653 



period the head occupies nearly a third of the whole length of the 

 embryo" (Foster and Balfour). 



In the second half of the second day, among the most important 

 changes are the appearance of the second and third cerebral vesi- 

 cles, the optic vesicles, while the " first rudiment of the ear is 

 formed as an involution of the epiblast on the side of the hind 



Third day. This day is one of the most eventful, as the rudi- 

 ments of so many important organs now first appear. First, the 

 embryo, now almost completely enveloped by the amnion, turns 

 around so as to lie on its left side. The heart, originally formed 

 under where the brain is destined to lie, moves backward into the 

 trunk, mid by this time (the third day) the neck has been formed, 

 in which appears the four branchial fissures, the most anterior 

 being formed first. It is these temporary fissures which corres- 

 pond to the branchial fissures of Amphioxus. "On account of 

 this resemblance— in fact by some assumed as an identity both in 



the branchial fissures (compare Fig. 288) and the vessels ^passing 



the |,:i>s;,^s between the gills of fishes, and the latter with the 

 vessels which supply the gills with blood" (Clark's Mind in Na- 

 ture, p. 311). 



In fact the embryo bird in some respects is now as far advanced 

 in organization as the Lancelet, and may be rudely compared with 

 that animal, though the incipient neck, head and brain are features 

 which the Lancelet lacks. 



The eye commences as a lateral outgrowth of the fore brain, 

 in the form of a stalked vesicle subsequently converted into the 

 optic nerve, while the lens is formed by an involution of the skin 

 of the body (outer germ-layer) over the front end of the optic 

 vesicle. The ear is also at first simply an involution of the outer 

 germ-layer (epiblast) forming a pit, or " otic vesicle," which is 

 destined to form the internal ear, containing the bones and other 

 parts of the inner ear. The nose* begins as two shallow pits 

 formed by the sinking in of the outer germ-layer. Each of these 

 pits is situated next to the olfactory vesicles (afterwards nerves), 

 but at first there is no connection between the pits and the nerves 

 as between the pits and the mouth, which is in fact not yet formed, 

 since it arises afterwards as an extension inward of the cleft be- 



