930 



OF GENERATION EMBRYONIC DEVELOPMENT. 



whilst in front it gives off the two aortse, which bend round the front end of 

 the foregut and gain its upper side, when they turn back and run in the me- 

 soblast on each side of the notochord to the tail. About the middle of the 

 embryo each gives off a large omphalo-mesenteric artery, which is distributed 

 over the pellucid and vascular areas, from whence the blood is returned by 

 the omphalo-mesaraic veins (Fig. 340, R.Of.A, L.0f\ At the close of the 

 first day the protrovertebrse begin to form in the uncleft portion of the meso- 

 blast near the middle line, which is called the vertebral plate. The first one 

 that makes its appearance is the body of the third or second cervical, others 

 soon becoming segmented off the vertebral plate both before and behind. 

 At this time the forepart of the medullary canal with its optic and cerebral 

 vesicles, which has hitherto been straight, begins to curve over the anterior ex- 



FIG. 334. 



Transverse section of the Embryo of a Fowl at the beginning of the third day of incubation X 90 

 100: ch, chorda dorsalis; uwh, undivided mesoblast, in which the protovertebra? are subsequently de- 

 veloped; hp, somatopleure ; df, intestinal fibrous lamina; dd, intestinal glandular lamina; dr, primitive 

 intestinal groove; h, corneal lamina (the line in the drawing extending a little too far); mr, medullary 

 tube (spinal cord); TO, muscular lamina ;p, pleuroperitoneal cavity; a/, fold of the amnios; ao, primitive 

 aorta; vc, vena cardinalis; un, Wolfflan body; ung, duct of the Wolffian body. 



tremity of the notochord, which is the commencement of the cranial flexure; 

 and so'on after the whole embryo becomes curved upon itself. On the third 

 day certain fissures or clefts, termed the visceral or branchial clefts, appear, de- 

 veloping in succession to the number of four from before backwards, and from 

 within outwards. The anterior border of each forms a visceral or branchial 



FIG. 335. 



B 



Progressive stages in the Segmentation of the Yolk of the Mammalian Ovum : A, its first division 

 into two halves; B, subdivision of each half into two; c, further subdivision, producing numerous 

 segments. 



fold, so that there are five folds. The first pair form the inferior maxillae, 

 and by a pair of buds they give off from their upper border, the superior 

 maxilhc. Other structures are formed in the remaining folds, which will 

 hereafter receive mention ( 795, 800). 



