ON EMBRYOLOGY AS AN AID TO ANATOMY 49 



is entirely within the substance of the mesoblast. 

 In the earliest stage of development the mesoblast 

 is solid and there is no coelom. In the trunk the 

 mesoblast very early splits into two layers which 

 become separated from each other and so give rise 

 to the coelom. It was formerly assumed to be a 

 sharp distinction between the trunk on the one 

 hand and the head and neck on the other, that in 

 the former the mesoblast splits in this manner and 

 that in the latter it does not do so. This distinction 

 is now known not to hold good, for the splitting 

 of the mesoblast has been shown to extend to the 

 head, but the cavities on each side do not meet in 

 the median ventral line. Again, in the trunk the 

 coelomic cavity becomes divided on either side into 

 an upper or dorsal part, and a lower or ventral 

 part. Each of the dorsal portions becomes cut up 

 transversely into a number of segments arranged 

 one behind the other in series, one such segment 

 occurring in each primary body segment or 

 protovertebra. In the head the coelomic cavity is 

 first cut up transversely by the visceral clefts into 

 a series of cavities, one in each visceral arch, and 

 then each cavity divides again into dorsal and 

 ventral portions. The ultimate result is the same as 

 in the trunk, but the order of division is different. 



Of these divisions of the coelom in the head, or 

 head cavities as they are called, we are only 

 concerned with the three most anterior. The 

 first head cavity the premandibular is in front 

 of the mandible and immediately behind the eye ; 

 the second the mandibular is situated in the 



D 



