DEVELOPMENT OF EMBRYO OF FOWL. 37 



be interesting to determine whether or not somites had the same 

 role in defective as in normal embryos. 



The numerical value of the somites in the chick seems to be 

 normally as follows : 



96-hour Chick. Eight-day Chick. 



I, 2, 3, Cephalic, hypoglossus region. 1 



4 to 1 6, Prebrachial, trunk. I to 13, prebrachial. 



17, 18, 19, Brachial. 14, 15, 16, brachial. 



20 to 25, Between wing and leg. 17 to 22, between wing and leg. 



26 to 32, Leg somites. 23 to 29, leg. 



33 to 35> Region of cloaca, 301032, region of cloaca. 



36 to 42, Caudal somites. 33 to 38, caudal. 



The count in the eight-day chick is really an enumeration of 

 nerves. For the four-day chick the exact number of the leg 

 somites was determined by comparison with the enumeration of 

 nerves of the eight-day chick. The wing somites may readily be 

 distinguished at four days, in entire mounts, by their size and by 

 the relatively large amount of mesenchyme formed opposite them. 

 In embryos of more than four days of age the enumeration by 

 nerves is much the easier ; and, as the brachial nerves (14, 15, 16) 

 are very much larger than their neighbors (Figs. 3 and 5, B.P.}, 

 it is simplest in determining the place of a postbrachial somite or 

 nerve to count only the somites back of the last brachial nerve. 

 Thus the postbrachial somites i 6 are between arm and leg ; 

 postbrachial 7-13 are leg-somites, etc. 



Experiment 125. 



In this experiment a very considerable part of the hind-end 

 was destroyed at the time of appearance of the tail-bud (see 

 operation-diagram), and the embryo was allowed to develop for 

 about four days more (91 hours). The egg was then reopened. 

 The vascular area covered at least three fourths of the yolk. 

 There was no allantois. The embryo lay in large part beneath 

 the blastoderm, but a large aperture in the latter towards the 

 hind-end of the embryo was filled by the amnion through the 

 transparent walls of which the embryo could be distinctly seen. 

 A large part of the blastoderm was removed with the embryo 

 and examination of the under surface was then made in salt solu- 



1 In this enumeration I have omitted again the incomplete anterior cephalic somite. 



