IV.] THE MESOBLASTIC SOMITES. 81 



the vitelline veins, uniting to form the heart h, already 

 established by the coalescence of two lateral halves which, 

 continuing forward as the bulbus arteriosus b.a., is lost in 

 the substance of the head just in front of the somatopleure 

 fold. 



HE. hind-brain ; MB. mid-brain ; p.v. and v.pl. mesoblastic 

 somites ; ch. front end of notochord ; mc. posterior part of 

 notochord ; e. parietal mesoblast ; pi. outline of area pellu- 

 cida ; pv. primitive streak. 



head two shallow pits are visible. They constitute the 

 first rudiments of the organ of hearing, and are known as 

 the avditory pits (Fig. 27, au.p.). 



The number of mesoblastic somites increases rapidly 

 by a continued segmentation of the vertebral plates of 

 mesoblast. The four or five pairs formed during the 

 first day have by the middle of the second increased to 

 as many as fifteen. The addition takes place from 

 before backwards ; and the hindermost one is for some 

 time placed nearly on a level with the boundary be- 

 tween the hind end of the trunk of the embryo, and 

 the front end of the primitive streak. For some time 

 the already formed somites do not increase in size, 

 so that at first the embryo clearly elongates by addi- 

 tions to its hinder end. 



Immediately behind the level of the last meso- 

 blastic somite there is placed an enlargement of the 

 unclosed portion of the medullary canal. This enlarge- 

 ment is the sinus rhomboidaUs already spoken of. It 

 is shewn in Fig. 23. On its floor is placed the front 

 end of the primitive streak. It is a purely embryonic 

 structure which disappears during the second day. 



In a former chapter it was pointed out (p. 27) 

 that the embryo is virtually formed by a folding 

 F. & B. 6 



