\I:I.ISHMI;M <.i i : \.\i. n>uM Off in;: i'.mv. 



The dilleiviu e i- i -t'-i a Me, first of nlL t or less extensive 



lt nJ.it in,, ,>(' nutritio ittk, the si^nitic.i : the 



na-cent organism is twofold. 



1-Yom a y.'/y.NW /,//(/'' '-. the nutritive yolk i- a rich 



source of energy which alone inake> it p:..v>il>]e for tin- emltrvolo^ie.il 



;> take place in iininterrupted sequence, until at length an 

 organism. \\ith an already relatively high organisation, l.'-ins its 



Independent existence. 



1 Voni a morphological point ofvi<-. on t he otlier hand, the yolk plays 

 the rule of ballast, which exerts a restrictive and modify in;: intluence 

 on tin* direct and tV o development of those organs \\liic-h are en- 

 \\itli thf reception and elaboration of it. Even at tin* \.-ry 

 of development we could see how the cleavage- process and 

 '.he forma tii n of the germ-layers were retarded, altered, and to a cert a in 

 nt even suppivssrd by the presence of yolk. In what follows we 

 shall a.irain have occasion to point out the same thing, how, owinij 

 to the presence of yolk, the normal formation of the intestinal canal 

 and of the body can be attained only gradually and by a ciivuit<ni> 

 proa 



In the second place, the great difference which the embryo 

 Vertebrates present is produced by the medium in which the eggs 

 underiro development. Eggs which, like those of water-inhabiting 

 Vertebrates, are deposited in the water, are developed in a more 

 simple and direct manner than those which, provided with a firm 

 shell, are laid upon the land, or than those which are enclosed in 

 the womb up to the time of the birth of the embryo-. 



In the two latter cases the growing organism attains its goal onlv 

 l>y very indirect ways. At the same time with the /- ///,/// /// . 

 there are also developed others which have no >ii;niticanre for the 



einlij-yonii- life, but which aerVQ during the eg^-stauv of e 

 ence either for the protection of the soft, delicate, and easily injured 



y, or for myj//vf//fM, or for nutrition. Tie > either und--riro 



metamorphosis at the en<l of embryonic life, or are 

 otVat birth a- u> "less and unimportant structures. Jlut inasmuch as 

 they are d -vel.ip.d out of the germ-layers, they ; ,re also properly to 

 be r?nr>trdfd as lu-lnii^ing immediately to the nism as 



bi inu it> cinlriitniic organs, and as such they too are to ! t reared in 

 morphological de.scrip: ion- 



The extensive material which has to he inaMer. d in this con- 

 nection I .shall pr s 'lit grouped into | 



In ti' ( we shall im|uiiv how the embryo overcomes the 



