4/2 NOTEa. 



4 



signification de Tceuf." Brussels, 1870. Hubert Lndwig, "On 

 Bgg-stnictnre in tbe Animal Kingdom " (" Ueber die Eibildung 

 in Thierreicbe." Wiirzburg, 1874). 



73 (i. 226). Discoidal cleavage {Segmentatio discoidalis) ol 

 Bird's eggs. Cf . Kolliker, " History of tbe Evolution of Man 

 and tbe Higber Animals " (" Entwickelungsgescbicbte des Men- 

 scben und der boberen Tbiere." 2nd edition, 1876, pp. 69-81 ; 

 Figs. 16-22). 



74 (i. 227), Disc-gastrula (Disco-gastrula) of Birds. Cf. 

 Rauber, "On tbe Place of tbe Cbick in tbe System of Evolu- 

 tion " (" XJeber die Stellung des Hiibncbens im Entwickelungs- 

 plan "). Leipzig, 1876. Foster and Balfour, " Tbe Elements of 

 Embryology." London, 1874. 



75 (i. 231). Bladder-gastrula (Perigastrula) of Articulated 

 Animals (^Arthrojpoda) . Cf. Bobretzky, "Russian Essay on tbe 

 Gerra-bistory of Astacus and Pal?emon." Kiew, 1873. Also my 

 own article on tbe gastrula and egg-cleavage. — " Jen. Zeitscbrift 

 fiir Naturwissenscbaft." Vol. ix. pp. 444-452, Plate VI. 



^Q (i. 234). Tbe Four-layer Tbeory, wbicb was first clearly 

 stated by Baer in 1837 (" Entwickelungsgescbicbte der Tbiere," 

 vol. ii. pp. 46, 68), and wbicb we bave bere carried out logically, 

 yet appears tbe only form of tbe germ-layer tbeory, wbicb, 

 on comparative observation of all bigber animals, supplies a 

 universal law of germination for all and at tbe same time meets 

 tbe inconsistent reputations of many observers. 



77 (i. 239). Caspar Friedricb Wolff first indicated tbe Four- 

 layer Tbeory (note 7Q^. Cf. tbe remarkable sentence, quoted at 

 vol. i. p. 45, from bis pregnant work on tbe formation of the 

 intestinal canal (note 19). 



78 (i. 24Q). Tbe Four Main Types of Gastmlation, wbicb 

 are diagrammatical ly distinguisbed in Plates II. and III., and in 

 Tables III. and IV. (vol. i. pp. 241, 242), are of course connected 

 by intermediate forms. Tbese are transitions botb between tbe 

 primordial and tbe unequal forms, and between tbe primordial and 

 tbe superficial forms ; similarly, tbe unequal form of egg-cleavage 

 is connected by twixt-f orms with tbe discoidal forms, which latter 



