GENERAL DEVELOPMENT OF THE EMBRYO. 



sack is in consequence a double one, formed of two limbs or 

 laminae, an inner limb constituted by 

 the splanchnopleure, and an outer limb 

 by the somatopleure. The relation of 

 these two limbs is shewn in the dia- 

 grammatic longitudinal section (fig. 

 105), and in the surface view (fig. 109) 

 the splanchnic limb being shewn at sf 

 and the somatic at so. Between the 

 two limbs, and closely adjoining the 

 splanchnopleure, is seen the heart (/if). 

 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 an- 

 terior part of the neural canal becomes 

 enlarged to form the first cerebral 

 vesicle, from which two lateral diver- 

 ticula rudiments of the optic lobes are almost at once given 

 off (fig. 1 08, op.v). By the stage figured the cephalic part of the 

 neural canal has become distinctly differentiated into a fore- 

 (/./?), a mid- (m.b) and a hind-brain (h.b) ; and the hind-brain is 

 often subdivided into successive lobes. In the region of the 

 hind-brain two shallow epiblastic invaginations form the rudi- 

 ments 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. 1 10. The enlarged part of the 

 neural tube, forming the hind-brain, is shewn at (Jib}. 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 notochord (c/t) is seen below the brain, and 

 below this again the crescentic foregut (a I). The commencing 

 heart (Jit}, 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 notochord is 



Fir,. 107. DORSAL VIEW 



OF THE HARDENED BLASTO- 

 DERM OF A CHICK WITH FIVE 

 MESOBLASTIC 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. pos- 

 terior part of the primitive 

 streak. 



