i9 Vertebrate Embryology 



ventral to the middle part of the neck (Fig. 

 72). Although the visceral clefts may be 

 seen, without difficulty, in favorable whole 

 mounts of chicks of the proper age, they are 

 quite small, and show better in horizontal 

 sections (Fig. 63), or in chicks that have 

 been dissected so as to lay open the cavity 

 of the pharynx. 



There has been considerable discussion in 

 regard to the visceral clefts, some workers 

 holding that none of the clefts actually open 

 to the exterior ; but it seems fairly certain 

 that all of the clefts normally open to the 

 exterior, except the last or most posterior 

 one. There is also some discussion as to 

 the time of opening and closing of the 

 branchial clefts : while there is probably a 

 good deal of individual variation in this re- 

 spect, it seems likely that none of the clefts 

 open to the exterior before the early part of 

 the third day, and that they have all closed 

 before the beginning of the sixth day. It 

 must not be forgotten that when we speak 

 of the development of, say, the fifth day, 

 we mean the average state of development 

 reached by chicks during that number of days 

 of incubation. 



