4/2 NOTES. 



signification de I'ceuf." Brussels, 1870. Hubert Ludwig, " On 

 Egg-structure in the Animal Kingdom " (" TJeber die Eibildung 

 in Thierreiclie." Wiirzburg, 1874). 



73 (i. 226). Disooidal cleavage {Segmentaiio discoidalis) of 

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

 and tlie Higher Animals " (" Entwickelungsgescbiclite des Men- 

 schen und der hoheren Thiere." 2nd edition, 1876, pp. 69-81 ; 

 Figs. 16-22). 



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

 Rauber, " On the Place of the Chick in the System of Evolu- 

 tion " (" TJeber die Stellung des Hiihnchens im Entwiokelungs- 

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

 Embryology." London, 1874. 



75 (i. 231). Bladder-gastrula {Perigast7-ula) of Articulated 

 Animals (Arthvpodcc). Cf. Bobretzky, "Russian Essay on the 

 Germ-history of Astacus and Palfemon." Kiew, 1873. Also my 

 own article on the gastrula and egg-cleavage.' — " Jen. Zeitschrif t 

 fiir Naturwissenschaft." Yol. ix. pp. 444-452, Plate VI. 



76 (i. 234). The Four-layer Theory, which was first clearly 

 stated by Baer in 1837 (" Entwickelungsgeschichte der Thiere," 

 vol. ii. pp. 46, 68), and which we have here carried out logically, 

 yet appears the only form of the germ-layer theory, which, 

 on comparative observation of all higher animals, supplies a 

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

 the inconsistent reputations of many observers. 



77 (i. 239), Caspar Eriedrich WolfE first indicated the Four- 

 layer Theory (note 76). Cf. the remarkable sentence, quoted at 

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

 intestinal canal (note 19). 



78 (i. 240). The Four Main Types of Gastrulation, which 

 are diagrammatically distinguished in Plates II. and III., and in 

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

 by intermediate forms. These are transitions both between the 

 primordial and the unequal forms, and between the primordial and 

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

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



