A VES. 



153 



perhaps the formation of the limbs. They appear as outgrowths 

 from a slightly marked lateral ridge (fig. 119 WR), which runs on the 



A. 



B. 



Fie;. 120. HEAD OF A CHICK FROM BELOW ON THE SIXTH AND SEVENTH DAYS OK 



INCUBATION. (From Huxley.) 



la. cerebral vesicles; a. eye, in which the remains of the choroid slit can still be 

 seen in A; (j. nasal pits; k. fronto-nasal process; I. superior maxillary process; 

 1. inferior maxillary process or first visceral arch; 2. second visceral arch; x. first 

 visceral cleft. 



In A the cavity of the rnouth is seen enclosed by the fronto-nasal process, the 

 superior maxillary processes and the first pair of visceral arches. At the back of it is 

 seen the opening leading into the throat. The nasal grooves leading from the 

 nasal pits to the mouth are already closed over and converted into canals. 



In B the external opening of the mouth has become much constricted, but it is 

 still enclosed by the frouto-nasal process and superior maxillary processes above, and 

 by the inferior maxillary processes (first pair of visceral arches) below. 



The superior maxillary processes have united with the fronto-nasal process, along 

 nearly the whole length of the latter. 



level of the lower end of the muscle plates for nearly the whole 

 length of the trunk. This ridge is known as the Wolffiau ridge. 

 The first trace "of the limbs can be seen towards the end of the third 

 day ; and their appearance at the end of the fourth day is shewn in 

 fig. 118 Fand EL. 



A section through the trunk of the embryo on the fourth day is 

 represented in fig. 119. The section passes through the region of the 

 trunk behind the vitelline duct. The mesentery (J\f) is very much 

 deeper and thinner than on the previous day. The notochord has be- 

 come invested by a condensed mesoblastic tissue, which will give rise 

 to the vertebral column. The two dorsal aortse have now completely 

 coalesced into the single dorsal aorta, and the Wolffian body has 

 reached a far more complete development. 



In the course of the fifth day the face begins to assume a less 

 embryonic character, and by the sixth and succeeding days presents 

 distinctive avian characters. 



The general changes which take place between the sixth day arid 

 the time of hatching do not require to be specified in detail. 



