1 84 



DEVELOPMENT DURING THE FOURTH DAY. 



Various other features in the development of the vascular 

 system, general mesoblast, etc., are also represented in these 

 sections. It may more especially be noted that there are at 

 first two widely separated dorsal aortae, which gradually approach 

 (figs. 115 and 116); and meeting first of all in front finally 

 coalesce (figs. 117 and 119) for their whole length. 



The general appearance of the embryo of the fourth day may 

 be gathered from fig. 118. 



FIG. 120. HEAD OF A CHICK FROM BELOW ON THE SIXTH AND SEVENTH DAYS 



OF INCUBATION. (From Huxley.) 



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

 seen in A; g. nasal pits; k. fronto-nasal process; /. superior maxillary process; 

 i. inferior maxillary process or first visceral arch; ?. second visceral arch; x. first 

 visceral cleft. 



In A the cavity of the mouth 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 fronto-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. 



The changes which have taken place consist for the most 

 part in the further development of the parts already present, and 

 do not need to be specified in detail. The most important event 

 of the day is perhaps the formation of the limbs. They appear 

 as outgrowths from a slightly marked lateral ridge (fig. 1 19, WR), 

 which runs on the level of the lower end of the muscle-plates for 



