142 



DEVELOPMENT DURING THE SECOND DAY. 



stop short somewhat in front of it. Underneath the neural canal may 

 be seen the notochord (fig. 109, ch) extending into the head, as far as 

 the base of the mid-brain. At the sides of the trunk are seen the 

 mesoblastic somites (p v), the outer edges of which mark the boundary 

 between the vertebral and lateral plates. A fainter line can be seen 

 marking off the part of the lateral plates which will become part of 

 the body-wall, from that which pertains to the yolk-sack. 



During the latter half of the second day, and during the third 

 day, great progress is made in the folding off of the embryo. Both 

 the head- and tail-ends of the embryo become quite distinct, and 

 the side-folds make such considerable progress that the embryo is 

 only connected with the yolk by a broad stalk. This stalk is double, 

 and consists of an inner splanchnic stalk, continuous with the walls 

 of the alimentary canal, and an outer somatic stalk, continuous with 

 the body-walls of the embryo. The somatic stalk is very much wider 

 than the splanchnic. (Compare fig. 121 E and F, which may be 

 taken as diagrammatic longitudinal and transverse sections of the 

 embryo on the third day.) A change also takes place in the position 



Fig. 108. Embbyo of the 

 chick between 30 and 36 hours 

 viewed from above as an opaque 

 OBJECT. (Chromic acid prepara- 

 tion.) 



f.h. front-brain; m.b. mid- 

 brain; h.b. hind-brain; op.v. optic 

 vesicle; au.-p. auditory pit; o.f. 

 vitelline vein ; p.r. mesoblastic 

 somite; m.f. line of junction of 

 the medullary folds above the me- 

 dullary canal ; s. r. sinus rhom- 

 boidalis ; t. tail-fold ; p.r. remains 

 of primitive groove (not satisfac- 

 torily represented) ; a.p. area pel- 

 lu<3ida. 



The line to the side between 

 p.v. and vi.f. represents the true 

 length of the embryo. 



The fiddle- shaped outline in- 

 dicates the margin of the pellucid 

 area. The head, which reaches 

 as far back as o.f., is distinctly 

 marked off; but neither the so- 

 matopleuric nor splanchnopleuric 

 folds are shewn in the figure ; the 

 latter diverge at the level of o.f., 

 the former considerably nearer the 

 front, somewhere between the 

 lines m.h. and h.b. The optic 

 vesicles op.r. are seen bulging out 

 beneath the superficial epiblast. 

 The heart lying underneath the 

 opaque body cannot be seen. The 

 tail-fold t. is just indicated; no 



distinct lateral folds are as yet visible in the region midway between head and tail. 



At m.f. the line of junction between the medullary folds is still visible, being lost 



forwards over the cerebral vesicles, while behind may be seen the remains of the" sinus 



rhomboidalis, s.r. 



