410 ZOOLOGY SECT. 



embrace the blastopore in Amphioxus. Both primitive streak and 

 medullary groove lie at right angles to the long axis of the egg, 

 the broad end of the latter being to the embryo's right. 



The blastoderm gradually extends peripherally, so as to cover 

 the yolk, and thereby becomes divisible into an embryonic portion, 

 from which the embryo is formed, and an extra- embryonic portion 

 which invests the yolk-sac, and takes no direct share in the forma- 

 tion of the embryo. The extension of the ectoderm and endoderm 

 takes place regularly and symmetrically, but the mesoderm, while 

 extending equally in the lateral and posterior regions, grows for- 

 wards in the form of paired extensions, which afterwards unite, so 

 that for a time there is an area of the blastoderm in front of the 

 head of the embryo, formed of ectoderm and endoderm only, and 

 called the pro-amnion (pr. am.}. 



At an early period the vertebral plate or dorsal portion of meso- 

 derm bounding the medullary groove (p. 114) becomes segmented 

 into protovertebrse (Figs. 1013, B, and 748, I>,pr. v.), and- the lateral 

 plaU or ventral portion of the same layer splits into somatic and 

 splanchnic layers with the ccelome between (Fig. 748, B). The 

 notochord (nch.) is developed in the middle line below the medullary 

 groove : sometimes it arises directly from the endoderm, as in 

 most of the lower forms, sometimes the mesoderm is formed as a 

 continuous plate, the axial portion of which is subsequently divided 

 off as the notochord. 



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. 1015). The body (Fig. 1014, A) becomes strongly flexed 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 the lower vertebrate 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 Apteryx, as well 

 as in some Carinatse, 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. 1014, B), the 

 clefts close, with the exception of the first, which becomes the tym- 

 pano-eustachian passage, and the head becomes characteristically 

 avian. The limbs are at first alike in form and size (A,/. /., h. /. ) 

 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), while the metatarsal region elongates and gives 

 rise to the equally characteristic foot. At the same time feather- 



