68 THE DEVELOPMENT OF THE CHICK 



to the apex of the tail flexure. But even then it is questionable 

 if this measurement is as accurate a means of classification as 

 the age of normally incubated embryos; particularly as the cer- 

 vical flexure is secondarily eliminated by raising of the head. 

 It is probable that the measurement from the tip of the head to 

 the apex of the cranial flexure (head-length) would be best for 

 classification of chick-embryos by measurement. This dimen- 

 sion may be readily taken, after the cranial flexure begins, 

 throughout the entire period of incubation. However, it has 

 been relatively little used up to the present time. 



The following tables give the chronology of development up 

 to the end of the fourth day, the period usually covered in labo- 

 ratory courses. For the later chronology the student is referred 

 to Keibel and Abraham's Normaltafeln zur Entwickelungsge- 

 schichte des Huhnes (Gallus domesticus), Jena, Gustav Fischer, 

 1900. In the various chapters of Part II, the later chronology 

 of the various organs is given here and there throughout the text. 

 It is believed that these references will be sufficient on the whole 

 to enable the student to determine what embryos to select for 

 the desired stage of most organs. The tables have been made 

 practically continuous from 1 s up to 41 s, because these cover 

 the period of development in which the primordia of most organs 

 are formed. They have been constructed mostly from entire 

 mounts. The corresponding tables in Keibel and Abraham's 

 work are noted by number in the right-hand column. 



Chronological Tables of the Development of the Chick 



I. Before Laying: 



1. Maturation and fertilization; found in the oviduct above the 



isthmus. 



2. Early cleavage up to about 44 superficial cells found in the isth- 



mus of the oviduct during the formation of the shell-membrane 

 (Kblliker). 



3. Later cleavage, formation of periblast and entoderm, etc., found 



in the uterus up to time of laying. 

 Data for the pigeon given in Chapter II ; see legends to figures. 



II. Incubation to Formation of the First Somite: 



The period may be divided in three parts: (1) before the appearance 

 of the primitive streak; (2) primitive streak formed but no head process; 

 (3) after the appearance of the head-process. These stages may be sub- 

 divided by time or by length of the primitive streak. 



