A VES. 



141 



cavity between these two layers lias already become pretty wide in 

 the lateral parts of the body of the embryo 

 and the area pellucida. The fold by which 

 the embryo is constricted off from the yolk- 

 sack is in consequence a double one, formed 

 of two limbs or laminae, an inner limb con- 

 stituted by the splanchnopleure, and an 

 outer limb by the somatopleure. The re- 

 lation of these two limbs is shewn in the 

 diagrammatic longitudinal section (fig. 105), 

 and in the surface view (fig. 109) the 

 splanchnic limb being shewn at sf and 

 the somatic at .<?o. Between the two limbs, 

 and closely adjoining the splanchnopleure. 

 is seen the heart (Jit). At the stage figured 

 the head is well marked off from the trunk, 

 but the first separation between the two 

 regions was effected at an earlier period, 

 on the appearance of the foremost somite 

 (fig. 107). Very shortly after the cephalic 

 region is established, and before the closure 

 of the medullary folds, the anterior part 

 of the neural canal becomes enlarged to 

 form the first cerebral vesicle, from which 

 two lateral diverticula — rudiments of the 

 optic lobes — are almost at once given off 

 (fig. 108, op.v). By the stage figured the cephalic part of the neural 

 canal has become distinctly differentiated into a fore- {f.h), a mid- (ni.h) 

 and a hind-brain [h.h); and the hind-brain is often subdivided into 

 successive lobes. In the region of the hind-brain two shallow epi- 

 blastic invaginations form the rudiments of the auditory pits {au. p). 



A section through the posterior part of the head of an embryo of 

 30 hours is represented in fig. 110. The enlarged part of the neural 

 tube, forming the hind-brain, is shewn at (hh). It is still connected 

 with the epidermis, and at its dorsal border an outgrowth on each 

 side forming the root of the vagus nerve is present (vg). The noto- 

 chord (ch) is seen below the brain, and below this again the crescentic 

 foregut (al). The commencing heart (ht), formed at this stage of 

 two distinct tubes, is attached to the ventral side of the foregut. 



On the dorsal side of the foregut immediately below the notocLord is 

 seen a small body (x) formed as a thickening of the hypoblast. This may 

 possibly be a rudiment of the subnotochordal rod of the Ichthyopsida. 



In the trunk (fig. 108) the chief point to be noticed is the com- 

 plete closure of the neural canal, though in the posterior part, where 

 the open sinus rhomboidalis was situated at an earlier stage, there 

 may still be seen a dilatation of the canal (fig. 108, s.r), on each 

 side of which are the tail-swellings ; while the mesoblastic somites 



Fig. 107. Dorsal view of 

 the haedened blastoderm of 

 a chick with five mesoblas- 

 TIC SOMITES. The medullary 



FOLDS HAVE MET FOR PART OF 

 THEIR EXTENT, BUT HAVE NOT 

 UNITED. 



a.pr. anterior part of the 

 primitive streak ; p.pr. posterior 

 part of the primitive streak. 



