

 '/> nn -layer is formed by means of gastrulation, the two middk 

 i>'rin-i<iiii'r>t /'// ///' fi.nmitti.H of tfie body-cavities, in ttxit tiro body-sacs 

 are evaginated from the ccelfi>f>n>t>. ,! </row out l< ' <1 separate 



tin' tin> jiriiiun'1/ <i>'rin-Jn >/>'i'8. There are, in the first place, animals 

 which are formed of two germ-layers, and possess in tin ir l-die> only 

 one cavity, a coelenteron, produced ly i n\ a iri nation (( 'o-lentei ata 

 and Ps.Midoc.rlia). and. sei-ondly. animals with four i:erm-la\ -er>. a 

 secondary intestine, and a body-cavity derived fn m the c.elenteron 

 an enter ocoel. To the t\\<> layered animals belong tin- ( u-lenterates 

 and the PseudocuAs. but all four layered animals are Enterocci'ls. 



From this standpoint we end. prove that hitherto there 



had been confused under the conception "middle irerm-layer" two 

 things which are genetically, morphologically, and historically 

 entirely different. 



