GQ J. Thos. Patterson. 



remained unsatisfactorily answered. Nearly all recent writers 

 acknowledge that the problem is far from being solved. Thus 

 Nowack, writing in 1902, admits that he has failed to make clear 

 the exact manner in which the entoderm takes its origin. He says, 

 "Ich bin leider nicht in der Lage, auf Grund meiner Praparate 

 eine absolut sichere Erkliirung liber die Entstehung des inneren 

 Keimblattes zu geben. Das aber kann ich mit aller Bestimmtheit 

 behaupten, dass das Entoderm nicht als eine Einstiilpung am Rande 

 des Blastoderms entsteht, wie es nach Duval der Fall sein soll."^ 

 The study of comparative embryology, nevertheless, would lead us 

 to expect to find this germ layer arising by a process of gastrulation. 

 Aside from a few descriptions of isolated stages, however, the theory 

 of gastrulation is supported, by actual observations, only in the work 

 of Duval ('84) ; but Duval's interpretation has been disputed on 

 the ground that he was probably misled through the use of path- 

 ological material (Kionka, '94, Barfurth, '95, Schauinsland '99) ; 

 and, as I have previously pointed out ('07, 6), this author's work 

 tends to support the idea of delamination. In this connection 

 the statement of Hertwig ('03) is of special interest, in that he has 

 often quoted Duval in support of gastrulation, but now says, "Der 

 Darstellung Duval's war ich in meinem Lehrbuch liingere Zeit ge- 

 folgt, halte sie aber jetzt nicht mehr fiir richtig und glaube, dass 

 die in Fig. 482'* am hinteren Rand der Keimhaut abgebildete Spalte 

 zwischen Embryonalzellen und peripherem Dottersyncytium durch 

 die Hartung oder beim Schneider kiinstlich erzeugt ist und mit 

 einer Gastrulation nichts zu thun hat."^ 



Before a complete history of the early development of the bird 

 can be written, therefore, it is necessary to give a detailed account, 

 not only of gastrulation itself, but also of the stages preceding and 

 immediately following it. Such an account is rendered possible 

 by the fact that the writer has been able to secure a close series of 

 stages covering this period of development. 



The results recorded here are the outcome of a line of investiga- 



^Loc. cit., p. 27. 



^Hertwig bere refers to Fig. 8, Piute I, of Duval ('84). 



^Loc. cit., p. 8G1. 



