XIII 



PHYLUM CIIORDATA 



439 



Gradually the embryo becomes folded off from the yolk-sac, as 

 in other large-yolked eggs ; but, owing apparently t(j the confined 

 space in which it is enclosed, it soon turns over so as to lie with 

 its left side against the yolk, and its right side facing the shell 

 (Fig. 1077). The body (Fig. 1076, A) becomes strongly Hexed so as 

 to bring the head and tail into contact, and the head soon acquires 

 a proportionally immense size, with very large projecting eyes. At 

 first the head is quite like that of one of the lower vertebrate 

 embryos, with protuberant brain-swellings (/. hr., m. hr., h. hr.), 

 large square mouth, ventrally placed nostrils connected by grooves 

 with the mouth, and three or four pairs of gill-slits. As in 



h.b, 



B 



7nn 



tzus at4..a.. 



J'hi 



Fig. 1070.— Gallus bankiva. Two stages in the development of the emhryo. nil. allautois ; 

 OA'ix. cut edge of amnioi; ; <oi. amis ; nu. ap. auditory aperture ; nu. .>•. auditory sac ; /. hr. fore- 

 brain ; /. /. fore-limb ; h. hr. hind-brain ; h. I, hind-limb ; lit. heart ; hi/, hj'oid arch ; m. h. mid- 

 brain ; inn. mandibular arch ; an. nostril ; t. tail. (After Duval.) 



Reptiles, there is never any trace of gills. In the Ostrich and 

 Apteryx, as well as in some CarinataB, an opercular fold grows 

 backwards from the hyoid arch, and covers the second and third 

 branchial clefts. Soon the margins of the mouth grow out into a 

 beak (Fig. 1076, B), the clefts close, with the exception of the first, 

 which gives rise to the tympano-eustachian passage, and the head 

 becomes characteristically avian. The limbs are at lirst alike in 

 form and size (A, /. I., h. I.), and the hands and feet have the 

 character of paws, the former with three, the latter with four 

 digits ; but gradually the second digit of the hand outgrows the 

 first and third, producing the characteristic avian manus (B), 



E E 2 



