embryo of seventeen segments 

 Transverse Sections 



47 



Transverse Section through the Fore-brain and Optic Vesicles (Fig. 43). — The 

 optic stalks connect the optic vesicles laterally with the ventral portion of the fore-brain. 

 Dorsally the section passes through the mid-brain due to the somewhat ventrally flexed head 



Mid-brail! 



Ccelom 

 Somalopleure 



Optic vesicle — -^4 ^'.i\:^^ 4 



mm: M^^ 



Ectoderm 



Mesenchyme 



Lens anlage 



Splanchnopleure 



Fig. 43. — Transverse section through the fore-brain of a thirty-eight-hour chick embryo. X 75. 



(cf. Fig. 42). We have alluded to the thickening of the lens placode. Note that there is now 

 a considerable amount of mesenchyme between the ectoderm and the neural tube. Layers 

 of mesoderm are present in the underlying blastoderm. 



Ectoderm 



Mesenchyme 



Descending aorta- 

 Ventral aorta 



Somatopleure. 



Mid-hrain 



Notochord 



Fore-gut 



Pharyngeal membrane 



Splanchnopleure 



Fig. 44. — Transverse section through the pharyngeal membrane of a thirty-eight-hour chick embryo* 



X75. 



Transverse Section through the Pharyngeal Membrane and Mid-brain (Fig. 

 44). — In the mid- ventral line the thickened ectoderm bends up into contact with the entoderm 

 of the rounded pharynx of the fore-gut. At this point the oral opening will break through. 



