426 



ZOOLOGY 



SECT. XIII 



At an early period the vertebral plate or dorsal portion of mesoderm 

 bounding the medullary groove becomes segmented into proto- 

 vertebraa (Fig. 1086, B, pr. v.), and the lateral plate or ventral portion 

 of the same layer splits into somatic and splanchnic layers with 

 the ccelome between (Fig. 1089, B). 



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

 in other large-yolked eggs ; but, owing apparently to 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. 1088). The body (Fig. 1087, A) becomes strongly flexed so as 

 to bring the head and tail almost 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 verte- 

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



large square mouth, 

 ventrally placed 

 nostrils connected 

 by grooves with the 

 mouth, and three or 

 four pairs of gill- 

 slits. As in Reptiles, 

 there is never any 

 trace of gills. In 

 the Ostrich and Ap- 

 teryx, as well as in 

 some Carinatae, 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. 1087, 

 B), the clefts close, with the exception of the first, which gives rise 

 to the tympano-eustachian passage, and the head becomes charac- 

 teristically avian. The limbs are at first alike in form and size 

 (A, f. L, h. L), 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 (5), while the metatarsal region 

 elongates and gives rise to the equally characteristic foot. At the 

 same time feather-papillae make their appearance, arranged in 

 narrow and well-defined pterylae. 



At an early period capillaries appear in the extra-embryonic 

 blastoderm between the opaque and pellucid areas, and give rise 

 to a well-defined area vasculosa (Fig. 1088, ar. vase.) : they are 

 supplied by vitelline arteries from the dorsal aorta, and their blood 

 is returned by vitelline veins which join the portal vein and take the 



FIG. 1088 Qallus bankiva. Egg with embryo and foetal 

 appendages, a. air-space ; all. allantois ; am. amnion ; 

 ar.vasc. area vasculosa ; emb. embryo ; yk. yolk-sac. (After 

 Duval.) 



