546 EMBRYOLOGY OF THE LOWER VERTEBRATES oh. 



day. The hind-gut is still closed posteriorly but the foregut late 

 in the third or during the fourth day becomes thrown into communi- 

 cation with the stomodaeum by rupture of the velar membrane. 

 The pituitary rudiment makes its appearance. The four gill-pouches 

 are formed and reach the ectoderm, the fourth in the closing hours 

 of the day, and the first or it may be the first two become perforate. 

 The thyroid rudiment makes its appearance and during the latter 

 half of the day becomes closed. The pulmonary rudiment develops 

 and becomes constricted off from the pharynx except at its front end. 

 About the beginning of the day the two liver rudiments appear and 

 during its course the process of anastomosis begins between the 

 branches which sprout out from them. During the latter half of 

 the day the pancreatic rudiments make their appearance — first the 

 dorsal, then the left ventral, then the right ventral. 



During the course of the day the mesoderm segments increase 

 from about 20 to 25 up to about 40. Early in the day the Wolffian 

 duct becomes tubular and in the latter half of the day it completes 

 its backward growth and reaches the cloaca. The germinal epithelium 

 becomes recognizable. 



The skeleton remains throughout the day purely notochordal. 



The heart retains its S-shape and during the latter half of the day 

 the atrial septum begins to develop. The two dorsal aortae begin 

 about the commencement of the third day to undergo their fusion to 

 form the definitive unpaired aorta. In addition to the first one or 

 two aortic arches which are already present the third makes its 

 appearance (Fig. 241, A, III, p. 550), then the fourth, and during the 

 latter half of the day the sixth, while the first becomes obliterated. 

 As regards the venous system the most important feature is the 

 assumption of the same general plan of the main trunks as is 

 characteristic of Fishes. 



Finally it should be noted that during this day the body of the 

 ejnbryo becomes enclosed within the amnion. 



It will be realized even from the bare summary that has been 

 given that the third day of incubation of the Fowl's egg is morpho- 

 logically the most important of all and the student will be well 

 advised to devote a good deal of time to making a detailed study of 

 embryos of this period. 



The Fourth Day of Incubation. — By the end of the fourth day 

 of incubation the blastoderm has spread about half-way round the 

 yolk. The vessels of the vascular area are conspicuous, though it is 

 to be noticed that the terminal sinus is becoming relatively less so 

 than it was during the third day. The folding off of the body of the 

 embryo has progressed greatly. By the extension backwards of the 

 head fold the region of the heart has become floored in on its ventral 

 side. Posteriorly the tail fold is deepening in a similar fashion. 

 Between head fold and tail fold the somatopleure of the embryonic 



body is prolonged ventralwards into a very short and wide tube the 



somatic stalk — the wall of which is reflected dorsalwards as the true 



