208 



EMBRYOLOGICAL TYPES 



tube, the neural crests come into existence, as in the frog. 

 Even at very early stages, the rudiments of the eyes may be 

 seen as outpushings to each side from the fore-brain. In 



Fig. 96. — Transverse sections through an embryo chick after 30 hours' 

 incubation. (From Jenkinson.) 



Taken in the planes shown by lines 4 and 5 on Fig. 94. 5 is anterior. 

 a, lateral dorsal aorta ; av, auditory vesicle ; c, coelom ; cv, cardinal vein ; 

 ec, ectoderm ; en, endoderm ; fg, foregut ; ht, heart endothelium ; htm, mus- 

 cular wall of heart ; kt, kidney tubule ; mg, neural groove ; ms, mesodermal 

 somite ; mt, nerve-tube ; n, notochord ; nc, neural crest ; nt, nephrotome ; 

 pc t pericardium ; so, somatopleur ; spl, splanchnopleur ; vv, vitelline vein. 



front, the neural tube remains open for a time at the neuropore, 

 which is situated at that part of the brain destined to become 

 the lamina terminalis. 



Amnion. — In front of the head of the embryo, a fold rises 



